CBSE Class 10 Chemistry Periodic Classification of Elements Worksheet Set F

PERIODIC CLASSIFICATION OF ELEMENTS

In the beginning of 18th century Joseph Louis Proust stated that hydrogen atom is the building material and atoms of all other elements are simply due to the combination of number of hydrogen atoms. (It is to be noted that at his time the atomic weight of all elements were given as whole numbers and the atomic weight of hydrogen was taken as one.)

DOBEREINER’S TRIADS
A German chemist Johann Wolfgang Dobereiner (1829) noted that there were groups of elements with three elements known as triads. Elements in each group or a triad possess with similar chemical properties. Dobereiner discovered that “the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements”. This statement is called the Dobereiner’s law of Triads.

♦ In this table, atomic mass of sodium is equal to arithmetic mean of atomic masses of lihtium and potassium. Similarly, atomic mass of strontium is equal to arithmetic mean of atomic masses of calcium and barium.

LIMITATION OF DOBEREINER’S TRIADS:
♦ All the then known elements could not be arranged in the form of triads.
♦ The law failed for very low mass or for very high mass elements. In case of F, Cl, Br, the atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
♦ As the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly valid.

NEWLANDS’ LAW OF OCTAVES
Newlands law of octaves states that when elements are arranged in the ascending order of their atomic masses they fall into a pattern in which their properties repeat at regular intervals.
Every eighth element starting from a given elements resembles in its properties to that of the starting element.

LIMITATION OF NEWLANDS’ OCTAVES:
♦ Newlands’ Octaves could be valid upto calcium only; as beyond calcium, elements do not obey the rules of Octaves.
♦ Newlands’ Octaves was valid for lighter elements only.
♦ It appears that Newlands did not expect the discovery of more elements than 56 which were discovered till his time.
♦ More than one element had to be placed in some of the groups; in order to place the elements having similar properties in one group. But in order to do so, he also put some dissimilar elements in same group.
♦ Iron; which has similar property as cobalt and nickel, was placed far from them.
♦ Cobalt and nickel were placed in the group with chlorine and fluorine in spite of having different properties.
♦ In spite of above limitations; Newlands was the first scientist who arranged the elements in order of their increasing relative atomic masses.

INTEXT QUESTIONS PAGE NO. 81

Question : Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Answer: Yes. Lithium, sodium and potassium; beryllium; magnesium and calcium are two triads that also exist in the columns of Newland’s octaves.

Question : What were the limitations of Dِ bereiner’s classification?
Answer: Please see above notes

Question : What were the limitations of Newlands’ Law of Octaves?
Answer: Please see above notes

MENDELEEV’S PERIODIC TABLE

Mendeleef arranged the elements known at that time in a chart in a systematic order in the increasing order of their atomic weights. He divided the chart into 8 vertical columns known as groups. Each group is divided into A, B sub groups. Each column contained elements of similar chemical properties.

The elements in the first column, for example, react with oxygen to form compounds with the general formula R₂O. For example, Li, Na and K when react with oxygen form compounds like Li₂O, Na₂O and K₂O respectively.

Elements of the second column react with oxygen to form compounds with the general formula RO. For example, Be, Mg and Ca when react with oxygen form BeO, MgO and CaO.
Mendeleef tried to explain the similarities of elements in the same group in terms of their common valency.

THE PERIODIC LAW:
Based on Mendeleeff’s observations regarding the properties of elements in the periodic table, a law known as the periodic law of the properties of elements was proposed.
“The law states that the physical and chemical properties of the elements are a periodic function of their atomic weights.”

SALIENT FEATURES AND ACHIEVEMENTS OF THE MENDELEEFF’S PERIODIC TABLE:

1. Groups and sub-groups: There are eight vertical columns in Mendeleeff’s periodic table called as groups. They are represented by Roman numerals I to VIII. Elements present in a given vertical column (group) have similar properties. Each group is divided into two subgroup ‘A’ and ‘B’. The elements within any sub-group resemble each other to great extent.
For example, sub-group IA elements called ‘alkali metals’ (Li, Na, K, Rb, Cs, Fr) resemble each other very much.

2. Periods: The horizontal rows in Mendeleeff’s periodic table are called periods. There are seven periods in the table, which are denoted by Arabic numerals 1 to 7. A period comprises the entire range of elements after which properties repeat themselves.

3. Predicting the properties of missing elements: Based on the arrangement of the elements in the table he predicted that some elements were missing and left blank spaces at the appropriate places in the table.

Mendeleef believed that some new elements would be discovered definitely. He predicted the properties of these new additional elements in advance purely depending on his table. His predicted properties were almost the same as the observed properties of those elements after their discovery.

He named those elements tentatively by adding the prefix ‘eka’ (eka is a Sanskrit numeral means one) to the name of the element immediately above each empty space. The predicted the properties of elements namely eka-aluminium, eka-boron, eka-aluminium and ekasilicon were close to the observed properties of Scandium, Gallium and Germanium respectively which were discovered later. 

4. Correction of atomic mass: the correct placement of elements in Mendeleeff’s periodic table helped in correcting the atomic masses of some elements like, Beryllium, Indium, Gold.
For example, At the time of Mendeleef, beryllium was given atomic weight 13.5.
Atomic weight = equivalent weight × valency
The equivalent weight of Be was found experimentally as 4.5 and its valency was understood as 3. Therefore atomic weight of beryllium was given as 4.5 × 3 = 13.5.
With this atomic weight it had to be placed in a wrong group in the table. He said that its valency should be only 2 and then its atomic weight then would be 4.5 × 2 = 9. If atomic weight of ‘Be’ is 9 it would fit in the second group and its properties practically are similar to Mg, Ca etc., of the second group elements. He also helped in the calculation of the correct atomic weights of ‘Indium’ and ‘Gold’ in this manner.

5. Some anomalous series of elements like ‘Te’ and ‘I’ were observed in the table. The anomalous series contained element with more atomic weight like ‘Te’ (127.6 u)
placed before the element with less atomic weight like ‘I’ (126.9 u). Mendeleeff accepted minor inversions in the order of increasing atomic weight when these inversions resulted in elements being placed in the correct groups.

It was the extraordinary thinking of Mendeleeff that made the chemists to accept the periodic table and recognise Mendeleeff more than anyone else as the originator of the periodic law.

LIMITATIONS OF MENDELEEFF’S PERIODIC TABLE:

1. Position of hydrogen: The position of hydrogen in the table is not certain because it can be placed in group IA as well as in group VIIA as it resembles both with alkali metals of IA group and halogens of VIIA group.
2. Anomalous pair of elements: Certain elements of highest atomic mass precede those with lower atomic mass.
For example, tellurium (atomic mass 127.6) precedes iodine (atomic mass 126.9).
Cobalt and nickel: argon and potassium which were placed in table by deviating the basis of classification (placement in ascending order of atomic masses).
For example, potassium (atomic mass 39) placed after argon (atomic mass 40).
Similar situation was found in pairs of cobalt and nickel and tellurium, iodine.
3. Dissimilar elements placed together: elements with dissimilar properties were placed in same group as sub-group A and sub-group B. For example, alkali metal like Li, Na, K etc.,
of IA group have little resemblance with coinage metals like Cu, Ag, Au of IB group.
4. Some similar elements separated: some similar elements like ‘copper and mercury’ and ‘silicon and thalium’ etc are placed in different groups of the periodic table.
5. Position of isotopes: isotopes of elements are placed in the same position in the table.

INTEXT QUESTIONS PAGE NO. 85

Question : Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:
K, C, AI, Si, Ba.
Answer:  Oxygen is a member of group VIA in Mendeleef’s Periodic Table. Its valency is 2. Similarly, the valencies of all the elements listed can be predicted from their respective groups. This can help in writing the formulae of their oxides.
(i) Potassium (K) is a member of group IA. Its valency is 1. Therefore , the formula of its oxide is K₂O .
(ii) Carbon (C) is a member of group IVA. Its valency is 4. Therefore, the formula of its oxide is C₂O₄ or CO₂.
(iii) Aluminium (Al) belongs to groups IIIA and its valency is 3. The formula of the oxide of the element is Al2O₃.
(iv) Silicon (Si) is present in group IVA after carbon. Its valency is also 4. The formula of its oxide is Si₂O4 or SiO₂.
(v) Barium (Ba) belongs to group IIA and the valency of the element is 2. The formula of the oxide of the element is Ba₂O₂ or BaO.

Question : Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table?
Answer: Scandium and germanium are the two elements that had been left by Mendeleef.

Question : What were the criteria used by Mendeléev in creating his Periodic Table?
Answer: The criteria used by Mendeleef were
(i) Physical and chemical properties of the elements.
(ii) Atomic masses in increasing order.

Question : Why do you think the noble gases are placed in a separate group?
Answer: Noble gases are also called inert gases because they have a complete octet and hence, are very stable. They do not react with other elements due to their stability. Since they all are unreactive, have complete octet and similar behaviour so they are placed in a separate group.

THE MODERN PERIODIC TABLE

Based on the modern periodic law, a number of forms of periodic table have been proposed from time to time but general plan of the table remained the same as proposed by Mendeleev.
The table which is most commonly used and which is based upon the electronic configuration of elements is called the long form of the periodic table. This is called the modern periodic table.

Long form of the periodic table is a chart of elements in which the elements have been arranged in the increasing order of their atomic numbers. This table consists of horizontal
rows called periods and vertical columns called groups.
♦ The modern periodic table has also been divided into four blocks known as s,p,d and f blocks. 

STUDY OF PERIODS
The horizontal rows are called periods. There are seven horizontal rows in the periodic table.
♦ First period (Atomic number 1 and 2): This is the shortest period. It contains only two elements (hydrogen and helium).
♦ Second period (Atomic number 3 to 10): This is a short period. It contains eight elements (lithium to neon).
♦ Third period (Atomic number 11 to 18): This is also a short period. It contains eight elements (sodium to argon).
♦ Fourth period (Atomic number 19 to 36): This is a long period. It contains eighteen elements (potassium to krypton). This includes 8 normal elements and 10 transition elements.
♦ Fifth period (Atomic number 37 to 54): This is also a long period. It contains 18 elements (rubidium to xenon). This includes 8 normal elements and 10 transition elements.
♦ Sixth period (Atomic number 55 to 86): This is the longest period. It contains 32 elements (ceasium to radon). This includes 8 normal elements, 10 transition elements and 14 inner transition elements (lanthanides).
♦ Seventh period(Atomic number 87 to 118): As like the sixth period, this period also can accomodate 32 elements. Till now only 26 elements have been authenticated by IUPAC.

STUDY OF GROUPS

♦ Vertical columns in the periodic table starting from top to bottom are called groups. There are 18 groups in the periodic table.
♦ First group elements are called alkali metals.
♦ Second group elements are called alkaline earth metals.
♦ Groups three to twelve are called transition elements .
♦ Group 1, 2 and 13 - 18 are called normal elements or main group elements or
♦ representative elements .
♦ Group 16 elements are called chalcogen family (except polonium).
♦ Group 17 elements are called halogen family.
♦ Group 18 elements are called noble gases or inert gases.
♦ The lanthanides and actinides which form part of the group 3 are called inner transition elements. 

CHARACTERISTICS OF MODERN PERIODIC TABLE

CHARACTERISTICS OF PERIODS
In a period, the electrons are filled in the same valence shell of all elements.
As the electronic configuration changes along the period, the chemical properties of the elements also change.
Atomic size of the elements in a period decreases from left to the right.
In a period, the metallic character of the element decreases while their non-metallic character increases.

CHARACTERISTICS OF GROUPS
♦ The elements present in 2 and 18 groups differ in atomic number by 8,8,18,18,32.
♦ The elements present in 13 – 17 groups differ in atomic number by 8,18,18,32.
♦ The elements present in 4 – 12 groups differ in atomic number by 18,32,32.
♦ The elements present in a group have the same number of electrons in the valence shell of their atoms.
♦ The elements present in a group have the same valency.
♦ The elements present in a group have identical chemical properties.
♦ The physical properties of the elements in group such as melting point, boiling point, density vary
♦ gradually.
♦ Atomic radii of the elements present in a group increases downwards.

ADVANTAGES OF THE MODERN PERIODIC TABLE
♦ The table is based on a more fundamental property ie., atomic number.
♦ It correlates the position of the element with its electronic configuration more clearly.
♦ The completion of each period is more logical. In a period as the atomic number increases, the energy shells are gradually filled up until an inert gas configuration is reached.
♦ It is easy to remember and reproduce.
♦ Each group is an independent group and the idea of sub-groups has been discarded.
♦ One position for all isotopes of an element is justified, since the isotopes have the same atomic number.
♦ The position of eighth group (in Mendeleev’s table) is also justified in this table. All transition elements have been brought in the middle as the properties of transition elements
are intermediate between left portion and right portion elements of the periodic table.
♦ The table completely separates metals from non-metals. The nonmetals are present in upper right corners of the periodic table.
♦ The positions of certain elements which were earlier misfit (interchanged) in the Mendeleev’s periodic table are now justified because it is based on atomic number of the elements.
♦ Justification has been offered for placing lanthanides and actinides at the bottom of the periodic table.

DEFECTS IN THE MODERN PERIODIC TABLE
♦ Position of hydrogen is not fixed till now.
♦ Position of lanthanides and actinides has not been given inside the main body of periodic table.
♦ It does not reflect the exact distribution of electrons of some of transition and inner transition elements.

INTEXT QUESTIONS PAGE NO. 90

 

A ssertion-Reason Type Questions (1 Mark)
For question numbers 1 and 2 two statements are given-one labeled as Assertion (A) and the other labeled
Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
(a) Both ‘A’ and ‘R’ are true and ‘R’ is correct explanation of the assertion.
(b) Both ‘A’ and ‘R’ are true but ‘R’ is not correct explanation of the assertion.
(c) ‘A’ is true but ‘R’ is false.
(d) ‘A’ is false but ‘R’ is true.

Question : Assertion: Increasing order of atomic masses could not be maintained in Mendeleev’s periodic table.
Reason: His periodic table helps in correction of atomic masses of some of the elements in the beginning.
Answer : (b) Both ‘A’ and ‘R’ are true but ‘R’ is not correct explanation of the assertion.

Question : Assertion: Dobereiner adjusted two elements in the same slot.
Reason: Dobereiner could not classify all the elements in triads.
Answer : (d) ‘A’ is false but ‘R’ is true

Question : How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
Answer: 1. The fundamental basis for Modern Periodic Table is atomic number and not atomic mass and hence, it is more accurate.
2. Properties of elements could be well explained when they were arranged according to their increasing atomic number in the Modern Periodic Table.
3. A separate group for noble gases could be created when noble gases were discovered.
4. Hydrogen has been given a unique position in the Modern Periodic Table at the top left corner because of its unique properties.

Q2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer: Magnesium (Mg) belongs to group 2 of Modern Periodic Table, known as alkaline earth metal family. The two other elements belonging to the same group are beryllium (Be) and calcium (Ca).
Calcium and magnesium; Beryllium and magnesium - This is because both of them have electronic configuration similar to Mg 

These elements belong to the same group and hence, will show similar properties.

Question : Name
(a) three elements that have a single electron in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.
Answer: 

Question : (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Answer:
(a) Li, Na, and K have one electron in their outermost shell.
Li 2, 1 (Atomic number 3)
Na 2, 8, 1 (Atomic number 11)
K 2, 8, 8, 1 (Atomic number 19)
(b) They both have completely filled outer shell. 

Question : In the Modern Periodic Table, which are the metals among the first ten elements?
Answer: Metals among the first ten elements are lithium and beryllium.

Question : By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Ga Ge As Se Be
Answer:
The position of the given elements in the Periodic Table is as.
Ga Ge As Se Br
(Most metallic) → (Least metallic)
On moving from left to right in a period, size decreases due to increase in effective nuclear charge and hence, tendency to lose electron, i.e., metallic character decreases. Thus Ga has maximum metallic characteristic or metallic nature.

EXERCISE QUESTIONS PAGE NO. 91 and 92

Question : Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table.
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
Answer: (c) On moving from left to right, the atomic number increases and hence, the nuclear charge increases. With the increase of nuclear charge, the force binding the electron increases so the atom lose the electrons with more difficulty, not easily.

Question : Element X forms a chloride with the formula XCl₂, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as
(a) Na
(b) Mg
(c) AI
(d) Si
Answer: (b) The formula of chloride is XCl₂ that means the valency of the element X is 2. The element having valency 2 will present in group 2. Out of given choices magnesium (Mg) belongs to group 2.

Question : Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?
Answer:
(a) Noble gases are the elements which have completely filled shells. The noble gas with two shells is Ne having atomic number 10 and electronic configuration 2, 8 both of which are completely filled.
(b) Electronic configuration 2, 8, 2 suggests that atomic number is 12 (2 + 8 + 2), magnesium (Mg) has atomic number 12.
(c) The element with three shells and form electrons in the valence shell will have electronic configuration 2, 8, 4. The atomic number of this element is 14 (2 + 8 + 4) so it will belong to group 14 hence, it is silicon (Si).
(d) Element with two shells with 3 electrons in the valence shell will exist in second period and will have the electronic configuration 2,3. The atomic number of this element will be 5 (2 + 3). So, it will be bron (B).
(e) The element has two shells. we know that first shell can have only 2 electrons, so according to the question there will be 4 electrons (double the number of electrons in first shell). The electronic configuration will be 2, 4 , so the atomic number is 6. Hence, the element is carbon (C).

Question : (a) What property do all elements in the same column of the Periodic Table as boron have in common?
(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Answer: (a) All elements of 13th group, in which boron is present, have 3 electrons in their valence shell (2, 8, 3).
(b) Fluorine belongs to 17th group. All elements of this group have 7 electrons in their valence shell (2, 8, 7). They all show a valency of 1 in their compounds.

Question : An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar? (Atomic numbersare given in parentheses.) N(7) F(9) P(15) Ar(18)
Answer:
(a) Atomic number of atom = 2 + 8 + 7 =17
(b) It will be similar to fluorine which is also having 7 electrons in its valence shell [F (2, 7)].

Question : The position of three elements A, B and C in the Periodic Table are shown below –
Group 16 Group 17
   ---            ---
   ---             A
  ---             ---
    B              C
(a) State whether A is a metal or non-metal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
Answer: (a) Since, A belongs to group 1 valence electrons so it is a non-metal because it will gain electron to complete its octet.
(b) C lies below A and in the same group. As we move down in a group, the size increases and electronegative character decreases. With the increase in electronegative character, the electron adapting tendency and hence the reactivities decrease so, C is less reactive than A.
(c) C is smaller than B in size because as we move left to right in a period atomic size decreases.
(d) As discussed in part (a) that element A has a tendency to gain electron to complete its octet.
It needs to take up one electron, so it will form anion (A⁻).

Question : Can the following group of elements be classified as Dobereiner’s triads? Explain by giving reasons.
(a) Na, Si, Cl (b) Be, Mg, Ca
Answer: (a) No, because atomic mass of Si = 58.5/2 = 29.25
which is nearly equal to 28 but these elements do not resemble with each other.
(b) Yes, atomic mass of Mg = 9 + 40 / 2 + = 24.5
which is nearly equal to 24 and these three elements resemble with each other.

Question : Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Answer: (a) Electronic configuration of nitrogen and phosphorus:
    K   L  M
N 2, 5
P 2, 8,  5
(b) N will be more electronegative than P as electronegativity decreases on going down a group in case of non-metals.

Question : Elements have been arranged in the following sequence on the basis of their increasing atomic masses: F, Na, Mg, Al, Si, P, S, Cl, K
(a) Pick up two sets of elements which have similar properties.
(b) The above given sequence represents which law of classification of elements?
Answer: (a) (i) F, Cl (ii) Na, K (b) Newlands’ law of octaves

Question : Write two drawbacks of Mendeleev’s periodic table.
Answer: (i) Isotopes challenged the basis of Mendeleev’s periodic table.
(ii) Increasing order of atomic masses could not be maintained in classifying all the elements

Question : What were the limitations of Newlands’ law of octaves?
Answer: (i) It was applicable upto Ca, i.e. lighter elements only.
(ii) New elements could not fit into Newlands’ octaves

Question : How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer: The number of valence electrons in an atom of an element tells us the group number. e.g., Na has atomic number 11
Electronic configuration of Na (11) = 
                                                    K  L  M
                                                    2  8  1
It has one electron in its last shell, thus it belongs to group I of the Periodic Table.
The number of shells in its electronic configuration tells the period number e.g., Na shows 3 shells K, L, M so it belongs to 3rd period of the Periodic Table.

Question : Use Mendeleev’s Periodic table to predict the formulae for the oxides of the following elements:
K, C, Al, Si, Ba
Answer: K₂O, CO₂, Al₂O₃, SiO₂, BaO

Question : In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Answer: 

From the electronic configuration written above we can see that element with atomic number 12 and 38 have 2 electrons in their last shell like calcium. So, they will resemble Ca in their physical and chemical properties.

Question : Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.
Answer: 

Mendeleev’s periodic table	Modern periodic table
1. Elements are arranged in the increasing order of their atomic masses.	1. Elements are arranged in the increasing order of their atomic numbers.
2. There are a total of 7 groups (columns) and 6 periods (rows).	2. There are a total of 18 groups (columns) and 7 periods (rows).
3. Elements having similar properties were placed directly under one another.	3. Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group.
4. The position of hydrogen could not be explained.	4. Hydrogen is placed above alkali metals.
5. No distinguishing positions for metals and non-metals.	5. Metals are present at the left hand side of the periodic table whereas nonmetals are present at the right hand side.

Question : (a) Why do we classify elements?
(b) What were the two criteria used by Mendeleev in creating periodic table?
(c) Why did Mendeleev leave some gaps in his periodic table?
(d) In Mendeleev’s periodic table, why was there no mention of noble gases like Helium, Neon and Argon?
(e) Would you place the two isotopes of chlorine, Cl-35 and Cl-37 in different slots because of their different atomic mass or in the same slot because their chemical properties are same? Justify your answer.
Answer : (a) It helps to study the properties of elements in a simpler way by studying the properties of 118 elements by studying properties of 18 groups and 7 periods.
(b) (i) Increasing order of atomic mass, (ii) Formula of oxides and hydrides of elements.
(c) These gaps were left for the undiscovered elements.
(d) Noble gases were not discovered by that time.
(e) They will be placed in same slot due to same properties

Question : On the basis of Mendeleev’s Periodic table, answer the following questions:
(a) Name the element which is in
(i) I group and III period. (ii) VII group and II period.
(b) Suggest the formula of the following:
(i) Oxide of nitrogen (ii) Hydride of oxygen
(c) In group VIII of the periodic table, why does cobalt with atomic mass 58.93 appear before nickel having atomic mass 58.71?
Answer : (a) (i) Na belongs to 1st group, IIIrd period. (ii) F belongs to VII group, IInd period.
(b) (i) N₂O₅ (ii) H₂O
(c) It is because Co, Rh, Ir resemble with each other and Ni, Pd, Pt resembles with each other.
Similarity in properties were preferred over increasing order of atomic masses

Question :Explain giving justification the trends in the following properties of elements, on moving from left to right in a period, in the Modern Periodic Table.
(a) Variation of valency (b) Change of atomic radius
(c) Metallic to non-metallic character (d) Electronegative character
(e) Nature of oxides
Answer : (a) Valency increases from left to right till middle, then decreases because valence electrons goes on increasing.
(b) Atomic radius decreases due to increase in effective nuclear change.
(c) Metallic character decreases, non-metallic character increases due to increase in tendency to gain electrons as atomic size decreases, effective nuclear change increases.
(d) Electronegative character increases due to increase in effective nuclear change.
(e) Acidic character of oxides increases, basic character of oxides decreases because metallic character decreases and non-metallic character increases.

Question :Given below are some elements of modern periodic table. Atomic number of elements are given in parenthesis:
A(4), B(9), C(14), D(19), E(20)
(a) Select the element that has one electron in outermost shell. Also write the electronic configuration of this element.
(b) Which two elements amongst there belong to the same group? Give reason for your answer.
(c) Which two elements amongst there belong to the same period? Which one of the two has bigger atomic radius?
Answer : (a) D(19) has one valence electron. Its electronic configuration is 2, 8, 8, 1.
(b) A(4), E(20) belong to same group because they have same number of valence electrons.
(c) A and B belong to same period, A is bigger than ‘B’. D and E also belong to same period, ‘D’ is bigger than E.

Question :What is meant by ‘group’ in the modern periodic table? How do the following changes occur on moving from top to bottom in a group?
(i) Number of valence electrons (ii) Number of occupied shells
(iii) Size of Atoms (iv) Metallic character of elements
(v) Effective nuclear charge experienced by valence electrons.
Answer : The vertical columns of periodic table are called Groups.
(i) Number of valence electrons remains the same.
(ii) Number of occupied shells goes on increasing.
(iii) Size of atoms increases down the group.
(iv) Metallic character of elements increases down the group.
(v) Effective nuclear charge decreases.

Question :The electronic configuration of an element ‘X’ is 2,8,6. To which group and period of the modern periodic table does ‘X’ belong .State it valency and justify your answer in each case.
Answer : X – 2, 8, 6
(a) Since ‘X’ has three energy shells and period number of an element is equal to the number of energy shells. So, X belongs to 3rd period.
(b) X has 6 valence electrons so, it belongs to group 16.
(c) Valency will be 2. To acquire noble gas configuration it will gain 2 electrons

Question :Write the number of periods and groups in the Modern Periodic Table. How does the metallic character of elements vary on moving (i) from left to right in a period, and (ii) down a group? Give reason to justify your answer.
Answer : In the Modern Periodic Table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
Metallic character: It is defined as the tendency of an atom to lose electrons
Across the period i.e., from left to right: Metallic character decreases.
Down the group i.e., from top to bottom: Metallic character increases.
Reason: Across the period, the effective nuclear charge increases, thus decreasing its atomic radius.
This favours the increase of electronegativity and therefore the tendency to lose electrons is less. This accounts for the decrease in the metallic character along the period.
But as we move down the group the number of shells keep on increasing and therefore the atomic size increases. This means that the electronegativity decreases. This enhances the ability to lose electrons and therefore the metallic character increases down the group.

Question :Write the names given to the vertical columns and horizontal rows in the Modern Periodic Table. How does the metallic character of elements vary on moving down a vertical column? How does the size of atomic radius vary on moving left to right in a horizontal row? Give reason in support of your answer in the above two cases.
Answer : In the Modern Periodic Table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
As we move down the group, the electrostatic attraction between the nucleus and the outer-most electron decreases due to increase in the distance between them. This happens because, on moving down the group, a new shell is added. So the valence electrons can be easily lost by the element. As we
know, metallic character is characterised by the ease of loss of electrons, thereby, metallic character increases on moving down the group in the Modern Periodic Table.
When we move across a period, the number of electrons in the same shell increases. This leads to greater electrostatic attraction between the nucleus and the outer-most electron. This increased attraction pulls the outer-most electron closer to the nucleus, thereby decreasing the atomic size.

Question :How is possible valency of element determined with the help of electronic configuration of its atom?
Determine the valency of ‘X’ whose atomic number is 15.
Answer : Valency is equal to number of valence electrons or 8–valence electrons. X has electronic configuration 2, 8, 5.
Its valency is equal to 3 because it can gain 3 electrons to become stable.

Question :Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electronic shells, but different number of electrons in their outermost shell. It was found that elements A and G combine to form an ionic compound. This compound is added in a small amount to almost all the vegetable dishes during cooking. Oxides of elements A and B are basic in nature, while those of E and F are acidic. The oxide of D is almost neutral. Based on the above information, answer the following questions:
(i) To which group or period of the periodic table do the listed elements belongs to?
(ii) What would be the nature of compound formed by the combination of elements B and F?
(iii) Which two of these elements could definitely be metals?
(iv) Which one of the eight elements is most likely to be found in gaseous state, at room temperature?
(v) If the number of electrons in the outermost shell of elements C and G are 3 and 7 respectively, then write the formula of the compound formed by the combination of C and G.
Answer : (i) They belong to the same period but different groups. ‘G’ belongs to Group 17. A and B belong to Group 1 and 2, ... their oxides are basic. ‘C’ belongs to Group 13 due to 3 valence electrons. ‘D’ belongs to
Group 14. E and F belong to group 15 and 16, respectively. ‘H’ belongs to group 18.
(ii) B and F will form an ionic compound.
(iii) A and B are metals.
(iv) G, H are gases at room temperature.
(v) CG₃

Question :Explain the basic character of oxides of elements down the group and across the period.
Answer : Basic character of oxides increases from top to bottom in a group because metallic character increases down the group due to increase in tendency to lose electrons.
Basic character of oxide decreases along a period from left to right because the atomic size decreases, tendency to lose electrons decreases, metallic character decreases

Question :(i) Which element can lose electrons most easily in 3rd period and why?
(ii) Why does tendency to lose electrons increases down the group?
(iii) What happens to the basic character of oxides down the group and why?
(iv) What happens to the acidic character of oxides along a period and why?
(v) Which group of elements can gain electrons most easily and why?
Answer : (i) ‘D’ is lithium, E is fluorine.
(ii) F belongs to group 17, 3^rd period.
H belongs to group 1, 4^th period.
(iii) They have the same number of valence electrons, D(3) 2, 1; G(11) 2, 8, 1; H(19) 2, 8, 8, 1.
(iv) DE is the formula of the compound.
(v) H is largest in size due to the presence of four shells

Question :Define atomic size. Give its unit of measurement. In the modern periodic table what trend is observed in the atomic radius in a group and a period and why is it so?
Answer : (i) E will form only covalent compounds.
(ii) B is non-metal with valency 2.
(iii) D is metal with valency 2.
(iv) ‘F’ has largest atomic radius.
(v) It belongs to noble gases.

Question :(a) How is valency of an element determined if its electronic configuration is known? Determine the valency of an element of atomic no. 9
(b) Given below are some elements of modern periodic table. Atomic number of elements are given in parentheses.
A (4), B (9), C (14), D (19), E (20)
(i) With the help of electronic configuration, find out which ore of the above elements will have one electron in its outermost shell.
(ii) Which two elements belong to the same group? Give reasons for your answer.
(iii) Which one of the above element belonging to the fourth period has bigger atomic radius and why?
Answer : (a) Valency is equal to number of valence electrons or 8— number of valence electrons.
F (9): 2,7
Its valency is equal 1 because it will become stable on gaining one electron.
(b) (i) A (4): 2,2 D (19): 2,8,8,1
B (9): 2,7 E (20): 2,8,8,2
C (14): 2,8,4
‘D’ has one valence electron.
(ii) ‘A’ and ‘E’ belong to same group because they have same number of valence electrons.
(iii) ‘D’ has larger atomic radius than ‘E’ because it has 19 protons which attract 19 electrons which is there less strongly than 20 protons can attract 20 electrons as in E.

Question :An element X of group 15 exists as a diatomic molecule and combines with hydrogen at 773 K, in the presence of a catalyst to form a compound ammonia, which has a characteristic pungent smell.
(i) Identify the element X. How many valence electrons does it have?
(ii) Draw the electron dot structure of diatomic molecule of X. What type of bond is formed in it?
(iii) Draw the electron dot structure for ammonia and what type of bond is formed in it?
Answer : (i) Na, it is due to larger atomic size and least effective nuclear charge.
(ii) It is due to increase in atomic size and decrease in effective nuclear charge.
(iii) Basic character of oxides increases down the group due to increase in metallic character.
(iv) Acidic character of oxides increases along a period due to increase in non-metallic character.
(v) Group 17 elements, due to smaller atomic size and more effective nuclear charge.

Question :(a) List any three observations which posed a challange to Mendeleev’s periodic table.
(b) How does the mettallic character of elements vary on moving from
(i) Left to right in a period.
(ii) From top to bottom in a group of the Modern periodic table? Give reason for your answer.
Answer : (a) (i) Increasing order of atomic mass could not be followed.
(ii) Isotopes cannot occupy different positions as they have same chemical properties but different atomic mass.
(iii) Position of hydrogen was not justified.
(b) (i) Metallic character decreases from left to right in a period because tendency to lose electrons decreases as effective nuclear charge increases.
(ii) Metallic character increases down the group from top to bottom because tendency to lose electron increases due to increase in atomic size and decrease in effective nuclear charge.

1 Define modern periodic law

2 What property do the elements of the same group have in common?

3 What was the basis of classification of elements according to Mendeleev and according to Modern periodic table?

4 Why does the reactivity of group 1 elements increase down the group?

5 How could the modern periodic table remove the anomalies of Mendeleev’s table?

6 Why is atomic number considered as a better basis for classification than atomic mass?

7 List the demerits of Doberiener’s triads.

8 What were the limitations of Newland’s law of octaves?

9 Define ionization energy, electron gain enthalpy

10 How does the valency of elements vary across the period and down a group?

11 What is the reason for the decrease in size of the atom across the period?

12 A metal X is in the first group of the periodic table. What will be the formula of its oxide?

13 An element Y is in the second period and the 16th group Is it a metal or a non-metal? What is its valency?

14 How does electronegativity vary down a group of Halogens?

15 Explain why

a)All elements of a group have similar chemical properties.

b) All elements of a period have different chemical properties

16 State all the demerits of Mendeleev’s periodic table?

17 The elements X, Y and Z belong to groups 2,14 and 16 respectively

a)Which two elements will form covalent bond

b)Which two elements will form an ionic bond?

18 An element A has an atomic number of 6.Another element B has 17 electrons in its neutral atom.

a) In which groups would you find these elements?

b) What type of bond is formed between A and B?

c) Suggest a formula of the compound between A and B.

19 An element has electron structure of 2,7

a) What is the atomic number of this atom?

b) To which of the following would it be chemically similar?

7N 15 P 17Cl 18 Ar

c) Why would you expect it to be similar?

20 The atomic numbers of three elements are given below.

Element Atomic number
A 5
B 7
C 10

a) Which element belongs to group 18, 13and 15?

b)To which period do these elements belong?

 

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