CBSE Class 10 Chemistry Periodic Classification Of Elements Worksheet Set C

Multiple choice questions

Very Short Answer Type Questions

Question. The electronic configuration of an element is 2, 8, 4. State its:
(A) group and period in the Modern Periodic Table.
(B) name and write its one physical property.
Answer : (A) As the number of valence electrons in the given element is 4, it belongs to Group 14 of the Modern Periodic Table.
As the number of occupied shells is 3 (since electrons are filled in K, L and M shells), the element belongs to third period.
(b) The given element is Silicon (atomic symbol: Si). It’s physical property is:
(i) Silicon is a solid
(ii) It is a semi-conductor
(iii) It exhibits allotropy
(iv) It has a metallic luster

Question. Can the following groups of elements be classified as Dobereiner s'triad?
(A) Na, Si, Cl
(B) Be, Mg, Ca
Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40.
Explain by giving reason.
Answer : For a group to be Dobereiner’s triad, the atomic mass of the middle element must be average of the atomic masses of the first and the third elements.
(A) No, Na, Si, Cl cannot be classified as Dobereiner’s triad because all these elements do not have similar properties, although the atomic mass of silicon is the average of the atomic masses of sodium (Na) and chlorine (Cl).

Atomic mass of Si = 23 +35 /2
= 58 /2 =29

(B) Yes, Be, Mg, Ca can be classified as Dobereiner’s triad because they have similar properties and the mass of magnesium (Mg) is roughly the average of the atomic mass of Be and Ca.
Atomic mass of Mg = 9+40 /2
= 49 /2 =24.5

Question. State the Modern Periodic Law. What is the number of groups and periods in the Modern Periodic Table?
Answer : The Modern Periodic Law states that ‘properties of elements are a periodic function of their atomic number.’
The number of groups in the Modern Periodic Table are 18 and the number of periods are 7.

Question. Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendeleev. [NCERT]
Answer : Eka-silicon is identified as germanium (Ge),which is placed in group 4 of Mendeleev’s periodic table. The valency of germanium is 4,so the chemical formula of its chloride must be
GeCl₄.
Eka-aluminium was later identified as gallium (Ga). It is placed in group 3 of Mendeleev’s periodic table. Hence, the valence of gallium is 3 and the formula of chloride would be GaCl3.

Question. Two elements X and Y have atomic numbers 12 and 16 respectively. To which period of the modern periodic table do these two elements belong? What type of bond will be formed between them and why? Also give the chemical formula of the compound formed.
Answer : Electronic configuration of X: 2,8,2, Y: 2,8,6 Both X and Y belong to 3rd period.
Ionic bond will be formed.
Reason: X will lose 2 electrons and Y will gain 2 electrons to complete their octet and become stable.
Formula is XY.

Question. Give reason: The system of classification into triads was not found to be useful.
Answer : The system of classification into Dobereiner’s triads was not found to be useful as he failed to arrange all the then-known elements in the form of triads of elements having similar chemical properties. He could identify only three triads.

Question. Consider the following : 20Ca, 8O, 16S, 4Be Which of the above elements would you expect to be in group 2 of the Modern Periodic table?
Answer : Consider the electronic configuration of elements:
Ca: 2, 8, 8, 2
O: 2, 6
S: 2, 8, 6
Be: 2, 2
As Ca and Be all have the same number of valence electrons (2), they belong to Group 2 of the Modern Periodic Table.

Question. Name the group number of the following elements, halogens, alkali metals, inert gases, hydrogen, in the Modern Periodic Table.
Answer : Halogens — group No. 17
Alkali metals — group No. 1
Inert gases — group No. 18
Hydrogen — group No. 1

Question. What happens to the valency of elements as we move from left to right in a Periodic Table?
Answer : As we move from left to right in a Periodic Table the valency first increases till 4 and then again decreases.
L → R Valency in a period → 1 2 3 4 3 2 1 0

Question. What happens to the metallic character as we move from top to bottom in a group?
Answer : The metallic character increases as we move from top to bottom as the tendency to lose electrons increases.

Question. The atomic number of ‘X’ is 17. Predict its
(a) valency, (b) formula of halide, (c) type of ion formed, (d) reactivity with respect to the other members of same group.
Answer : ‘X’ has atomic number 17
∴ Electronic configuration → 2, 8, 7
(a) Valency = 1
(b) Formula of halide = HX
(c) Type of ion formed = Negative ion (Anion).
(d) Reactivity = Most reactive among those elements which lie below X in a group.

Question. What are the merits of Mendeleev’s Periodic Table?
Answer : Merits of Mendeleev’s Periodic Table are:
(i) Mendeleev left some gaps in his table. Predicted the chemical properties of these 3 elements which were discovered later and had same properties as predicted by Mendeleev, they were gallium, germanium and scandium.
(ii) He arranged the elements very systematically in periods and groups.

Question. Carbon with atomic number 6 and silicon with atomic number 14 belong to same group although carbon is non-metal and silicon is semi-metal.
Answer : Carbon with atomic number 6, shows electronic configuration 2, 4. Silicon with atomic number 14 shows electronic configuration 2, 8, 4.
Both the elements have same valence electrons, hence they are placed in same group.

Question. How does the valency of an element be determined, if its electronic configuration is known? What will be the valency of an element with atomic number 9?
Answer : Valency is equal to the number of valence electrons when valence electrons are from 1 to 4 or 8 – no. of
valence electrons when valence electrons are from 5 to 8.
F(9): 2, 7; It can gain 1 electron to become stable, so its valency = 1.

Question. Three elements ‘X’, ‘Y’ and ‘Z’ having atomic numbers 11, 7 and 6 respectively react with oxygen to form their oxides.
(a) Arrange these oxides in increasing order of their basic nature.
(b) Give reason for your answer.
Answer : X(11): 2, 8, 1; Y(7): 2, 5; Z(6): 2, 4
(a) Y < Z < X
(b) ‘X’ is metallic in nature, therefore it will form basic oxide. ‘Y’ and ‘Z’ are non-metals will form acidic oxides. ‘Y’ will form more acidic oxide than ‘Z’ because it is more non-metallic in nature

Question. How does electronegativity of an element change as we go down a group and across a period? Give reason.
Answer : Electronegativity decreases down the group due to increase in atomic size and decrease in effective nuclear charge.
Electronegativity increases along a period due to decrease in atomic size and increase in effective nuclear charge.

Question. Given below are four elements with their atomic numbers:
Element Atomic     Number
          A                 16
          B                  11
          C                   3
          D                 14
(a) Identify the element which belong to same group of Modern Periodic Table.
(b) Arrange the given elements in decreasing order of atomic size.
(c) Write the formula of the oxide of ‘B’.
(d) Which of the above element is a metalloid?
Answer : a) ‘B’ and ‘C’ belong to same group. (b) B > D > A > C
(c) B₂O (d) ‘D’ is a metalloid.

Question. Which is bigger (i) O or F, (ii) N or P and why?
Answer : O is bigger in size than F due to less effective nuclear charge.
P is bigger in size than N due to more number of shells.

Question. Two elements ‘M’ and ‘N’ belong to Group I and II respectively and are in the same period of the periodic table. How do the following properties of M and N vary:
(a) size of their atoms (b) their metallic characters
(c) their valencies in forming oxides (d) formulae of their chlorides
Answer : (a) Size of ‘N’ is smaller than ‘M’.
(b) ‘M’ is more metallic than ‘N’.
(c) Valency of ‘M’ is 1 and valency of ‘N’ is 2.
(d) MCl and NCl₂ are the formulae of their chlorides.

Question. Calcium is an element with atomic number 20
(i) Will it be a metal/non-metal? (ii) What will be its valency?
(iii) What would be formula of its chloride?
(iv) Will it be larger/smaller than K?
Answer : (i) It is a metal. (ii) Its valency is equal to 2.
(iii) CaCl₂ is the formula of its chloride. (iv) It will be smaller than K.

Question. How does the metallic character of elements changes along a period of the periodic table from left to right and why?
Answer : It decreases due to decrease in atomic size and decrease in tendency to lose electrons.

Question. What is meant by periodicity of properties of elements? Why are the properties of elements placed in the same group of periodic table similar?
Answer : The repetition of similar properties of elements after a certain interval of elements is called periodicity of properties.
Elements of the same group have same number of valence electrons, same valency and therefore posses similar chemical properties.

Question. Give reasons for the following:
(a) Lithium atom is smaller than sodium atom.
(b) Chlorine (Atomic number 17) is more electronegative than sulphur (Atomic number 16).
Answer : (a) It is because Li(2, 1) has two shells whereas Na(2, 8, 1) has three shells.
(b) Chlorine is smaller in size and has more effective nuclear charge than sulphur, therefore it is more electronegative.

Question. In the periodic table, how does the tendency of an atom to lose electrons changes on moving from (i) left to right across a period?, (ii) top to bottom in a group?
Answer : (i) It decreases due to increase in effective nuclear charge.
(ii) It increases due to decrease in effective nuclear charge.

Long Answer Type Questions

Question. Two elements A and B belong to group 1 and 2 respectively in the same period. Compare them with respect to:
(a) Valency (b) Size of atom (c) Formula of oxide
(d) Nature of oxide (e) Metallic character
Answer : Group 1 2
Elements A B
(a) Valency A → 1, B → 2
(b) Size of atom → A is bigger atom than B.
(c) Formula of oxide → A₂O, BO
(d) Nature → Basic
(e) Metallic character → A is more metallic than B.

Question. Give the characteristics of a period.
Answer : In a period as we go from left to right:
(a) Valence electrons → Goes on increasing
1, 2, 3, 4, 5, 6, 7, 8.
(b) Valency → Valency first increases and then decreases
1, 2, 3, 4, 3, 2, 1, 0.
(c) Size of atom → Size of atom goes on decreasing
(d) Metallic character → Decreases
(e) Non-metallic character → Increases

Question. Name the following elements
(a) Two shells, both of which are completely filled.
(b) Three shells with 2 valence electrons.
(c) Group 1, two shells.
(d) Group 17, period 3.
(e) Metal, with valency 3 group number 13 period 3.
Answer : (a) Two shells → K L
Filled →                              2, 8
Atomic number → 10
                                                                ∴ Element — Neon.
(b) Three shells →     K, L, M
2 Valence electron → 2, 8, 2
Atomic number →12                                ∴ Element = Magnesium
(c) Group 1
2 shells →                           K L
Electronic configuration → 2, 1
Atomic number → 3                                  ∴ Element = Lithium
(d) Group 17
Period → 3,                        K L M
Electronic configuration → 2, 8, 7
Atomic number → 17,                                ∴ Element = Chlorine
(e) Group → 13
Valency → 3
Period → 3                         K L M
Electronic configuration → 2, 8, 3
∴ Atomic number → 13,                                ∴ Element = Aluminium.

Question. On the baiss of the table of Mendeleev’s Periodic Table
(a) Name the element which is in
(i) I group and 3rd period. (ii) VII group and 2nd period.
(b) Suggest the formula for the following:
(i) Oxide of nitrogen (ii) Hydride of oxygen
(c) In group VIII of Periodic Table, why does cobalt with atomic mass 58.93 appear before nickel having atomic mass 58.71?
(d) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his Periodic Table?
(e) Using atomic masses of Li, Na and K, find the average atomic mass of Li, and K and compare it with the atomic mass of Na. State the conclusion drawn from this activity.
Answer : (a) (i) Sodium (ii) Fluorine
(b) (i) N₂O₅ (ii)H₂O
(c) Co resembles with Rh and Ir whereas ‘Ni’ resembles with Pd and Pt.
(d) Germanium and scandium

Question. (a) Which 2 criteria did Mendeleev use to classify the elements in his table.
(b) State Mendeleev’s Periodic law.
(c) Why could no fixed position be given to hydrogen in Mendeleev’s Periodic Table.
(d) How and why does the atomic size vary as you go:
(i) From left to right along a period
(ii) Down a group? 
Answer : (a) (i) Increasing order of atomic mass and similarities in chemical properties of elements.
(ii) The formula of oxides and hydrides formed by elements.
(b) Mendeleev’s Periodic Law → Properties of elements are periodic functions of their atomic masses.
(c) Hydrogen had no fixed position in Mendeleev’s Periodic table because it resembles alkali metal by forming positive ions and resembles halogens by forming diatomic molecule.
(d) (i) Atomic size decreases from left to right, as the valence electrons are attracted by the nucleus due to increase in the nuclear force.
(ii) The atomic size increases from top to bottom in a group because the number of shells keep on increasing therefore distance between nucleus and valence electrons increases.

Question. Mendeleev ′ predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminum.
(a) Name the elements which have taken the place of these elements
(b) Mention the group and the period of these elements in the Modern Periodic Table.
(c) Classify these elements as metals, non-metals or metalloids
(d) How many valence electrons are present in each one of them?
Answer : (a) Eka- silicon is Germanium (Ge)
Eka- aluminium is Gallium (Ga)
(b) Eka- silicon – group 14 & period 3
Eka- aluminium – Group 13 & period 3
(c) Eka- silicon - metalloid
Eka- aluminium – metal
(d) Valence electron present in Eka- silicon is 4
Valence electron present in Eka- aluminium is 3

Question. (a) Electropositive nature of the element(s) increases down the group and decreases across the period
(b) Electronegativity of the element decreases down the group and increases across the period
(c) Atomic size increases down the group and decreases across a period (left to right)
(d) Metallic character increases down the group and decreases across a period.
On the basis of the above trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9.
(a) Name the most electropositive element among them
(b) Name the most electronegative element
(c) Name the element with smallest atomic size
(d) Name the element which is a metalloid
(e) Name the element which shows maximum valency
Answer : (a) Lithium (3) (b) Fluorine (9) (c) Fluorine (9)
(d) Boron (5) (e) Carbon (6). Its valency is 4.

Question.29. An element X which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of Ferrous sulphate crystals and are the major air pollutants.
(a) Identify the element X
(b) Write the electronic configuration of X
(c) Write the balanced chemical equation for the thermal decomposition of Ferrous sulphate crystals?
(d) What would be the nature (acidic/ basic) of oxides formed?
(e) Locate the position of the element in the Modern Periodic Table.
Answer :

Question. An element X of group 15 exists as diatomic molecule and combines with hydrogen at 773 K in presence of the catalyst to form a compound, ammonia which has a characteristic pungent smell.
(a) Identify the element X. How many valence electrons does it have?
(b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it?
(c) Draw the electron dot structure for ammonia and what type of bond is formed in it?
Answer : 

Question. a) In this ladder (Figure 5.2) symbols of elements are jumbled up. Rearrange these symbols of elements in the increasing order of their atomic number in the Periodic Table.
(b) Arrange them in the order of their group also.

Answer : (a) H, He, Li, Be, B, C, N, O, F, Ne, Mg, Al, Si, P, S, Cl, Ar, K, Ca
(b) Group 1:H, Li, Na, K
Group 2: Be, Mg, Ca
Group 13: B. Al
Group 14: C, Si
Group 15: N. P
Group 16: 0, S
Group 17: F. U
Group 18: He, Ne, Ar

1. How many elements are known to us presently?

2. Why did the scientists feel the need to classify elements?

3. What was the basis of ‘triads’ formed by Dobereiner? Give an example.

4. What was Newlands ‘Law of Octaves’?

5. Describe in brief the classification made by Mendeleyev.

6. Why did Mendeleyev leave some gaps in his periodic table?

7. Give 2 examples of such elements which could fit into gaps left by Mendeleyev.

8. Besides Gallium, which other elements have since been discovered to fill the gaps left by Mendeleyev in his periodic table?

9. What is the basis of the Modern classification?

10. Why have Mg, Ca and Sr been kept in the same group of the periodic table? Which group is it?

11. An element has electronic configuration 2, 8, 7.

  (a) What is its atomic number?

  (b) Name another element to which it has similar properties.

  (c) Is it smaller or bigger than another element with atomic no. 16?

12. Why has hydrogen been placed in the first group of the periodic table although it is a non-metal?

13. How does the atomic radius change in a period and a group?

14. What change is observed across a period and down a group in the following:

  (a) Electron affinity (b) Ionization energy What is the unit of each?

15. Why are cations smaller than their corresponding atoms?

16. Why is the 18th gp of the periodic table called the zero group?