CBSE Class 10 Science Periodic Classification Of Elements Notes

PERIODIC TABLE & PERIODICITY IN PROPERTIES    

DEFINITION :

A periodic table may be defined as the table giving the arrangement of all the known elements according to their properties so that elements with similar properties fall with the same vertical column and elements with dissimilar properties are separated.

EARLY ATTAMPTS TO CLASSIFY ELEMENTS :

(a) Metals and Non - Metals :

Among the earlier classification, Lavoisier classified the elements as metals and non-metals. However, this classification proved to be inadequate. In 1803, John Dalton published a table of relative atomic weights (now called atomic masses). This formed an important basis of classification of elements.

(b) Dobereiner’s Triads :

(i) In 1817, J.W.Dobereiner a German Chemist gave this arrangement of elements.                 

(A) He arranged elements with similar properties in the groups of three called triads.

(B) According to Dobereiner the atomic mass of the central element was merely the arithmetic mean of atomic masses of the other two elements.

For e.g.

(ii) Limitations of Dobereiner’s Classification:

(A) Atomic mass of the three elements of some triads are almost same.

e.g.      Fe, Co, ni          and Ru, Rh, Pd

(B) It was restricted to few elements, therefore discarded.

(c) Newlands’ Law of Octaves :

In 1866, an English chemist, John Newlands, Proposed a new system of grouping elements with similar properties. He tried to correlate the properties of elements with their atomic masses. he arranged the then known elements in the order of increasing atomic masses. He started with the element having the lowest atomic mass (hydrogen) and ended at thorium which was the 56th element. He observed that every eight element had properties similar to that of the first.

Thus, Newlands suggested that when the elements are arranged in the order of increasing atomic masses, the properties of every eight element are a repetition of that of the first element 

Newland called this relation as a law of octaves due to the similarity with the musical scale.

(i) Newlands’ arrangement of elements into ‘Octaves’:

(ii) Limitations of law of octaves : The law of octaves has the following limitations : 

(A) The law of octaves was found to be applicable only upto calcium. It was not applicable to elements of higher atomic masses. 

(B) Position of hydrogen along with fluorine and chlorine was not justified on the basis of chemical properties. 

(C) Newlands placed two elements in the same slot to fit elements in the table. He also placed some unlike elements under the same slot. For example, cobalt and nickel are placed in the same slot and in the column of fluorine, chlorine and bromine. But cobalt and nickel have properties quite different from fluorine, chlorine and bromine. Similarly, iron which has resemblances with cobalt and nickel in its properties has been placed for away from these elements. 

Thus, it was realized that Newlands’ law of octaves worked well only with lighter elements. Therefore, this classification was rejected.

(d) Mendeleev’s Periodic Table :

The major credit for a systematic classification of elements goes to Mandeleev. He has been trying to group the elements on the basis of some fundamental property of the atoms. When Mendeleev started his work, only 63 elements were known. He examined the relationship between atomic masses of the elements and their physical and chemical properties.

Among chemical properties, Mendeleev concentrated mainly on the compound formed by elements with oxygen and hydrogen. he selected these two elements because these are very reactive and formed compound with most of the elements known at that time. The formulae of the compounds formed with these elements (i.e. oxides and hydrides) were regarded as one the basic properties of an element for its classification.

(i) Mendeleev’s periodic law : This law states that the physical and chemical properties of the elements are the periodic function of their atomic masses. This means that when the elements are arranged in the order of their increasing atomic masses, the elements with similar properties recur at regular intervals. Such orderly recurring properties in a cyclic fashion are said to be occurring periodically. This is responsible for the name periodic law or periodic table. 

(ii) Merits of Mendeleev’s periodic table : Mendeleev’s periodic table was one of the greatest achievements in the development of chemistry. Some of the important contributions of his periodic table are given below : 

(A) Systematic study of elements : He arranged known elements in order of their increasing atomic masses considering the fact that elements with similar properties should fall in the same vertical column. 

(B) Correction of atomic masses : The Mendeleev’s periodic table could predict errors in the atomic masses of certain elements were corrected. For example, atomic mass of beryllium was corrected from 13.5 to 9. Similarly, with the help of this table, atomic masses of indium, gold, platinum etc. were corrected. 

(C) Mendeleev predicated the properties of those missing elements  from the known properties of the other elements in the same group. Eka-boron, eka – aluminium and eka - silicon names were given for scandium, gallium and germanium (not discovered at the time of Mendeleev). 

Properties predicted by Mendeleev for missing elements and those found experimentally were almost same. 

(D) Position of noble gases : Noble gages like helium (He), neon (Ne) and argon (Ar) were mentioned in many studies. However, these gases were discovered very late because they are very inert and are present in extremely low concentrations. One of the achievements of Mendeleev’s periodic table was that when these gases were discovered, they could be placed in a new group without disturbing the existing order.

(iii) Limitations of Mandeleev’s periodic table : Inspite of many advantages, the Mendeleev’s periodic table has certain defects also. Some of these are given below -        

(A) Position of hydrogen : Position of hydrogen in the periodic table is uncertain. It has been placed in 1A group with alkali metals, but certain properties of hydrogen resemble those of halogens. So, it may be placed in the group for halogens as well/ 

(B) Position of isotopes : Isotopes are the atoms of the same element having different atomic masses. Therefore, according to Mendeleev’s classification these should be placed at different places depending upon their atomic masses. For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties. 

(C) Anomalous pairs of elements “ In certain pair of elements, the increasing order of atomic masses was not obeyed. In these, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses.

For example : 

1. The atomic mass of argon is 39.9 and that of potassium 39.1 But argon is placed before potassium in the periodic table.
2. The positions of cobalt and nickel are not in proper order. Cobalt (at. mass = 58.9 ) is placed before nickel (at. mass = 58.7)
3. Tellurium (at. mass = 127.6 ) is placed before iodine (at. mass = 126.9)/

(D) Some similar elements are separated, in the periodic table. For example copper (Cu) and mercury (Hg), barium (Ba) and lead (Pb). On the other hand some dissimilar elements have been placed together in the same group.

e.g. : Copper (Cu), silver (Ag) and gold (Au) have been placed in group 1 along with alkali metals. Similarly, manganese (Mn) is placed in the group of halogens.

(E) Cause of periodicity : Mendeleev could not explain the cause of periodicity among the elements.

MODERN PERIODIC TABLE : 

(a)Introduction : 

In 1913, an English physicist, Henry Moseley showed that the physical and chemical properties of the atoms of the elements are determined by their atomic number and not by their atomic masses. Consequently, the periodic law was modified.

(b) Modern periodic Law (Moseley’s Periodic Law):

“Physical and chemical properties of an element are the periodic function of its atomic number”. The atomic number gives us the number of protons In the nucleus of an atom and this number increases atomic number, is called periodicity.

(c) Cause of Periodicity :

The periodic repetition of the properties of the elements is due to the recurrence of similar valence shell (outermost shell) electronic configuration (ns¹) and therefore, have similar properties.

Modern periodic table is based on atomic number, not an atomic mass.

(e) Long Form of Periodic Table : 

(i) The long form of periodic table is based upon Modern periodic law. Long form of of periodic table is the contribution of Range, Werner, Bohr and Bury. 

(ii) This table is also referred to as Bohr’s table Since it follows Bohr’s scheme of the arrangement of elements into four types based on electronic configuration of elements.  

(iii) Long form of periodic table consists of horizontal rows (periods) and vertical columns (groups). 

(f) Description of Periods : 

(i) A horizontal row  of a periodic table is called a period .

(ii) There are  seven periods numbered as 1,2,3,4,5,6 and 7. 

(iii) Each period starts with an alkali metal having outermost shell electronic configuration ns1.

(v) Each period ends with a noble gas with outermost shell electronic configuration ns2 np6 except helium having outermost electronic configuration 1s2.

(vi) Each period starts with the filling of a new energy level. 

(A) 1^st period : This period is called very short period because this period contains only 2elements H and He.

(B) 2^nd and 3^rd periods : These periods are called short periods because these periods contain 8 elements. 2^nd periods starts from 3Li to ₁₀Ne and 3^rd period starts form ₁₁Na to ₁₈Ar.

(C) 4^th and 5^th periods : These periods are called long periods because these periods contain 18 elements. 4^th period starts from ₁₉K to ₃₆Kr and 5^th period start from ₃₇Rb to ₅₄Xe. 

(D) 6^th period : This period is called very long period. This period contains 32 elements. Out of the 32 elements 14 elements belong to Lanthanoid series (₅₈Ce to ₇₁Lu). 6^th period starts from ₅₅Cs to ₈₆Rn. 

(E) 7^th period : This period is called as incomplete period. It contains 25 elements. out of the 25 elements 14 elements belong to Actinoid series (₉₀Th to ₁₀₃Lr.) 7^th period starts from ₈₇Fr to ₁₁₁Rg.                 

Modern period table consists of seven periods and eighteen groups.

Different elements belonging to a particular period have different electronic configurations and have different number of valence electrons. That is why elements belonging to a particular period have different properties.

(g) Description of Groups  

 (i) A vertical column of elements in the periodic table is called a group.                  

(ii) There are eighteen groups numbered as 1,2,3,4,5,---------13,14,15,16,17,18. 

(iii) A group consists of a series of elements having similar valence shell electronic configuration and hence exhibit similar properties.

 e.g. : Li, Na, K belong to the same group and have 1 electron in their valence shell.

(iv) The group 18 is also known as zero group because the valency of the elements of this group is zero.

the elements of 18^th or zero group are called noble gases.

(v) The elements present in groups 1,2,13 to 17 are called normal representative elements. 

(vi) Elements of group 1 and 2 are called alkali metals and alkaline earth metals respectively                       

(vii) Elements present in group 17 are called halogens 

Elements present in a period have different properties, while elements present in a group have similar properties.

(h) Merits of Long Form of Periodic Table L

(i) The long form of periodic table is based on atomic number. Atomic number is a more fundamental property of a element as compared to atomic mass.

Not two elements can have the same atomic number. 

(ii) In the long form of periodic table, different isotopes can be placed at the same place because they have same atomic number. On the other hand, isobars such as Ar (40) and Ca (40) have to be placed at different places due to their different atomic numbers.

(iii) The long form of periodic table can explain why all the elements in a group have similar properties while the elements in a period have different properties.

The basis for periodicity of elements is the similar electronic configuration of the outermost shell of elements of the same group. The similar electronic configuration of the elements are repeated at regular intervals so the properties of the elements are also repeated at regular intervals.

(iv) It is easy to remember and reproduce the table.

(i) Limitations of Long Form of Periodic Table :

(i) Position of hydrogen is not accurate. 

(ii) Inner transition elements (lanthaoids and actinoids ( have been given separate positions below in the periodic table.

PERIODICITY IN PROPERTIES :

(i) The electronic configuration of the atoms display periodic variations with increase in atomic number.

(ii) The elements exhibit periodic variations of physical and chemical properties.

Following are some the important properties of the elements -

(a) Valency          (b) Atomic size      (c) Metallic and non - metallic character

(a) Valency :

(i) The valency of an element may be defined at the combining capacity of the element.

(ii) The electrons present in the outermost shell are called valence electrons and these electrons determine the valency of the atom.

Valiancy of an element is determined by the number of valence electrons in an atom of the element.    

The valency of an element = number of valence electrons

 (when number of valence electrons are from 1 to 4)

The valency of an element         - 8 - number of valence electrons.         

(when number of valence electrons are more than 4)       

(iii) Variation of valency across a period : The number of valence electrons increases from 1 to 8 on moving across a period. They valency of the elements with respect to hydrogen and chlorine increases from 1 to 4 and then decreases from 4 to zero. With respect to oxygen valency increases from 1 to 7.

Variation of valency of elements of second and third periods :

Valency of elements changes in a period. 

 Elements of third period :

(iv) Variation of valency along a group : On moving down a group, the number of valence electrons remains the same and, therefore, all the elements in a group exhibit the same valency.

e.g. All the elements of group 1 have valency equal to 1 and those of group 2 have valency equal to 2.

Valency of elements remains same in a group. 

(b) Atomic Size : 

The term atomic size refers to the radius of an atom. In general atomic size may be considered as the distance between the centre of the nucleus and the outermost shell of an isolated atom. 

(i) Variation of atomic size in a period : Within each period, the atomic radii decrease with increase in atomic number.

e.g. Atomic radii decrease from lithium to fluorine in the second period.

Reason : The decrease of atomic radii along a period can be explained on the basis of increase in nuclear charge. On moving from left to right across the period, the nuclear charge increases progressively by one unit but the additional electron goes to the same shell. As a result the electrons are pulled closer to the nucleus by the increased nuclear charge. This causes a decrease in atomic size.

The values given in the table, show abrupt increase  in the atomic size of Ne. This is due to the reason that the values for other elements are covalent radii whereas that for Ne it is Vander Waals radius because it does not form covalent bond due to its stable configuration.

Covalent radii is taken when electrons are shared between two elements, while Vander Waals radii is taken in case of gases.

(ii) Variation of atomic radii within a group : The atomic radii increase from top to bottom within a group of the periodic table.

Reason :

In moving down a group, the nuclear charge increases with increase in atomic number, but at the same time there is a progressive increase in the number of energy levels. Since, the effect additional energy level is more pronounced than the effect of increased nuclear charge, the distance of the outermost electron from the nucleus increases on going down the group.

1. Atomic radii increase down the group.       
2. Atomic radii decrease across the period.

METALLIC AND NON - METALLIC CHARACTER :

 (a) Metals : 

The metals are characterised by their nature of readily giving up the electrons. 

(i) Metals comprise of more than 75% of all known elements and most of them appear on the left hand side of the periodic table.           

(ii) Metals are usually solid at room temperature (except mercury). 

(iii) They have high melting and boiling points and are good conductors of heat and electricity. 

(b) Non - Metals : 

(i) Non-metals do not lose electrons but take up electrons to form corresponding anions.

(ii)  Non-metals are located at the top right hand side of the periodic table.

(iii) Non- metals are usually solids or gases (except bromine whish is liquid) at room temperature with low melting and boiling points.

(iv) They are poor conductors of heat and electricity.

Carbon is an exception as one of its allotropic forms, graphite is a good conductors of electricity.

(c) Metalloids (Semimetals ) : 

(i) Some elements lying at the border of metallic and non- metallic behaviour possess the properties that are characteristics of both metals and non - metals. These elements are called semimetals or metalloids.

(ii) The metalloids comprise of the elements B, Si, Ge, As, Sb, Te and Po.

(iii) Variation of metallic character across a period :

Metallic character decreases along a period due to increase in ionisation energy.

Non - metallic character increases with increase in atomic number across a period. 

(iv) Variation of metallic character along a group : Metallic character increases on going down a group from top to bottom. This can be explained in terms of decrease in lionisation energy on going down a group fro top to bottom. 

NOTE : Metals generally form cations by losing electrons from their outermost shell, while non - metals generally form  anions by accepting one or more electrons.

e.g. Alkali metals form M+ ions by losing one electron, while alkaline earth metals form M⁺ ions by losing two electrons from their outermost shell.

Metallic character decreases and non-metallic character increases across a period from left to right, while metallic character increases and non-metallic character decrease down the group

Top concepts

1. Dobereiner’s triads: Johann Wolfgang Dobereiner, a German chemist, classified the known elements in groups of three elements on the basis of similarities in their properties. These groups were called triads.

* Characteristic of Dobereiner’s  Triads:

a. Properties of elements in each triad were similar.

b. Atomic mass of the middle elements was roughly the average of the atomic masses of the other two elements.

2. Newlands’ Law of Octaves: John Newlands, an English scientist, arranged the known elements in the order of increasing atomic masses and called it the ‘Law of Octaves’. It is known as ‘Newlands’ Law of Octaves’Characteristics of Newlands’ Law of Octaves

a. It contained the elements from hydrogen to thorium

b. Properties of every eighth element were similar to that of the first element

* Limitations of Newlands’ law of Octaves:

a. The law was applicable to elements upto calcium (Ca) only

b. It contained only 56 elements. Further it was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future.

c. In order to fit elements into the table. Newlands’ adjusted two elements in the same slot and also put some unlike elements under same note. For example cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements. Iron, which resembles cobalt  and nickel in properties, has been placed differently away from these elements 

3. Mendeleev’s Periodic Table: Dmitri Ivanovich Mendeleev, a Russian chemist, was the most important contributor to the early development of a periodic table of elements wherein the elements were arranged on the basis of their atomic mass and chemical properties.. 

* Characteristic of Mendeleev’s Period Table:

a. Mendeleev arranged all the 63 known elements in an increasing order of their atomic masses.

b. The table contained vertical columns called ‘groups’ and horizontal rows called ‘periods’.

c. The elements with similar physical and chemical properties came under same groups.

* Mendeleev’s Periodic Law: The properties of elements are the periodic function of their atomic masses.

* Achievements of Mendeleev’s Periodic Table:

a. Through this table, it was very easy to study the physical and chemical properties of various elements. 

b. Mendeleev adjusted few elements with a slightly greater atomic mass before the elements with slightly lower atomic mass, so that elements with similar properties could be grouped together. For example, aluminum appeared before silicon, cobalt appeared before nickel.

c. Mendeleev left some gaps in his periodic table. He predicted the existence of some elements that had not been discovered at that time. His predictions were quite true as elements like scandium; gallium and germanium were discovered later

d. The gases like helium, neon and argon, which were discovered later, were placed in a new group without disturbing the existing order

 Limitations of Mendeleev’s Periodic Table :

a. He could not assign a correct position to hydrogen in the periodic table

b. Positions of isotopes of all elements was not certain according to Mendeleev’s periodic table

c. Atomic masses did not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements

4. Modern Periodic Table: Henry Moseley gave a new property of elements, ‘atomic number’ and this was adopted as the basis of Modern Periodic Table’.

* Modern Periodic Law: Properties of elements are a periodic function of their atomic number

* Position of elements in Modern Periodic Table:

a. The modern periodic table consists of 18 groups and 7 periods

b. Elements present in any one group have the same number of valence electrons. Also, the number of shells increases as we go down the group.

c. Elements present in any one period, contain the same number of shells. Also, with increase in atomic number by one unit on moving from left to right, the valence shell electron increases by one unit

d. Each period marks a new electronic shell getting filled

* Trends in the Modern Periodic Table:

(i) Valency: Valency of an element is determined by the number of valence electrons present in the outermost shell of its atom

• Valency of elements in a particular group is same

• Valency of elements in a particular period increases by one unit from left to right with the increase in atomic number by one unit

(ii) Atomic Size: Atomic size refers to the radius of an atom.

* In a period, atomic size and radii decreases from left to right. This is due to increase in nuclear charge which tends to pull the electrons closer to nucleus and reduces size of atom

* In a group, atomic size and radii increases from top to bottom. This is because on moving down, new shells are added. This increases distance between outermost electrons and nucleus which increases the size of atom

(iii) Metallic and Non- metallic Properties:

* The tendency to lose electrons from the outermost shell of an atom, is called metallic character of an element

* Metallic character decreases across a period and increases down the group

* The tendency to gain electron in the outermost shell of an atom, is called non- metallic character of an element

* Non-metallic character increases across a period and decreases down the group

* Elements intermediate between metal and non-metals that show characteristic of both metals and non-metals are called as semi-metals or metalloids

5. Metals have a tendency to loose electrons while forming bond. Hence they are electropositive in nature

6. Non-metals have a tendency to gain electrons while forming bond. Hence they are electronegative in nature

7. Oxides formed by metals are generally basic and oxides formed by nonmetals are generally acidic

PERIODIC CLASSIFICATION OF ELEMENTS

• Elements : Substances containing atoms of only one type. eg. Na,Au,Mg etc.

There are around 118 elements known to us.

•  Elements are classified to make the study easy.
•  Dobereiner’s Traids :When the elements were written in order of increasing atomic masses the atomic mass of the middle was the average of the atomic mass of the other two elements. eg. Elements AtomicMass

          Ca          40.1

          Sr          87.6

          Ba        136.3

Limitations : Only three triads were recognised from the elements known at that time.

•  Atomic mass of an element is the relative mass of its atom as compared widh the mass of a Carbon-12 atom taken as 12 units
•  Newland’s law of octaves :

– Based on increasing atomic mass of elements.

– When elements are arranged it was found that every eighth element had properties similar to that of the first. eg properties of sodium and Lithium are the same.

Limitations :
– Applicable only upto Calcium
– Properties of new elements couldn’t fit in it.
– It some cases properties of the elements were not same as defined by octave.
– Worked well onlywith lighter elements.

Mendeleev's periodic law :– The properties of elements are the periodic function of their atomic mass.
Mendeleev's periodic table based on the chemical properties of elements.
Contain eight vertical columns called groups and seven horizontal rows called periods formMendeleev’s peridic table.
Achievements ofMendeleev’s Periodic table
– Elements with similar properties could be grouped together
– Some gaps were left for the undiscovered elements.
– Noble gases could be placed without disturbing the existing order.

Limitations:
– No fixed position for hydrogen
– No place for isotopes
– No regular trend in atomic mass.
Modern Periodic Table 
Modern Periodic Law : Properties of elements are a periodic function of their atomic number.
– Atomic Number – denoted by Z and equals to the no. of protons in the nucleus of an atom.
– Modern periodic table contains 18 vertical columns known as groups and 7 horizontal rows known as periods.
– Elements in a group have valence electrons
– No. of the shells increases as we go down the group.
– Elements in a period have same number of shells.
– Each period marks a new electronic shell getting filled.
– No. of elements placed in a particular period depends upon the fact that how electrons are filled into various shells.

– Maximum no. of electrons that can be accomodated in a shell depend on the formula 2n² where n is the no. of the given shell.
eg. k shell – 2 x (1)² = 2 elements in the first period L shell – 2 x (2)² = 8 elements in the second period.
– Position of the element in the periodic table tells about its reactivity.

Trends in the Modern Periodic Table
– Valency : No. of valence electrons present in the outermost shells.
– Atomic Size : Atomic size refers to radius of an atom.
– Atomic size or radius decreases in moving from left to right along a period due to increase in nuclear charge
– Atomic size increases down the group because new shells are being added as we go down the group.

Metallic Character : Metallic character means the tendency of an atom to lose electrons.
– Metallic character decreases across a period because the effective nuclear charge increases that means the tendency to lose electrons decreases.
– Metals are electropositive as they tend to lose electrons while forming bonds.
– Metallic character increases as we go down a group as the effective nuclear charge is decreasing. Non metals are electronegative. They tend to form bonds by gaining electrons.
– Metals are found on the left side of the period table while non-metals are towards the right hand side of the periodic table.
– In the middle we have semi-metals or metalloid because they exhibit some properties of both metals and non metals.
– Oxides of metals are basic in nature while oxides of non-metals are acidic in nature.
(Refer the table given on side page)

EXERCISE
(Question Bank)

Element: A substance that cannot be separated into simpler substances and that singly or in combination constitute all matter.

Periods: It is a horizontal row of periodic table.

Groups: It is a vertical column of the periodic table.

Atomic mass: Atomic mass of an element is the sum of protons and neutrons.

Atomic radius: It is defined as the distance from the centre of nucleus to the outermost shell of the atom. It is generally expressed in Picometer (pm).

Periodicity: The recurrence of similar physical and chemical properties of elements when arranged in a particular order.

Valency: It is defined as the combining capacity of an atom of an element to acquire the next inert gas configuration.

Electronegativity: The relative tendency of an atom to attract shared pair of electrons towards itself.

Electron affinity: The amount of energy released when an electron is added to an isolated gaseous atom.

Ionisation energy: The amount of energy required to remove most loosely bonded electron from an isolated gaseous atom.

Isotopes: Atoms of same element having similar atomic number but different atomic mass.

Isobars: Atoms of different elements having different atomic number but same atomic mass.

PERIODIC CLASSIFICATION OF ELEMENTS

Gist of the lesson:

Classification of elements:the arrangement of element in such manner that element with similar properties are grouped together while elements with dissimilar properties are separated .

Early attempt to classify elements:

DOBEREINER’S TRIADS:

He arranged the elements with similar properties in a group of three known as triad in such a manner that the atomic mass of the middle element was approximately the average of the other two elements

LIMITATIONS:

Only three triads were identified from the element known at that time .hence this classification was not useful.

NEWLAND’S LAW OF OCTAVES :

He arranged the element in the order of increasing atomic masses starting with hydrogen(least atomic mass ) and ended with thorium having atomic mass 56 . According to him ,the properties of every eighth element are similar to the first element . Iit was compared to music notation sa,re ,ga ,ma, pa ,da ,ni ,sa,and thus the name Newlands law of octaves(notes of music ).

LIMITATIONS:

1. It was applicable only for lighter element having atomic mass upto 40 amu ,i.e.upto calcium .

2. He believed that only 56 elements existed in nature but later on more element were discovered whose properties did not fit into Newland law of octaves.

3. Some elements having different properties were grouped together like cobalt and nickel have been placed with halogens . Due to above limitations, Newland law of octave was rejected

MANDELEEV’S PERIODIC TABLE :

He arranged the elements in order of increasing atomic masses , similarity in physical and chemical properties of element . properties of hydrides and oxides of different element were

studied and elements with similar properties were grouped together . He classified the elements in table consisted of vertical columns called groups and horizontal rows

called periods . there were 7 groups in table and group is subdivided into subgroups A and B except group 7 which has three sets of elements in 4th , 5th , 6th period.

LIMITATIONS OF MENDELEEV, PERIODIC TABLE :

1. Position of hydrogen was not assigned correctly .

2. No separate position has been given to isotopes of an element .

3. Some element having higher atomic mass are placed before the elements with lower atomic mass .

MODERN PERIODIC TABLE :

Mosely modified the Mandelleve‘s periodic table by taking atomic number as the fundamental property instead of atomic mass.

Modern periodic table consists of 18 vertical columns known as group , and 7 horizontal rows known as periods .

GROUPS:

Elements in group one are called alkali metal s.

Elements in group 2 are called alkaline earth metals .

Elements in group 17 are called halogens .

Group 18 element are called inert gasses or noble gases.

Significance of group in the periodic table is that an element in a group has same number of valance electron ,valency and thus identical chemical properties.

PERIODS
1ST PERIOD – 2 elments and is called very short period .
2nd PERIOD- 8 elements and are called short period .
3rd PERIOD – 8 elements and are called short period .
4th PERIOD – 18 elements and are called long period .
5th PERIOD – 18 elements and are called long period .
6th PERIOD – 32 elements and are called very long period .
7th PERIOD- incomplete period .

The number of shell present in the element indicates the period to which it belongs .

VALENCY :
It is defined as the combing capacity of an atom of an element to acquire noble gas configuration.

it is equal to the number of electron lost , gained or shared during the formation of chemical compound .

ATOMIC SIZE / ATOMIC RADII:
It is defined as the distance from the centre of the nucleus to the outer most cell of atom . it is generally expressed in pico meter ( pm) .

On moving down the group the atomic radii increases.

Because on moving down the group a new energy shell is added which increases the distance between the outermost electron and the nucleus . All though the nucleus charge also increases , but it is compensated by the additional shell being added thus , increasing the size of the atom .

Across the period the atomic radii decrease. Due to the increase nuclear charge , the pull on the electron increases and hence, they are pulled closer to the nucleus thus, decreasing the atomic size .

Oxides and its nature. Metal reacts with oxygen to form oxides by loss of electrons. These oxides on dissolution in water from bases.

 REACTIVITY OF ELEMENTS.

Down the group reactivity of metal increases as the tendency to lose electron increases due to the increased atomic size .
Reactivity of non metals decreases down the group because of the increased atomic size and the tendency to gain electron decreases.
On moving across the period the reactivity first increases due to the decreased in the metallic character and increase in nonmetallic character.