Class 10 Science Periodic Classification of Elements NCERT Notes

Periodic Classification of Elements Class 10 Science CBSE NCERT Notes

The arranging of elements into different groups on the basis of the similarities in their properties is called classification of elements.

The classification of similar elements into groups makes the study of elements easier. There are about 118 different elements known so far. Out of these 118, only 94 are naturally occurring.

Early attempts at classification of elements

The earliest attempt to classify elements was grouping the then known elements (about 30 elements) into two groups called metals and non metals.

The defect in this classification was that it had no place for metalloids (elements which have properties of both metals and non metals) which were discovered later.

Dobereiner’s Traids

When elements were arranged in the order of increasing atomic masses, groups of three elements (known as traids), having similar chemical properties are obtained.

The atomic mass of the middle element of the triad was roughly the average of the atomic masses of the other two elements. For example,

Elements	Atomic Mass
Ca	40.1
Sr	87.6
Ba	137.3

Limitations: Only three traids were recognized from the elements known at that time.

Li	Ca	Cl
Na	Sr	Br
K	Ba	I

Newlands’ Law of Octaves

Newland classified the elements in the increasing order of their atomic masses into groups of eight elements called octaves like the notes of music. He found that when the elements were arranged in the increasing order of their atomic masses into octaves then there was similarity of properties in every eighth element.

Limitations

It was applicable upto calcium (for lighter elements only).
Properties of new discovered elements did not fit into the law of octave.
To fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties.

Mendeleev’s Periodic Table

When elements are arranged in the order of increasing atomic masses, the element with similar properties occur at regular intervals. The properties of elements are a periodic function of their atomic masses.

The formulae of the oxides and hydrides formed by the elements was also the basis for the classification of the elements.

Mendeleev’s periodic table has 6 horizontal rows called periods and 8 vertical rows called groups. The groups 1 to 7 had two sub groups called A sub group and B sub group. Group 8 had 3 rows of elements. Elements having similar properties were placed in the same groups. There are some spaces left vacant in the table to accommodate the elements to be discovered in future.

Merits of Mendeleev’s periodic table

Elements were classified on a more fundamental basis of their atomic masses and properties.
Spaces were left vacant to accommodate the elements to be discovered in future.
It could predict the properties of the elements which helped in the discovery of new elements.
The inert gas elements discovered later could be placed in a separate group without disturbing the table.

Defects of Mendeleev’s periodic table

Some elements are not arranged in the increasing order of their atomic masses. Co is placed before Ni, Te is placed before I etc.
Position of hydrogen is not clear because it shows properties similar to metals as well as non metals.
The position of isotopes of elements is not clear.

The Modern Periodic Table

In the modern periodic table elements are arranged in the increasing order of their atomic numbers in the form of a table having 7 horizontal rows of elements called periods and 18 vertical rows of elements called groups.

Periods

There are 7 periods of elements as:

First period has 2 elements H and He called very short period.
Second period has 8 elements Li to Ne called short period.
Third period has 8 elements Na to Ar called short period.
Fourth period has 18 elements K to Kr called long period.
Fifth period has 18 elements Rb to Xe called long period.
Sixth period has 32 elements Cs to Rn called very long period.
Seventh period has 28 elements from Fr to atomic number 114 called incomplete period.
14 elements each of he sixth and seventh periods are placed separately at the bottom of the table.

The 14 elements of the sixth period from La to Lu are called Lanthanides. and the 14 elements of the seventh period from Ac to Lr are called Actinides.

Groups

There are 18 groups of elements divided into 9 main groups. They are I, II, III, IV, V, VI, VII, VIII and 0 groups. The groups I to VII has two sub groups each called A – sub group and B – sub group. Group VIII has 3 rows of elements and 0 group has one row of elements.

The A sub group elements are called normal elements.
The B sub group elements are called transition elements.
Lanthanides and Actinides are called inner transition elements.
Group 1 (IA) elements are called alkali metals
Group 2 (II A) elements are called alkaline earth metals.
Group 17 (VII A) elements are called halogens.
Group 18 (0 group) are called noble gases.

In a group all the elements have the same number of valence electrons.

Group I elements have 1 valence electron,
Group II elements have 2 valence electron,
Group III elements have 3 valence electrons etc.

In a period all the elements contain the same number of shells.

Trends in the Modern periodic table

The physical and chemical properties of an element mainly depend upon its outer electronic configuration. Since the outer electronic configuration changes as as we go from left to right in a period therefore, within the same period, elements show a gradation both in their physical as well as chemical properties.

These properties which show a regular gradation on moving from top to bottom within the same group or from left to right along a period are called ‘atomic properties. for example, valency, atomic size, metallic or non-metallic character, etc.

Valency

Valency is the combining power if an element with other atoms when it forms a chemical compound.

Valency is equal to number of electrons gained or lost or shared to complete etc octet or doublet.

On moving from left to right in each period, the valency of elements increases from 1 to 4 and then decreases to 0.

Third Period Elements	Na	Mg	Al	Si	P	S	Cl	Ar
Valency	1	2	3	4	3	2	1	0

Valency remains the same down in a group.

Atomic size

Atomic size refers to the radius of an atom. It may be visualized as the distance between the centre of the nucleus and the outermost shell.

Atomic size or radius of an atom decreases as we move from left to right in a period because due to large +ve charge on the nucleus, the electrons are pulled in more close to the nucleus and size decreases. For example,

Third Period Elements	Na	Mg	Al	Si	P	S	Cl
Atomic radii (Pm)	186	160	143	118	110	104	99

Atomic size increases as we move down the group because new shells are being added and this increases the distance between nucleus and outermost electron.

Metallic Character

Metallic character means the tendency of an atom to lose electron.
Metals occupy the left hand side of the periodic table.
On moving left to right in a period, the metallic character of an element decreases because the effective nuclear charge increases. It means tendency to lose electron 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-metallic Character

Non-metals are electronegative as they tend to form bonds by gaining electrons.
Non-metals occupies the right side of the periodic table.
Non-metallic character increases across a period because due to increase in effective nuclear charge that means tendency to gain electron increase.
Non-metallic character decreases as we move down a group due to decrease in effective nuclear charge experienced by the valence electron thus the tendency to gain electron decreases.
In the middle of periodic table we have semi-metals or metalloid because they exhibit some properties of metals and non-metals.
Oxides of metals are basic in nature while oxides of non-metals are acidic in nature.