Modern Periodic Table

Modern periodic table: In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property and on the basis of this, he modified Mendeleev’s periodic law as “physical and chemical properties of the elements are a periodic function of their atomic number”. This is called modern periodic law. When the elements were arranged in the increasing order of the atomic number, the obtained table is called modern periodic law.

Features of Modern Periodic Table:

This table has 18 vertical columns, known as groups and 7 horizontal rows, known as periods.

Features of Groups:

The groups are not divided into sub-groups.

The elements present in a group have the same number of valence or similar electrons.

The elements present in a group have the same valency.

The number of shells increases as we go down the group.

The elements present in a group have identical chemical properties.

The physical properties of the elements such as melting point, boiling point, density in a group vary gradually.

Features of Periods:

Elements of a period do not have the same number of valence electrons but they contain the same number of shells.

The number of valence shell electrons increases by one unit as the atomic number increases by one unit on moving from left to right in a period. Therefore, the atoms of different elements with same number of shells are placed in the same period.

As the number of valence shell electrons changes, the chemical properties of the elements also change.

Different periods have different number of electrons which can be explained on the basis of filling of electrons into various shells.

We can explain the number of elements in the periods based on how electrons are filled into various shells. Maximum number of electrons that can be accommodated in a shell is given by the formula, $2n^2$ . Where, n = number of given shell from the nucleus.

K-shell= 2 x (1)2 = 2, hence the first period has 2 elements and is called very short period.

L-shell= 2 x (2)2 =8, hence the second period has 8 elements and is called short period.

M-shell= 2 x (3)2 = 18 but the outermost shell can have only 8 electrons, so the third period also has only 8 elements thus, it is also called short period. Fourth and fifth periods have 18 elements and are called long periods.

Sixth and seventh periods have 32 elements and are also called long periods.

Some Facts:

The identities of some elements included in the seventh period have not been fully established. It can also be regarded as an incomplete period.

Block of Periodic Table:

The periodic table is divided in four blocks:

1. s-block elements: It includes group I (alkali metals) and group 2 (alkaline earth metals) elements.

2.p-block elements: It includes group 13 to 18 elements.

3.d-block elements: It includes group 3 to group 12. These are also called transition elements (in between s-block and p-block elements).

4.f-block elements It includes 14 elements after lanthanum (La, 57), called lanthanoides and 14 elements after actinium (Ac, 89), called actinoides.