The modern periodic law states that, “**The physical and chemical properties of the elements and their compounds are periodic functions of their atomic number.**”

In modern periodic table, elements are arranged according to their increasing atomic numbers.

**Structure: **

The modern periodic table consists of horizontal rows called periods and vertical columns called groups. These are discussed below:

**Periods: **

A horizontal row in the periodic table is called periods. In terms of electronic structure of an atom a period constitutes a series of elements whose atoms have the same number of electron shells. i.e. principal quantum number (n). There are seven periods—

**The first Period:**Corresponding to n=1 is unique because it contains only two elements. This is because first energy shell has only one orbital, which can accommodate only two electrons. It contains Hydrogen (1s^{1}) and Helium (1s^{2}).**The second Period:**contains 8 elements because of n=2, there are four orbital (one 2s and three 2p). These four orbital have a capacity of eight electrons. Therefore, the second period has 8 elements. It starts from lithium (z=3) and ends at neon (z=10).**The third Period:**Corresponding to n=3, there are nine orbital one 3s, three 3p and five 3d. 3d-orbital are filled after filling 4s-orbital. Hence, this period involves the filling of only four electron (3s and 3p) and contain eight elements from sodium (z=11) to argon (z=18).**The fourth Period:**Corresponding to n=4 involves the filling of one 4s and three 4p-orbital. In between 4s and 4p-orbital, five 3d-orbital are filled which have high energy in between these orbitals. Thus, in all nine orbital are 0 to be filled, therefore, there are eighteen elements in the fourth period from potassium (z=19) to krypton (z=36).**The fifth Period:**Like the fourth period it also consists of 18 elements. If begins with rubidium (z=37) to Xenon (z=54).**The sixth Period:**Contains 32 elements, (z=55 to 66) and successive electrons enter into 6s, 4f, 5d and 6p orbital. It starts from cesium (z=55) and ends at radon (z=86).**The seventh period:**Though expected to have 32 elements, it is incomplete and contains only 24 elements at present.

**Groups: **

The vertical columns in the periodic table are called groups. There are 18 groups in the periodic table. The groups Ito VII are divided into sub-groups A and B. The groups are numbered as IA, IIA, IIIA, IVB, VB, VIB, VIIB, VIII, IB, IIB, IIIA, IVA, VA, VIA, VIIA and zero groups. All the elements in a group have similar properties.

The first two groups on the extreme left and last six groups on the extreme right involve the filling of s- and p-orbital respectively. These groups represent the main groups of periodic table and are numbered as 1, 2, 13, 14, 15, 16, 17 and 18 corresponding to general configuration of ns^{1}, ns^{2}, ns^{2}np^{1}, ns^{2}np^{2}, ns^{2}np^{3}, ns^{2}np^{4}, ns^{2}np^{5} and ns^{2}np^{b} respectively.

There two more rows at the bottom of the table. These rows consist of 14 elements after lanthanum (z=57) and fourteen elements which follow actinium (z=89). These are lanthanides and actinides and are all together called inner transition elements or rare earth metals.

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