Modern periodic law : The physical and chemical properties of elements are the periodic functions of their atomic number (or) electron configuration.
i) The modern periodic table has eighteen vertical columns known as groups and seven horizontal rows known as periods.
ii) Elements are arranged in the order of increasing atomic numbers in periods.
iii) In groups the elements are placed having similar electronic configuration or having similar number of electrons in their outermost shells.
iv) Basing on which subshell, the differentiating electron enters, the elements are classified as s, p, d and f - block elements.
s - block elements : The elements with valence shell electronic configuration ${{ns}^{1}}$ and ${{ns}^{2}}$ are called s - block elements.
p - block elements : The elements with valence shell electronic configuration from ${{ns}^{2}}$${{np}^{1}}$ to ${{ns}^{2}}$${{np}^{6}}$ are called p-block elements.
The s and p elements together known as Representative elements, d - block elements : The elements with valence shell electronic configuration ${{ns}^{2}}$${{np}^{6}}$ (n - 1) ${{d}^{1}}$ to ${{ns}^{2}}$${{np}^{6}}$(n - 1) ${{d}^{10}}$ are called d - block elements. These are also called as Transition elements.
f - block elements : The elements in which f-orbital are being filled in their atoms are called f-block elements. These elements are also called ‘Inner Transition elements’.
Inert gases : The elements with complete outermost shell configuration (${{ns}^{2}}$${{np}^{6}}$) are known as inert gases. He, Ne, Ar, Kr, Xe and Radon do not react with any other element. So these are called inert gases.
First period contains 2 elements.
2nd and 3rd periods contains 8 elements each.
4th and 5th periods contains 32 elements.
7 period is incomplete.
The elements from $Ce _{ 58 }$ to $Lu { 71 }$ are called Lanthanoids.
These elements are 4f block elements.
The elements from $Th{ 90 }$ to $Lr _{ 103 }$ are called Actinoids. These elements are 5f- block elements.
Lanthanoids and Actinoids are shown separately at the bottom of the periodic table.