What is periodicity? What is its cause?
The repetition of the properties of elements after certain intervals when the elements are arranged in order of increasing atomic numbers is known as periodicity.
Cause of periodicity: We know that the physical and chemical properties of the elements depend upon the number of electrons present in their outermost shell. Elements having similar outer electronic configurations have similar properties. When the elements are arranged in the periodic table on the basis of their increasing atomic numbers, similar valence shell electronic configurations are repeated after certain regular intervals of atomic numbers i.e. 2, 8, 8, 18, 18, 32 (magic numbers). This repetition of similar valence shell electronic configurations of the elements in a group is the cause of periodicity of the elements in the periodic table. For example, all the members of alkali metal family, which have definite gaps of atomic numbers (8, 8, 18, 18, 32), have similar physical and chemical properties because they have similar valence shell electronic configuration i.e. ns1 as shown below:
| Element | Atomic No. | Electronic configuration |
| Lithium (Li) | 3 | [He]221 |
| Sodium (Na) | 11 | [Ne]103s1 |
| Potassium (K) | 19 | [Ar]18 4s1 |
| Rubedium (Rb) | 37 | [Kr]36 5s1 |
| Cesium (Cs) | 55 | [Xe]54 6s1 |
| Francium (Fr) | 87 | [Rn]86 7s1 |
In a similar manner, all the halogens i.e. elements of group 17 have similar outer electronic configuration (ns2 np5) and as such possess similar properties.
