Question

The first three elements of Group I have the following atomic structures
a) Lithium : 3 protons , 4 neutrons; 2, 1 electrons
b) Sodium : 11 protons, 12 neutrons ; 2, 8, 1 electrons
c) Potassium : 19 protons, 20 neutrons; 2, 8, 8, 1 electrons
Which of the following features causes them to have similar properties ?

Answer 1

Electronic configuration of Lithium: $1s^{2}2s^1$
Electronic configuration of Sodium: $1s^{2}2s^{2}2p^{6}3s^1$
Electronic configuration of Potassium: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$
The outermost shell of Li, Na and K has 1 electron. Hence they have similar properties and they belongs to the group IA.

THEORY
Alkali metals:
Electronic configuration:
1. One valence electron $\left(n{s}^1\right)$ outside the noble gas core
2. Most electropositive metals
3. Readily lose electron to give unipositive $M^{+}$ ions
4. Hence, never found in free state in nature.