CBSE Class 10 Science 2019 · Paper 31/1/1 · Question 3(a)

The Modern Periodic Table consists of groups and periods. The number of valence electrons decides the group: Li ($2,1$), Na ($2,8,1$) and K ($2,8,8,1$) all have $1$ valence electron and lie in group 1. The number of shells decides the period: Na ($2,8,1$) to Ar ($2,8,8$) all have $3$ shells and lie in period 3. Since both the group and the period are fixed by the electronic configuration, the structure of the table is based on it.