Question

Explain why all the periods in the periodic table do not have the same number of elements in them

Answer 1

it is due to the valence electron

Answer 2

All the periods in the periodic table do not have the same number of elements in them as the number of elements in a period is determined by the quantum numbers.

For example: The first period has two elements only since the 1s orbital gets filled.

The second period has eight elements only since the two orbitals- 2s and 2p get filled. This happens for all the periods.

Therefore, it depends on the Principle Quantum number and Azimuthal quantum number.