How can a period, group and block of the element be determined?

The group, period and the block of the element can be determined on the basis of its electronic configuration.

i. Period: The principal quantum number of the valence shell corresponds to the period of the element.

e. g. The principal quantum number (n) of the valence shell (3s¹) of Na (1s² 2s² 2p⁶ 3s¹) is 3.

This corresponds to third period.

ii. Block: The subshell in which the last electron enters, corresponds to the block of the elements (with exception being He). 

e. g. The subshell 3d (in which the last electron enters) for Sc (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹ 4s²) corresponds to d block.

iii. Group: The group of the element is determined on the basis of number of electrons present in the outermost or penultimate [next to outermost, i.e. (n-1)] 

shell:

• For s-block elements, group number = number of valence electrons.
• For p-block elements, group number = 18 – number of electrons required to complete octet.
• For d-block elements, group number = 2 + number of (n-1)d electrons.