An atom of element X has an electronic configuration of 1s²2s²2p⁶3s²3p³. In which group of the periodic table is X?

The configuration is 1s²2s²2p⁶3s²3p³. The outer shell is the third shell. It has 3s² and 3p³, so 2 + 3 = 5 outer electrons.

The number of outer electrons gives the group, so X is in group 5. It has 15 electrons in all, so it is phosphorus.