Atoms of two adjacent elements in the third period are in the ground state. an atom of element a has only s electrons in its valence shell. an atom of element b has at least one p electron in its valence shell. identify elements a and b.

Explanation:

A is Magnesium, B is Aluminium both are adjacent element and lie in third period.

Magnesium with atomic number of 12 consist two s electrons in it's valence shell in ground state whereas, Aluminium which has atomic number of 13 consist three electrons in it's valence shell in the ground state out of which two are s electrons and only one p electron.