Question

what are 1a, 3b, and 7a examples of on the periodic table? groups. numbers periods rows

Explanation:

In the periodic table, elements are arranged in rows (periods) and columns (groups). Groups are vertical columns. 1A, 3B, and 7A are vertical columns (groups). Rows (periods) are horizontal. Numbers is a very general term not specific to the context here.

Answer:

groups.