Question

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

Explanation:

On the periodic table, elements are organized into vertical columns called groups. The notation like 1A, 3B, 7A is used to label these groups. Rows are called periods and are numbered (e.g., period 1, period 2). "Numbers" is too vague in this context.

Answer:

groups.