Question

what information on the periodic table is used to complete electron dot diagrams?
the number before the a at the top of a column shows how many dots to draw in the diagram.
the number before the b at the top of a column shows how many dots to draw in the diagram.
the number at the left of each row shows the number of valence electrons that are in atoms of elements in that row.
the number at the left of each row shows how many dots to draw above the symbol for elements in that row.

Explanation:

Electron dot diagrams represent valence electrons of an atom. For main-group elements (labeled with A in column headers), the number before A (e.g., 1A, 2A) equals the number of valence electrons, which determines the number of dots in the diagram. Row numbers indicate electron shells, not valence electrons or dots. B-labeled columns are transition metals, whose valence electrons don’t follow this simple A-column rule.

Answer:

The number before the A at the top of a column shows how many dots to draw in the diagram.