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 (Lewis dot diagrams) represent the valence electrons of an atom. In the periodic table, for main - group elements (groups labeled with A), the group number (the number before the A at the top of a column) indicates the number of valence electrons. For example, group 1A elements have 1 valence electron, group 2A elements have 2 valence electrons, and so on up to group 8A elements which have 8 valence electrons. The number of dots in an electron - dot diagram is equal to the number of valence electrons.

• Option 2: Groups labeled with B are transition metal groups. Transition metals do not follow the simple rule of having the group number equal to the number of valence electrons for electron - dot diagram construction in the same straightforward way as main - group (A - group) elements.
• Option 3: The number at the left of each row (period number) indicates the principal energy level of the outermost electrons, not the number of valence electrons.
• Option 4: Similar to option 3, the period number (number at the left of each row) is not related to the number of dots (valence electrons) in the electron - dot diagram.

Answer:

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