What information on the periodic table is used to complete electron dot diagrams?

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

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

c. the number at the left of each row shows the number of valence electrons that are in atoms of elements in that row.

d. the number at the left of each row shows how many dots to draw above the symbol for elements in that row.

A.

The number before the A at the top of a column is called the group number and it’s equal to the number of electrons that atom has in the last level. Those are the electrons that we draw in the diagram (the rest of the electrons are inner electrons). Example: F has a total of 9 electrons, which are 7 external electrons and 2 internal electrons, and it belongs to group VIIA. So we will draw 7 dots for F.

Not all atoms have an A at the top of a colum. Others have a B instead. But in that case, that number is not related to the external electrons. For example: Ti has 22 electrons, which are 2 external and 20 internal, and it belongs to group IV B. For these elements, we focus on the oxidation number to study their compounds, and we usually don’t make dot diagrams for those compounds. These are called transition elements.

The number of row is called the period number and it shows how many electron levels the atom has. For example: K is in period 4, and it has: 2 electrons in the first level, 8 electrons in the second level, 8 electrons in the third level and 1 electron in the fourth level, which makes a total of 19 electrons. The fourth level is the outer level. As the fourth level has 1 electron, we would draw 1 electron in the dot diagram.