Question

What is the otal number of principle energy levels that are completely filled in an atom of magnesium in the ground state?
Is it 2 or 4?

Answer 1

Look at the Periodic Table. The number of filled shells is always equal to the number of periods *before* the period you're element's in.

Mg is in Period 3, so it has 2 filled shells.

Remember, the Periodic Table is your friend when it comes to electron configuration. Almost everything about the electron configuration of an element can be deduced from where it lies in the Periodic Table -- and you'll almost always have a PT handy, even on an exam.