What is the electron shell configuration of magnesium?

• A. 2,8,2
• B. 2,8,1
• C. 2,7,2
• D. 2,8,3

Answer: (A)

Placing electrons follows filling the lowest-energy shells first, and each shell has a maximum capacity. Magnesium has 12 electrons, so you start by filling the first shell with 2 electrons. The second shell can hold up to 8, so it takes all 8. That uses 10 electrons, leaving 2 to go into the next shell. So the distribution by shells is 2 in the first, 8 in the second, and 2 in the third, written as 2, 8, 2 (or 1s2 2s2 2p6 3s2 in orbital notation).

This matches the total number of electrons for magnesium (12) and the typical filling order. Other patterns would miscount the total or place electrons in a higher shell before the second shell is filled to its capacity.