Outer electron

Sodium has atomic number 11, so a neutral sodium atom has 11 electrons. Its electronic structure is 2,8,1: three occupied shells place it in Period 3, and one outer electron places it in Group 1. Conversely, calcium’s position in Period 4 and Group 2 tells you that its atom has four occupied shells and two outer electrons.

For groups 1–7, the number of outer electrons gives the group. Magnesium and calcium each have two outer electrons, so both are in group 2. Chlorine, 2.8.7, has seven outer electrons and is in group 7.