Chapter 8 - Electron Configurations and Periodicity - Questions and Problems - Page 330: 8.31

Please see the work below.

According to Pauli principle there can be only two electrons with the same spin in a given orbital. The first six elements of the periodic table will have the electronic configuration as follows. $1s^2$ $1s^2$ $1s^3$ $1s^4$ $1s^4 2s^1$ $1s^4 2s^2$