Referring to the periodic table as needed, write electron configurations for all the elements in the third period.

 The third period begins with sodium and ends with argon. The atomic number Z of sodium is 11, and so a sodium atom has 11 electrons. The maximum number of electrons in the 1s, 2s, and 2p orbitals is ten, and so the eleventh electron of sodium occupies a 3s orbital. The electron configuration of sodium is 1s²2s²2p_x²2p_y²2p_z²3s¹.