Question

Why does the third period of the periodic table contain 8 elements?

Answer 1

The third period contains 8 elements because it corresponds to the filling of the 3s and 3p subshells (of the third shell) which take a total of 8 electrons, two in the 3s and 6 in the 3p subshell. Each additional electron in these subshells represents a different element eg: 1 electron, $3s^1$, corresponds to sodium, 3 electrons, $3s^2,3p^1$, correspond to aluminium, 7 electrons, $3s^2,3p^5$, to chlorine.