The atomic number of the element which does not usually form compounds because its outermost shell is full is: 20 18 8 16 19

The element that does not usually form compounds because its outermost shell is full is a noble gas. Noble gases have complete valence electron shells, making them chemically inert under standard conditions.<br /><br />Among the given options, the atomic number 18 corresponds to the element Argon (Ar), which is a noble gas with a full outer electron shell.<br /><br />Therefore, the correct answer is:<br /><br />18