Dear student, 1)For mono-atomic gases, 1 mole atom occupies 22.4 it's at NTP 2)Mole = Volume/22.4 (At S.T.P or N.T.P.) so according to this formula 1 Mole of gas occupies 22.4 litres at N.T.P. suuppose diatomic gas is O2 1 mole of O2 gas occupies 22.4 lit at N.T.P. since O2 consist of two atoms means 1 mole of O2 contains 2 mole atom of oxygen. thus we can say since 1 mole of O2 or 2 mole atoms of oxygen occupies = 22.4 litres at N.T.P. therefore 1 mole atom of oxygen occupies = 22.4/2 = 11.2 litres at N.T.P. regards