Dear student,
Atomicity will define the number of atoms present in an element or molecule.
Monoatomic = 11, 12
Diatomic = 1,9
triatomic = 3,4,8
Tetraatomic = 5, 6
Polyatomic = 7, 10
Monoatomic means one atom, diatomic means two atoms, triatomic means three atoms and so on.
Regards