Atomicity means the number of atoms present in a molecule or an ion or a compound or an element. So, atomicity of oxygen in oxygen gas is 2, and in ozone gas is 3.
Atomicity of molecule KMnO4 (potassium permanganate) is 6.
Molecules or substance with one atom in a molecule is called monoatomic substance. O2 gas is called diatomic. Ozone is called triatomic substance.
Atomicity has important effects on the dipole moments, charge, reactivity, size, bond angles, energy of molecules and even on gamma factor in the equation PV = gamma RT or the equation gamma = ratio of specific heats.
Atomicity has effect on physical properties of substances like modulus of elasticity and even on speed of sound in that medium.