Atomicity is defined as:
The number of atoms constituting a molecule is known atomicity.
Elements one atom are called monoatomic. Elements with 2 atoms in their molecule are called diatomic, and those with many atoms are called polyatomic.
Examples of monoatomic elements include helium, neon, argon, etc.
Examples of diatomic elements include oxygen, chlorine, hydrogen, etc.
Examples of polyatomic elements include sulphur, phosphorous, etc.