The combination of an atom with other atom is bonding. Or break apart from other substance is breaking of interaction.
The bonding or interaction (inter-atomic) are due orbitals and electrons present in these orbitals
The atom consists of
a) a central nucleus which has proton and neutrons: they do not take part in chemical reactions
b) the part other than nucleus which has shells, sub shells and orbitals
These orbitals have electrons.
So the outer part of nucleus takes part in any combination with other substance
