An atom consists of one nucleus, protons and neutrons are inside it. The electrons will be outside the nucleus, within their shells. Hope Darekmoses, you will understand.
The nucleus are held by the strong nuclear force of attraction. There can be K,L,M,N shells. The number of electrons in each each shell can be derived by the formula 2n².
The mass of an atom is the sum of protons and neutrons. The electrons are responsible for the reactivity of the atom, as the valency is depending ypon the number of atoms.
Therefore, an atom consits of a nucleus in the center, surrounded by the electrons, in their own shells.
Hope this will help you.