An atom is the smallest piece of an element which maintains the chemical properties of that element. It is made up of smaller bits of particles called subatomic particles. There are three different types of subatomic particles: protons, neutrons and electrons.
The nucleus of an atom is at the center and it contains protons and neutrons. The proton carries a positive charge but the neutron is neutral and carries no electric charge. The nucleus of the atom is surrounded by electrons which are negatively charged and they are arranged in shells referred to as energy levels.
Atomic number refers to the number of protons in the atom. Atoms have no electrical charge overall because the number of protons which carry positive charge in the atom is the same as the number of electrons which carry negative charge.