An atom is the smallest particle of an element which may or may not have an independent existence. It is the building blocks of elements. All atoms combine to form molecules. There are numerous atoms in elements.
An atom consists of protons,neutrons and electrons. There is a nucleus further inside the atom which contains the protons and neutrons.
Protons carry positive charge and have unit mass,i.e., 1 a.m.u. (atomic mass unit). Neutrons carry no charge/neutral and have unit mass.
The atom is surrounded by orbits or shells around which electrons revolve. Electrons have negative charge and have unit mass.
In a neutral atom, the number of protons is equal to number of electrons. But it happens not in all the atoms of elements.