Answer:
(1) Atom: An atom is any particle of matter at its most basic level which contains at least one proton.
(2)Proton: A proton may be defined as that fundamental particle which carries 1 unit positive charge and has a mass nearly equal to that of hydrogen atom.
(3)Electron: An electron is a negatively charged subatomic particle. It can be either free (not attached to any atom), or bound to the nucleus of an atom.
(4)Neutron: A neutron is a neutral subatomic particle and is present in the nucleus of the atom. They are usually denoted by N and net charge associated with it is zero. It is present in the nucleus and proton and neutron together are called nucleons. Total number of protons and neutrons gives the atomic mass of an element.