What is nucleus

Nucleus:-

         The nucleus lies in the center of an atom. It has particles in it called (protons and neutrons). Around the nucleus there are electrons revolving in the circular paths called orbits or shells.

            The size (which is infact diameter) of the nucleus is between 1.6 fm (10⁻¹⁵ m) (for a proton in light hydrogen) to about 15 fm. Although it is only a very small part of the atom, the nucleus has most of the mass. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus with a very small contribution from the orbiting electrons.

What is Neutron

Neutron:-

            A neutron is a particle of an atom which is found in the nucleus of an atom except simple hydrogen.  Its name is neutron as this particle does not have any charge.   Neutrons are extremely dense.  A single neutron would have a mass of only 1.675 ? 10-27 kilogram.
          The number of neutron in nucleus is called the atomic number.  This number gives each element its unique identity.  In the atoms of any particular element, for example carbon, the number of protons in the nuclei is always the same, but the number of neutrons can change.

What is Proton

Proton:-

      A proton is a particle of an atom which has a positive electric charge that lies within the nucleus of an atom. The proton has +1 charge (alternatively, 1,602 x 10^-19 coulombs), exactly the opposite of the charge -1 containing the electron.

       In a neutral atom, the number of electrons and protons are always equal. While when an atom gain or loses an electron then it becomes an ion (cation or anion). In an ion the number of electrons and protons do not remain same.

What is electron

Electron:-

        Electron is a negatively charged particle in an atom. Electrons are found around the nucleus in circular paths called Shells.
        Electrons have no known components or substructure. Therefore they are generally considered to be elementry particles.

            In electrical conductors, current flow results from the movement of electrons from atom to atom. 
          The charge on a single electron is considered as the unit electrical charge. It is assigned negative polarity. The charge on an electron is equal, but opposite, to the positive charge on a proton.