It has a hard central core known as nucleus.It contains two types of particles one is known as proton and carries positive charge,the other is neutron (discovered by Chadwick in 1932) which is electrically neutral i.e. It carries no charge though it is as hearvy as proton.The protons and neutrons are very closely held together with tremendous nuclear forces.Revolving round the relstively massive nucleus,in more or less elliptical orbits (or shells) are infinitesimally small particles known as electrons.These electrons carry the smallest negative charge and have a negligible mass.The mass of as electron is approximately 1/1840 that of a proton.