a) The atoms can be defined as the basic unit of matter. Initially, it has been assumed that the atom is indivisible, but scientist J.J. Thompson first suggested that the atom can be divided and has discovered the elementary particle “electron”. The atom has been made of electron, proton and neutron. There atom has the nucleus in its center that consists the proton and neutron. The electron moves around the nucleus in a series of level. Among all the atomic particles, the electron has the negative charge, the proton has the positive charge and the neutron is a chargeless particle (Reynolds, 2018).