All atom of the same element are alike.An atom has particles concentrated in the centre called nucleus. The nucleus consists of two types of particles- namely protons and neutrons. Collectively. protons and neutrons are known as nucleus. Surrounding the protons and neutrons are electrons. The protons have a positive charge while the neutrons do not have any charge as they are neutral. Both neutron and proton have a relative mass of 1. The electron carries one negative electric charge. The electron has a relative mass of 1/1800. The electrons move rapidly around the nucleus. The first two electrons nearest to the nucleus circulate in an elliptical path. These two electrons are constantly in motion in the fixed path. This path is known as the first electron shell and can hold at most two electrons. Any other electron will have to go on to the second shell. The second shell can hold at most 8 of electrons which are constantly circulating in pairs. Once the second shell is full with 8 electrons, the electrons move to the third shell which can also hold at most 8 shells. The number of protons in the nucleus is the atomic number. For an atom, the number of protons is equal to the number of electrons. The number of neutrons plus protons in the nucleus is known is the nucleon number or relative atomic mass.