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Discovery of neutrons

Discovery of neutrons:
According to rutherford,the nucleus of an atom consists of protons and the entire mass of the atom is due to its nucleus.In other words,the number of protons must be equal to its mass on a.m.u.scale.But he found that the mass of protons in the nucleus of many elements was far less than their own respective masses.For example,the mass of helium is 4 a.m.u. but it has only two protons of mass 2 a.m.u. to account for this,rutherford predicted the existence of neutral particles in the nucleus of the atom having a mass equal to that of a proton but carrying no charge.This prediction was confirmed by james chadwick in 1932.He observed that when the beryllium foil is bombarded with alpha particles,a radiation is emitted which consists of uncharged particles (as these are not affected by electric and magnetic fields). chadwick named these particles as neutrons. Each neutron possesses a mass of 1.67*10-24g(1 unit mass) which is nearly the same as that of a proton.Thus,neutron like protons and electrons was recognised as another fundamental particle of all matter having mass of 1.67*10-27kg or 1.67*10-24g.
A neutron may be defined as a sub-atomic particle which carries no charge but has a mass equal to that of hydrogen atom or proton.
From the above discussion,it follows that an atom consists of two parts viz.
(a).Nucleus,and
(b).Extranuclear part.
(a). Nucleus: It is the central,small but massive part of the atom which contains protons and neutrons.since protons are positively charged while neutrons are neutral particles,the net charge on the nucleus is positive and is called the nuclear charge. since electrons have a negligible mass,so the entire mass of the atom is due to its nucleus.The charge on the nucleus is equal to number of protons present in it.
(b).Extranuclear part:The portion of an atom around the nucleus in which electrons revolve in certain fixed orbits is called extranuclear part.Since atom as a whole is electrically neutral,the number of electrons in the extranuclear part is equal to the number of protons in the nucleus.Further, as stated above,electrons possess negligible mass, so the extranuclear part does not contribute towards the mass of an atom but is mainly responsible for its size.
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