Protons and Neutrons



(a) Atomic or Proton number z:After it had been established that the negatively charged electron was one of the basic constituents of all atoms, the search began for a positively charged counterpart.

Experiments on the bombardment of atoms by suitable high-speed particles revealed that in certain cases a particle with the properties of a hydrogen ion ejected from the nucleus of an atom. This provided fairly conclusive evidence for the belief that such particles are one of the fundamental particles from which atoms are made. As they were the lighest and simplest positively charged particles then known, they were called protons (from the Greek protos meaning first).

The proton, denotd by the symbol p, has a charge equal in magnitude but opposite in sign to that of an electron and its rest mass is 1836 times the rest mass of the electron.

The number allotted to an element in the periodic table was called its '''atomic number. '''The significance of this term was not apparant until the nuclear theory of the atom had been proposed and evidence obtained about the structure of the nucleus.

The atomic or proton number Z of an atom is the number of protons in its nucleus.

An atom is normally electrically neutral and so the atomic number is also the number of electrons in a neutral atom of the element. Hydrogen Z = 1 has 1 proton and 1 electron, while uranium with Z = 92 has 92 protons and 92 electrons.

(b) Mass or nucleon number A:



Since the atomic number of an element is about half its relative atomic mass, it follows that if the mass of an atom is due to its nucleus, then there must be other constituents besides protons in the nucleus. The possibility of the existence with a similar mass to that of the proton, was suggested in 1920. This particle, named the neutron, symbol n, remained undiscovered until 1932, partly on account of the difficulty of detecting a particle which, being uncharged, is not deflected by electric or magnetic fields and produces no appreciable ionisation in its path.

Neutrons can also be expelled from certain nuclei by bombardment and there is now no doubt that they are basic constituents of matter. The rest mass of the neutron is 1839 times that of the electron. Protons and neutrons are collectively called nucleons.

The mass or nucleon number A of an atom is the number of nucleons in the nucleus.

It follows that if N is the neutron number of a nucleus, i.e. the number of neutrons it contains, then

A = Z+N

The simplest nucleus is that of hydrogen which consists of 1p; in symbolic notation it has superscript that gives the neutron number and a subscript that gives the proton number (see diagram). Helium has 2p and 2n, giving A= 4 and Z = 2, etc. Atom X is represented by superscript A, subscript Z.

The neutron is written superscipt 1, subscript 0 since it has A =1 and zero charge, i.e Z=0; proton can be written superscript 1, subscript 1 and the electron superscipt 0, subscript -1.