As has been stated the nucleus consists of protons and neutrons, the nucleons.

The number of protons is called the Atomic Number and is designated by the letter Z. All atoms of a par­ticular element have an unique atomic number — the number of protons defining the element. Elements that occur in nature have atomic numbers ranging from 1 for hydrogen to 92 for uranium, the heaviest element that occurs naturally. Elements with atomic numbers greater than 92 have been produced artifi­cially — the transuranic elements. It will be seen in subsequent chapters that ^the element plutonium, atomic number 94, has a particular significance in nuclear power generation and is inevitably produced in an operating nuclear reactor. (Strictly, plutonium should also be referred to as a naturally occurring element. It has been found at different locations around the world where fortuitous conditions at some time in the history of the earth allowed uranium de­posits at these places to behave as a nuclear reactor and thus produce plutonium which is still present but in very small quantities.)

The number of nucleons in a nucleus is called the Mass Number and is designated by the letter A. Therefore,

A = Z + N

where N is the number of neutrons.

It is usual to denote an element as Ac where:

ZE

• E is the chemical symbol for the element.

• Z is the atomic number = number of protons.

• A is the mass number = number of nucleons

= total number of protons and neutrons.

Atoms of a particular element which, by definition, have the same number of protons in their nucleus may however have different numbers of neutrons. For example, the nucleus of the element uranium has by definition 92 protons but can have 142 or 143 or 146 neutrons, represented by 234 It, 235 , r or 238 rr

92 U 92 u 92 U’ Again, a chlorine nucleus — which must have 17 protons — can have 18 or 20 neutrons: 35 r,, 37 r]

17U 17L

Atoms with the same atomic number Z but different mass number A are called isotopes. 234 n, 235 ,, and

92 U 92 U

chemical symbol gives the same information as the atomic number, may be written in
the form U-234, U-235, U-238 respectively) are there­fore all isotopes of uranium. Similarly Cl-35 and Cl-37 are isotopes of the element chlorine. Appendix A lists the naturally occurring isotopes of the elements, from hydrogen to uranium, and gives other useful infor­mation (technetium and promethium are included but they are not found in nature). It will be noted in the appendix that for stable elements of low mass number the number of neutrons is about equal to the number of protons. For the heavier elements, however, the number of neutrons increases rather faster than the number of protons until there are about one and half times as many neutrons as protons in the uranium nucleus. This is shown graphically in Fig 1.2.

One may qualitatively understand why heavier ele­ments have disproportionately more neutrons than protons by considering what holds the nucleus to­gether. The gravitational forces between the nucleons are very small indeed and negligible compared to the electrostatic repulsion forces between the positively charged protons, tending to disrupt the nucleus. How­ever there is a strong attractive force between the nucleons — between proton and proton, proton and neutron, neutron and neutron — known as the strong nuclear force. The effect of having neutrons mixed w’uh the protons means that the latter are further apart on average and therefore the electrostatic repul­sion force, which is inversely proportional to the square of the distance of separation between the charges, is reduced. Equally importantly the total attractive force is increased by the presence of the neutrons. The heavier the element the more necessary it is to reduce the electrostatic forces and increase the nu­clear forces.