We will start this topic with a summary of the particles mentioned so far (Table 15.1).

^(a) The numbers given for the masses are approximate.
^(b) $e = 1.60 \times {10^{ - 19}}C$.

All nuclei, except the lightest form of hydrogen, contain protons and neutrons, and each nucleus is described by the number of protons and neutrons that it contains.
- Protons and neutrons in a nucleus are collectively called nucleons. For example, in a nucleus of gold,  there are 79 protons and 118 neutrons, giving a total of 197 nucleons altogether.
- The total number of nucleons in a nucleus is called the nucleon number (or mass number) A.
- The nucleon number is the sum of the number of neutrons and protons in the nucleus, or $A = N +Z$
(where A = nucleon number, N = neutron number and Z = proton number).
The unit used to measure masses at this level is the unified atomic mass unit (u).
$1 u$ is defined as being one-twelfth of the mass of a carbon-12 atom.
An isolated proton has a mass of $1.00727646677 u$ and an isolated neutron has a mass $1.008665 u$. You can see that there is a discrepancy between the sum of the masses of the protons and neutrons in a carbon-12 atom and the sum of the masses of six isolated protons and six isolated neutrons The reasons for these discrepancies are explored in detail in Chapter 29.
A specific combination of protons and neutrons in a nucleus is called a nuclide.
The nucleus of any atom can be represented by the symbol for the element (shown here as X) along with the nucleon number A and proton number Z:

$\begin{array}{*{20}{c}}
{nucleon\,number\, \to \,A}\\
{proton\,number\, \to \,Z}
\end{array}\,X$

For example:
Element       Symbol          Nucleon number A    Proton number Z            Represented as:
oxygen         O                    18                                 8                                       $\begin{array}{*{20}{c}}
{\,16}\\
{\,8}
\end{array}\,O$
gold             Au                  197                               79                                       $\begin{array}{*{20}{c}}
{197}\\
{\,79}
\end{array}\,Au$
uranium      U                    238                               92                                        $\begin{array}{*{20}{c}}
{238}\\
{\,92}
\end{array}\,U$
The proton and nucleon numbers of some common nuclides are shown in Table 15.2.

Isotopes

Although atoms of the same element may be identical chemically, their nuclei may be slightly different.
The number of protons in the nucleus of an atom determines what element it is: helium always has two protons, carbon six protons, oxygen eight protons, neon 10 protons, radium 88 protons, uranium 92 protons and so on.
However, the number of neutrons in the nuclei for a given element can vary. Take neon as an example.
Three different naturally occurring forms of neon are:
$\begin{array}{*{20}{c}}
{20}\\
{10}
\end{array}\,Ne\,\,\,\,\,\,\,\begin{array}{*{20}{c}}
{21}\\
{10}
\end{array}\,Ne\,\,\,\,\,\,\,\begin{array}{*{20}{c}}
{22}\\
{10}
\end{array}\,Ne$
The first has 10 neutrons in the nucleus, the second 11 neutrons and the third 12 neutrons. These three types of neon nuclei are called isotopes of neon. Each isotope has the same number of protons (for neon this is 10) but a different number of neutrons. The word ‘isotope’ comes from the Greek isotopos (same place), because all isotopes of the same element have the same place in the Periodic Table of elements.
Isotopes are nuclei of the same element with different numbers of neutrons but the same number of protons.
Any atom is electrically neutral (it has no net positive or negative charge), so the number of electrons surrounding the nucleus must equal the number of protons in the nucleus of the atom. If an atom gains or loses an electron, it is no longer electrically neutral and is called an ion.
For an atom, the number of protons (and hence the number of electrons) determines the chemical properties of the atom. The number of protons and the number of neutrons determine the nuclear properties. It is important to realise that, since the number of protons, and therefore the number of electrons, in isotopes of the same element are identical, they will all have the same chemical properties but very different nuclear properties.
Hydrogen has three important isotopes, $\begin{array}{*{20}{c}}
1\\
1
\end{array}\,H$ (sometimes called protium), $\begin{array}{*{20}{c}}
2\\
1
\end{array}\,H$ (deuterium) and $\begin{array}{*{20}{c}}
3\\
1
\end{array}\,H$ (tritium)
(Figure 15.7).

Protium and deuterium occur naturally, but tritium has to be made. Deuterium and tritium form the fuel of many fusion research reactors. Hydrogen is the most abundant element in the Universe (Figure 15.8), because it consists of just one proton and one electron, which is the simplest structure possible for an atom.

The different numbers of neutrons in the isotopes of an element means that the isotopes will have different relative atomic masses. There are differences too in some of their physical properties, such as density and boiling point. For example, heavy water, which is water containing deuterium, has a boiling point of ${104^ \circ }C$ under normal atmospheric pressure.
Table 15.3 gives details of some other commonly occurring isotopes.