​Sub-atomic particles
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Atoms are made from three sub-atomic particles. These are called protons, neutrons, and electrons. The table shows the mass and electric charge of each sub-atomic particle.

It is difficult to understand the properties of sub-atomic particles if you look at their actual masses and charges. Instead, it is often more useful to look at their masses and charges compared to the proton. These numbers are called the relative mass and the relative charge. The table shows the relative mass and relative charge of each sub-atomic particle.

​​The word ‘nuclear’ means anything to do with the nucleus. The nuclear atom has a nucleus at its centre, with electrons arranged in energy levels around it. The nucleus is made of protons and neutrons (also called nucleons). Most of the mass of the atom is found in the nucleus. This is because the protons and neutrons in the nucleus have much more mass than the electrons. 
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​Any atom has an equal number of protons and electrons. This means that atoms are neutral overall. For example, fluorine atoms each have nine protons and nine electrons. The nine positive charges carried by the protons in the nucleus are balanced by the nine negative charges carried by the electrons. Because neutrons are neutral, there can be any number of neutrons in the nucleus without making any difference to the overall charge of the atom.

Atomic number and mass number
The atomic number of an atom is the number of protons in its nucleus. It is also called the proton number. The atomic number is shown by the symbol Z. Every element has its own unique atomic number. All the atoms of a particular element have the same atomic number.

The mass number of an atom is the number of sub-atomic particles in its nucleus. The mass number is shown by the symbol A. It is the number of neutrons added to the number of protons. The full symbol for an atom is written like this:

The mass number of an atom is the number of sub-atomic particles in its nucleus. The mass number is shown by the symbol A. It is the number of neutrons added to the number of protons. The full symbol for an atom is written like this:

\[\begin{array}{*{20}{c}}{({\bf{MassNumber}}){\bf{A}}}\\{({\bf{AtomicNumber}}){\bf{Z}}}\end{array}Chemical Symbol\]

The atomic number is shown as a subscript and the mass number as a superscript. You can work out the number of neutrons in an atom by subtracting the atomic number from the mass number. So for \(F\begin{array}{*{20}{c}}{19}\\9\end{array}\) the number of neutrons is 19 - 9 = 10. The number of protons (and the number of electrons) in the atom is 9.

Isotopes

Isotopes are atoms that have the same number of protons but different numbers of neutrons in their nuclei. For example, \(Cl\begin{array}{*{20}{c}}{35}\\{17}\end{array}\) and \(Cl\begin{array}{*{20}{c}}{37}\\{17}\end{array}\) are both isotopes of chlorine. Their atoms each contain 17 protons. But every \(Cl\begin{array}{*{20}{c}}{35}\\{17}\end{array}\) atom has 35 - 17 = 18 neutrons, and every \(Cl\begin{array}{*{20}{c}}{37}\\{17}\end{array}\) atom has 37 - 17 = 20 neutrons. The different isotopes of an element are chemically identical to each other because they contain the same number of protons and electrons.