The Structures of Atoms and Ions

So far, we have learnt about three particles: protons, neutrons and electrons.

These three particles can combine together to form larger particles called atoms and ions.

The diagram below shows an atom and an ion. As you can see they have very similar structures. In fact, there is only one difference in structure between them. Before we discuss that difference, we will first look at the things they have in common.

An atom and an ion. Both of these particles are made of protons, neutrons and electrons.

Atoms and ions both have a nucleus

At the centre of every atom or ion is a structure called the nucleus, which is a ball of protons and neutrons held together by a very strong force. The number of protons and neutrons in the nucleus varies from one atom or ion to the next.

The plural of nucleus is nuclei.

Atoms and ions both have shells of electrons around the nucleus

Around the nucleus, there are electrons. The electrons orbit around the nucleus in a similar way to satellites orbiting around the earth.

The paths that the electrons follow as they orbit the nucleus are called shells. In diagrams of atoms, circles are usually drawn to represent the shells. There are not actually circles around the atom, they just represent the paths of the electrons as they orbit around the nucleus.

The structure of an atom. The nucleus, in the centre, is made of protons and neutrons. The electrons orbit around the nucleus. These features are also found in ions.

In both atoms and ions, electrostatic attraction keeps the electrons in orbit

The electrostatic force keeps the electrons in orbit around the nucleus. The nucleus is positively charged because of the protons which make it up (the neutrons have no effect on the charge, because they are neutral). The electrons are negatively charged. Therefore, there are forces of electrostatic attraction between the electrons and the nucleus.

These forces of attraction keep the electrons in orbit around the nucleus and stop them from flying away from the atom. This is similar to the way that satellites are kept in their orbits around the earth by the force of gravitational attraction.

The forces of electrostatic attraction within an atom. The nucleus is positively charged and the electrons are negatively charged. Therefore there are forces of electrostatic attraction between them. This keeps the electrons in orbit around the nucleus.

In an atom, the number of electrons equals the number of protons. This is not the case in an ion.

The difference between atoms and ions is that in an atom, the number of electrons is equal to the number of protons, whereas in an ion the number of electrons is different to the number of protons.

The diagram below shows atoms on the left and ions on the right. The structures of the two types of particle are so similar that at first glance they seem to be the same. However, once you count the electrons and protons the difference becomes clear. In all of the atoms, the number of electrons equals the number of protons. In all of the ions, the number of electrons is either greater than or less than the number of protons.

A selection of atoms and ions.

Atoms are neutral and ions are positively or negatively charged

From a structural point of view we could think of atoms and ions as different versions of the same thing. When it has an equal number of protons and electrons we call it an atom. When it has unequal numbers of protons and electrons we call it an ion.

However, this seemingly minor difference has a big consequence in terms of charge. For atoms, because the numbers of protons (which are positive) and electrons (which are negative) are equal, the atom is neutral. Whereas for ions, there is an imbalance in charge. If the ion has more protons than electrons then it is positively charged. If it has more electrons than protons then it is negatively charged.

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