Ions and ionisation

An ion is a charged atom or molecule. It becomes charged if it loses or gains one or more electron. This can happen in some chemical reactions, when the material is dissolved in water, when it it heated to a very high temperature, when exposed to ionising radiation  or subject to a strong electric field. 

Ionisation in an electric field and sparks

An electric field is created between two differently charges surfaces. The field is particularly strong around a sharply curved surface or point. If the field is sufficiently strong between the two charged surfaces in a gas because the potential difference is very high and/or they are very close together, outer electrons can break free of the atom or molecule. When this happens the electrons accelerate towards the positive surface, colliding with other molecules and in turn ionising them. This causes an avalanche of free electrons and ions and a spark. The video below explains that with the help of animated graphics and practical demonstration.

Sparks, electric fields and ionisation

Ionising radiation

Ionising radiation includes that emitted from radioactive sources, that is alpha, beta and gamma radiation. In the electromagnetic spectrum ultra violet light and all waves of a shorter wavelength (X rays and gamma radiation) can cause ionisation. In living tissue this can be harmful since the ionised molecules may cause a cell to die or, rarely but dangerously, they may alter its structure to cause cancer. Perversely the same radiation can be used to kill cancer cells and thus can be an effective treatment for some forms of cancer.

Radioactive sources are commonly detected and measured by the ionisation they cause. In particular the Geiger Counter using a Geiger Muller tube is a particularly common and useful instrument. The video below explains how radioactive sources cause ionisation and how that is used in a Geiger Counter.


Ionising radiation and the Geiger Counter

Ionic bonding

Ionic bonding is the force of attraction between a positively charged metal ion and a negatively charged non-metal ion. When metals and non metal react together metals form positive ions because the atom loses one or more electrons to become stable. The transfer of this/these to the non metal which also allows it to achieve stability. They do this by creating a full outer electron energy level. The resulting molecule is very stable since the two oppositely charged ions are very strongly bonded together. To break these bonds requires a very high temperature but they can easily be separated by dissolving in water. The water becomes electrically conductive because the ions will move in solution. The current will then cause chemical changes – this is electrolysis.

Using graphic simulations and video the tutorial explains the process and how the ions dissociate in solution.

Ions and ionic bonding