Ionisation

In solids electric current is a flow of electrons but in liquids and gases the electric current is a flow of electrons and ions.


An ion is a charged atom and these can be either positive or negative. Figure 1 shows you a diagrammatic view of what the two types of ion are like.

Ions are made when atoms or molecules (linked groups of atoms) are literally torn apart to give positive and negative ions or positive ions and free electrons. This may sound a bit violent but you can do it by simply putting a voltage across the gas or liquid. This is what happens in electrolysis where a voltage is applied to two metal plates in a liquid.

In the electrolysis of copper sulphate the liquid is ionised to give positive copper ions and negative sulphate ions. (Cu⁺⁺ and SO₄^--). Two electrons are torn from the copper atoms and added to the sulphate ions.

The positive copper ions are then attracted to the negative electrode while the negative sulphate ions are pulled towards the positive electrode (See Figure 2).

Ionisation of water will give you positive hydrogen ions (H⁺) at the cathode and negative ions (OH^-) at the anode.

Applying a high voltage across a gas such as air will ionise the air molecules allowing a current to flow through it and giving a spark. The spark is the movement of the positive and negative ions through the air. You will have seen the ionisation of air on a large scale as lightning. (See: Lightning)

Ionisation can also be produced by charged particles such as alpha or beta radiation colliding with an atom or by the impact of gamma radiation on the atom. The ions formed in one collision can move on to collide with other atoms – ionising them to give what is called an avalanche of ions.

(See: Radioactivity and ions)