What is an Air Ion?

In order to keep things simple, we shall use the example of a helium atom

This has just two electrons (shown blue)  orbiting around a central nucleus consisting of  protons and neutrons.

(most other substances are far more complicated. Oxygen, for example,  has eight each of  electrons, protons and neutrons)

As they have no electrical charge, we can ignore the neutrons and just visualise the protons (shown in red) at the centre.

It can now be seen that the atom has two electrons  orbiting  two protons. Or, simply put, two negative charges orbiting two positive charges.

These charges cancel out and so the atom has no overall electrical charge.

However, the negatively charged electrons are very much lighter  (in fact, almost two thousand times lighter) and under some circumstances can quite easily become dislodged

For example. Imagine the helium gas being blown by a fan, though an air duct. The atoms will be jostling around and banging into each other. This can easily knock one of the very lightweight electrons out of its orbit.

This now leaves two positive charges to one negative charge leaving the atom with an overall positive charge. Now it is a positive ion.

The dislodged electron has to go somewhere, so it clings on to the nearest atom, giving it an extra negative charge and thereby turning it into a negative ion.

Because it is a gas and the atoms are in motion, they quickly get separated.

The distance becomes too great for the electron to jump back and so the two oppositely charged ions are left drifting in the air.

This situation is very unstable and the electron tries to return to it's original situation. At the first opportunity it will jump back off  and fill a vacant space on another atom of the opposite charge.

The separation of electrical charges in this way, is what gives rise to "static" electricity.

Millions of dislodged electrons jump back to "earth" - causing anything from a small spark if it's off your car, for example - right up to the dimensions of a full lightning strike.