Ionisation

As you know an atom of an element is composed of a heavy positively charged central nucleus surrounded by a cloud of orbiting electrons. The size of the positive charge on the nucleus and the number of orbiting electrons determines the type of element. (See Figure 1).


We can show the state of the electrons in orbit round the nucleus by an energy level diagram. The energy of each electron is shown on the vertical axis. A simplified version of an energy level diagram is shown in Figure 2. The electrons are spread through the energy levels. Notice that no electron can have an energy state between the levels.



In hydrogen there is just one orbiting electron. The electron is usually in its unexcited or ground state - level 1 (Figure 3(a).



If energy is put into the atom in the form of radiant energy or by an inelastic collision with a charged particle (such as another electron) this electron is raised to a higher energy level and is said to be excited and in an excited state. (In Figure 3 (b) the electron has been raised to level 3 and the colliding electron has lost some energy).

After a short time (usually less than a millisecond) this electron falls back to a lower energy state. It loses energy in the form of a photon of radiation that is emitted from the atom (Figure 3(c)). Large electron energy drops give photons of higher energy and therefore higher frequency. To raise an electron in hydrogen from level 1 to level 3 requires 19.4x10^-19 J or 12.1 eV

The electron is shown falling back to its ground state although it does not have to do this. It could fall back to level 2 and then later on back to level 1.

If the collision with an incoming electron is sufficiently violent an electron within the atom can be given enough energy to raise it to the level marked with an infinity symbol in the diagrams. (Figure 4 (a and b))

If it reaches this level, or above, it will escape from the atom altogether. This level is called the ionisation level and the process is called ionisation.

The removal of one (or more) electrons will leave the atom with a net positive charge – it has become a positive ion. To ionise hydrogen requires 21.8x10^-19 J or 13.6 eV. This assumes that the electron starts off in its ground state.