Doc Brown's chemistry notes on atomic and electron structure

9. The effect of ionizing radiation on atoms - positive ion formation via electromagnetic radiation

9. The effect of ionizing radiation on atoms

The higher energy uv, and both X-ray and gamma radiations have enough energy to cause complete ionisation of an atom.

Ionisation has already been explained on this page, but a reminder won't go amiss!

An atom is ionised if it completely loses one or more electrons to form a positive ion.

The process is illustrated below for the formation of a monopositive ion of sodium from a sodium atom.

In terms of electrons, the change is ...

This represents the ionisation of a sodium atom to form a positive sodium ion and a free electron:

Na ==> Na⁺  +  eŻ  (electron configuration change of sodium atom 2.8.1 ==> sodium ion 2.8)

In this case the incoming EM radiation must have sufficient energy to promote the electron all the way up the energy levels until it is completely free of the attraction of the positive nucleus - so the atom has been ionised.

A positive ion is formed because there are now less negative electrons on the atom than positive protons - so there is a surplus of positive charge on the atom.

The more electrons 'knocked off' the bigger the positive charge on the ion.

i.e. the charge on the ion can be +, 2+, 3+ etc. by knocking off 1, 2 or 3 electrons etc. The more electrons knocked off, the bigger the positive charge on the ion.

(Note: Using X-rays, you can knock off all 92 electrons from a uranium atom, element 92, ₉₂U, to form the U⁹²⁺ ion, but this far too extreme for GCSE students!, but very exciting to contemplate!)

More on ionisation and ionising radiation in the following GCSE level physics notes

Alpha, beta & gamma radiation - properties of 3 types of radioactive nuclear emission & symbols ,dangers of radioactive emissions - health and safety issues and ionising radiation

and Electromagnetic radiation, sources, types, properties, uses (including medical) & dangers 

Quizzes - practice exam questions

• Some basic practice exam questions on atomic structure (at end of index page)

• GCSE level Foundation (easier) Atomic Structure multiple choice QUIZ

• GCSE level Higher (harder) Atomic Structure multiple choice QUIZ

• gap-fill worksheet on Atomic Structure

• matching pair quiz on Atomic and electronic structure 1. fundamental particles

• matching pair quiz on Atomic and electronic structure 2. periodic table