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SITEMAP   School Physics Notes: Electromagnetic spectrum 10. Ionising effects of EMR

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Electromagnetic spectrum: 10. The excitation of atoms by electromagnetic radiation causing ionisation, more on the dangers of ionising radiation

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INDEX of physics notes: Properties and uses of electromagnetic radiation

10. Ionisation: excitation of atoms and more on the dangers of ionising radiation

- ionising radiation and dangers, electronic changes in atoms, emission of visible light photons

KEY: EM shorthand for electromagnetic (radiation);  shell = electronic energy level;  outer means furthest from the nucleus;  excited means an atom in a more energised unstable electronic state. A photon is a little packet of EM wave radiation (see quantum theory for beginners on EM wave interactions!).

(i) Reminders on the electronic structure of atoms and how electrons can absorb energy

(c) doc bThe electrons of atoms and molecules occupy a series of specific electronic energy levels (shells) at increasing distance from the nucleus. The electrons fill the lowest available energy levels nearest the nucleus - the most stable arrangement. The electron arrangement of potassium is shown on the right where the electrons fill three inner levels and two in the outer shell (see atomic structure). It is the outer shell electrons that are the most easily lost in chemical reactions (see Alkali Metals) or here due to ionising radiation (below).

The EM uv, X-ray and gamma radiations have enough energy to promote outer shell electrons to a higher level forming an electronically 'excited' atom or molecule. This can only happen if the EM radiation has the specific amount of energy - if it isn't the right energy, the electron can't absorb the energy and move to a higher level (section (b) below). Sometimes the energy of the EM radiation is sufficient to move an electron all the way up the energy levels and completely remove it from the atom or molecule creating a positive atom or molecule (section (c) below).

(ii) Excitation of atoms by absorbing uv, X-rays or gamma EM radiation which then give out (emit) EM radiation

When the EM radiation has less energy than that required for ionisation, you still 'excite' an atom into a more energised state by promoting an electron from the outer shell to another, but higher level shell (which may or may not be empty). The excited atom is unstable and will 'relax' back to its normal stable state by emitting EM radiation photons.

The diagram above illustrates the process:

1. An electron in an outer shell absorbs incoming EM radiation energy and is promoted up to the next higher empty shell (in this case, but can be a partially filled shell). This called electron 'excitation'.

2. The atom is now in an 'excited' state and unstable because the electron has gained excess energy and if possible the atom would like to return its original stable state.

3. So, the promoted electron now loses energy and drops back down to the 'stable' original level stabilising the atom.

The excess energy is lost as EM radiation and the photon emitted has the same energy as that absorbed by the electron in the first place.

See section on flame emission spectroscopy and flame colours notes and make the connection!

and Atomic structure, history, definitions, examples and explanations including isotopes

In general, when the electrons in electronically excited atoms or molecules fall from a higher shell to a lower shell EM radiation is emitted in the form of visible light, ultraviolet light or X-rays depending on the energy difference of the levels.

The higher the level (shell) the electron is promoted too, the greater the energy and frequency of the EM radiation emitted when the electron falls down to lowest possible level.

However, these processes are complicated with many different electron transitions possible.

The further you are from the nucleus, the closer the energy levels become e.g. an electron falling from the 5th level to the 4th level releases a more energetic higher frequency photon, than an electron falling from the 6th level to the 5th etc.

If you can raise an electron to the highest possible level (which amounts to 'infinity' since there an infinite number of levels theoretically), it has sufficient energy to overcome the attraction of the nucleus. In other words the atom loses an electron and forms a positive ion - because with one negative electron charge less, there is now a surplus of positive proton charge in nucleus. This process is described further with diagrams in the next section (c).

NOTE danger! The excited atoms that can be dangerous and promote chemical reactions you might not wish to happen (e.g. in living cells) and also release their excess energy as heat.

(iii) Ionisation by EM radiation of atoms to form a positive ion

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

An atom is ionised if it completely loses one or more electrons.

This is a bit more energetic than 'excitation' described above.

The energy carried by ultraviolet light radiation, X-rays and gamma radiation is sufficient to cause ionisation of atoms by knocking off negative outer shell electrons to form a positive ion - the atom has been ionised - see diagrams and explanation below.

  ==  high energy uv/X-ray/gamma ray photon  ==>  +    +    electron-

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 from 2.8.1 ==> 2.8, as in chemistry notes!)

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 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!)

Dangers of ionising radiation

From (b) we see that excited atoms that can be dangerous and promote chemical reactions you might not wish to happen.

BUT, ions can be even more destructive on cells and break chemical bonds and cause even more genetic damage.

This is the essence of the dangers of ionising radiation - burns and cell DNA damage (mutations) leading to cell death or rogue multiplication of mutated cancer cells.

(iv) Although NOT needed for GCSE science level - a note on 'Quantum Theory' hmm!!! 

You can skip to (v) dangers of ionising radiation!

A photon is a tiny packet of EM radiation energy, it has both particle and wave properties. You can think of it as a little bullet of energy in wave-like form (~).

The diagram above in (b) illustrates an example of the scientific theory we call quantum physics. It describes the interaction and exchange of energy between the electrons of atoms and molecules with photons of EM radiation. The diagram actually shows one atom of sodium interacting with one photon of EM radiation. Obviously overall you are dealing with trillions of atoms and photons of EM radiation, BUT, it occurs at an individual atom level - at the quantum level - so, what's a quantum?

A photon is sometimes called a 'quantum' of energy - this term and 'quanta' (plural of quantum) are derived from the early theoretical ideas of quantum physics which suggested, (correctly), that energy changes involve the exchange of tiny packets of energy called 'quanta'.

e.g. as you are reading this web page, trillions of visible light photons (quanta of EM radiation energy of wavelength 400-700 nm) from the screen are hitting the retina at the back of your eye to excite the molecules in the receptor cells! Here the 'excitation' effect doesn't lead to emitted EM radiation, but does create a nerve signal to the brain - a tiny quantum energy transfer per photon.

We are all quantised at the atomic and molecular level - scary!!!

So, reading section (b) for your GCSE exam, is probably your first encounter with (perhaps the last!), of what some scientists regard as the most successful theory of all science of all time - quantum theory!

(v) More on the dangers of ionising radiations

High energy ultraviolet light, and even higher energy X-rays and gamma radiation are all types of ionising radiation - these EM waves carry enough energy to remove electrons from atoms and molecules.

These ionised atoms and molecules are very reactive and can cause all sorts of reactions to happen in cells that would not have otherwise occurred.

These reactions may be harmful to the life of a cell - it can be damaged or killed.

These reactions can cause mutations in the cell DNA that can lead to cancer.

If the ionising radiation kills cells, but not too many, you can survive without any long-term effect.

A very high dose of e.g. gamma radiation, can kill so many cells and damage others that your immune system is overwhelmed and you suffer from radiation sickness and your life is in peril.

BUT, if the radiation just damages a cell and causes a DNA mutation, this can be carried forward by uncontrolled cell division and if the damaged cells are cancerous, then a tumour can grow in your body with potentially fatal consequences

Exposure to high levels of ionising radiation can be quite dangerous to us and other animals and pants.

For more specific details on dangers see ultraviolet light  *  X-rays  *  gamma radiation

INDEX of notes: Properties and uses of electromagnetic radiation

Keywords, phrases and learning objectives for electromagnetic radiation spectrum

Be able to describe the excitation of atoms by electromagnetic radiation causing ionisation and link to this to the dangers of ionising radiation.


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INDEX of notes: Properties and uses of electromagnetic radiation