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
The 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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mobile phone in 'landscape' mode?
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INDEX of notes: Properties and
uses of
electromagnetic radiation
