All substance are made of atoms.
These have electrons (e) around the outside,
and a nucleus in the middle.
The nucleus consists of
protons (p) and neutrons (n),
and is extremely small.
(Atoms are almost entirely made of empty space!)
In some types of atom, the nucleus is unstable,
and will decay into a more stable atom. This radioactive
decay is completely spontaneous.
It's not the same as what happens
in a nuclear power station (where neutrons whizz around and
hit uranium nuclei, causing them to split).
This form of Lithium is NOT radioactive
- it's just an example of a simple atom.
Most radioactive substances have many more particles in their
nucleus.
You can heat the substance up, subject it
to high pressure or strong magnetic fields - in fact, do pretty
much whatever you like to it - and you won't affect the rate of
decay in the slightest.
That's what we've believed for some time - but recent
research makes us wonder if this is true after all. There is some
evidence of slight seasonal variations in the rates of radioactive
decay of some isotopes, which may be due to temperature differences.
Or may be nothing at all. Or may be due to something else we don't
know about yet. For now, for GCSE exams, let's go along with the
idea that the rate of radioactive decay does not depend on external
factors.
When
an unstable nucleus decays, there are three ways
that it can do so.
It may give out:-
an alpha particle (we use
the symbol )
a beta particle (symbol
)
a gamma ray (symbol )
Many radioactive substances emit
particles and
particles as well as
rays.
In fact, you won't find a pure source:
anything that gives off rays
will also give off
and/or
too.
That's the basic information. Click here for more details:
Alpha
particles
Alpha particles are made of 2 protons
and 2 neutrons.
This means that they have a charge of
+2, and a mass of 4 (the mass is measured in "atomic mass units",
where each proton & neutron=1)
We can write them as ,
or, because they're the same as a helium nucleus, .
Alpha particles are relatively slow and
heavy.
They have a low penetrating power - you
can stop them with just a sheet of paper.
Because they have a large charge, alpha particles
ionise other atoms strongly.
More about
particles:
Beta
particles
Beta particles have a charge of minus
1, and a mass of about 1/2000th of a proton. This
means that beta particles are the same as an electron.
We can write them as
or, because they're the same as an electron, .
They are fast, and light.
Beta particles have a medium penetrating
power - they are stopped by a sheet of aluminium
or plastics such as perspex.
Beta particles ionise
atoms that they pass, but not as strongly as alpha particles
do.
More about
particles
Gamma
rays
Gamma rays are waves, not particles.
This means that they have no mass and no charge.
So we sometimes write .
Gamma rays have a high penetrating power
- it takes a thick sheet of metal such as lead, or
concrete to reduce them significantly.
Gamma rays do not directly ionise other atoms, although they may cause atoms to emit
other particles which will then cause ionisation.
We don't find pure gamma sources - gamma rays
are emitted alongside alpha or beta particles. Strictly speaking,
gamma emission isn't 'radioactive decay' because it doesn't
change the state of the nucleus, it just carries away some
energy.
More about rays:
Summary
Alpha particles are easy to stop, gamma
rays are hard to stop.
Particles that ionise other atoms strongly have
a low penetrating power, because they lose energy each
time they ionise an atom.
Radioactive decay is not affected by external conditions.
You need to know the information in
this table:-
Type
of Radiation
Alpha particle
Beta particle
Gamma ray
Symbol
or or
or
(can look different,
depends on the font)
Mass(atomic mass units)
4
1/2000
0
Charge
+2
-1
0
Speed
slow
fast
very fast (speed
of light)
Ionising
ability
high
medium
0
Penetrating
power
low
medium
high
Stopped
by:
paper
aluminium
lead
We
talk about "radioactive isotopes" - but what's
an isotope?
For a start, just because something is called an isotope
doesn't necessarily mean it's radioactive.
You can think of different isotopes of an atom being
different "versions" of that atom.
Consider a carbon atom.
It has 6 protons and 6 neutrons - we call it "carbon-12"
because it has an atomic mass of 12 (6 plus 6).
If we add a neutron, it's still a carbon atom, but it's
a different isotope of carbon.
One useful isotope of carbon is "carbon-14",
which has 6 protons and 8 neutrons. This is the atom
we look for when we're carbon
dating an object.
So isotopes of an atom have the
same number of protons, but a different number of neutrons.
Video clip: Radioactive
isotopes Showing the types of radioactivity and
their penetrating power
)
)
)
,
or, because they're the same as a helium nucleus, 
.
or, because they're the same as an electron, 
.
.
