Topic 6

What are the uses of and dangers of emissions from radioactive substances?

The Basic Atom

All matter is made up of atoms.  Although we can go smaller than atoms, we will do that later.  The basic atom consists of a nucleus surrounded by electrons going round the nucleus in orbit.  Electrons are negatively charged.  Here is a Lithium atom:

The nucleus consists of:

• Protons which are positively charged

• Neutrons that have no charge.

The protons and neutrons are the nucleons.

Elements are often written like this:

A is the total number of nucleons.

Z is the total number of protons.

To work out the number of neutrons we take away the number of protons from the number of nucleons:

No of neutrons = A - Z

If the number of electrons is the same as the number of protons, the atom carries zero overall charge.  It is described as neutral.

Question 1  How many protons, neutrons, and electrons are there in this atom?  ANSWER

The nucleus is very tiny, about 1/10 000 the size of an atom.  It is the equivalent to the size of a pea on the floor of your school dining hall.

If we change the number of electrons, the atom is charged.  It becomes an ion:

• Remove an electron, the overall charge is positive.  We have a positive ion.

• Add an electron, we have a negative ion.

Ions are NEVER made by adding or taking away protons.

Isotopes

Look at this atom:

It has six protons, six neutrons and six electrons.  It is an atom of carbon.

Now look at this atom:

Question 2  What is the same and what is different between this atom and the last atom?  ANSWER

Isotopes have the same number of protons, but different numbers of neutrons.  If we change the number of protons, we change the element completely.

Isotopes have the same chemical properties as the normal element.

Now answer Question 3

Radioactivity

Some isotopes have an unstable nucleus.  This means that at some point the nucleus will decay into a more stable nucleus.  When it does so, it will emit some kind of radiation:

• Alpha decay

• Beta decay

• Gamma radiation.

Alpha and beta decays result in the emission of a particle.  Gamma radiation is an electromagnetic wave of very short wavelength (see Topic 5).

Let us look at how each of these radiations occur:

Alpha Decay

An alpha particle is a helium nucleus.  It consists of 2 neutrons and 2 protons (i.e. 4 nucleons).

  An alpha particle is a helium nucleus, NOT a helium atom.

Question 4.  How can you tell that an alpha particle is not a helium atom?  ANSWER

When there is alpha decay the nucleon number goes down by 4 and the proton number by 2.

Question 5.  Polonium has a proton number of 84 and a nucleon number of 210.  It decays by alpha decay to lead.  What is the proton number and nucleon number of lead?  ANSWER

If we send alpha particles through the poles of a magnet (a magnetic field), we find that they are deflected.  This means that they are charged.  If we pass them between a positively charged plate and a negatively charged plate (an electric field), we find that they are attracted to the negatively charged plate.  This means they are positively charged.

Beta Decay

A beta particle is a high speed electron which is ejected from the nucleus.  A neutron turns into a proton and the electron is ejected.  It has nothing to do with the electrons surrounding the atom.

In beta decay the nucleon number stays the same, but the proton number goes up by 1.  Because the proton number changes, the element changes.

Question 6  Carbon-14 decays by beta decay.  What does it decay to?  ANSWER

If we send alpha particles through the poles of a magnet (a magnetic field), we find that they are deflected in the opposite direction to alpha particles.  This means that they are charged.  If we pass them between a positively charged plate and a negatively charged plate (an electric field), we find that they are attracted to the positively charged plate.  This means they are negatively charged.

Gamma Radiation

Gamma rays are very short wavelength and highly energetic electromagnetic radiation. They are given off by very energetic or excited nuclei when some other decay has occurred. Cobalt-60 is a common source of gamma rays.

Gamma radiation does not in itself alter the nucleon and proton numbers.

Gamma rays are not affected by electric or magnetic fields.

Half-Life

Radioactive decay is a random process.  If you look at a nucleus, it might decay within ten seconds, or twenty two million years.  Since there are many billions of nuclei, a random decay pattern is seen.

Radioactive decay is NOT influenced by any chemical or physical process, however violent.  You could dissolve the element in the most powerful acid, or vaporise it at extreme temperatures, but the decay would NOT be affected.

Each radioactive isotope decays in its own way and has its own half-life which is defined as:

the time taken for half the original number of atoms to decay.

This is shown on the graph:

  .

If it takes 4 days for half the atoms to decay:

• after 4 days, 1/2 are left over;

• after 8 days,  1/4 are left over;

• after 12 days, 1/8 are left over.

This is called exponential decay.

  A common bear trap is to say that after two half-lives, all the atoms have decayed.  This is wrong.

Question 7.  What fraction of the original number of atoms is left over after 4 half-lives?  ANSWER

Some half lives are extremely short, much less than 1 second.  Some are very long, about 4500 million years.

Properties of Radiation

The table shows some properties:

Alpha particles are stopped by a few cm of air, while beta particles have a range of several metres in air.  This means that an alpha source can be used safely with minimal shielding.  Your skin will stop alpha particles.

Alpha particles are intensely ionising.  Being quite big and moving fast, they collide frequently with other atoms, knocking off electrons, causing ionisation.  They rapidly lose their energy.  Eventually they stop and then pick up two stray electrons to become helium atoms.  All the Earth's helium atoms are thought to come from alpha decay.

Uses and Hazard of Radiation

Now answer Question 8

Now try the Crossword in Question 9.

Now try the Topic Quiz

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