Topic  9

What happens to radioactive substances when they decay?

We have covered much of this in Unit 1 Topic 6, but we will go over the stuff again as a revision exercise.

The Atom

You will remember that 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.

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

The protons and neutrons have very nearly the same relative mass.  The neutron has slightly more mass than the proton, but at this level we are going to say that the relative mass of both the proton and the neutron is 1.  The mass of a proton or neutron in kilograms is about 1.6 × 10^-27 kg.

The mass of an electron is about 1/1800 the mass of a proton.  The mass of an electron is about 9.1 × 10^-31 kg.

The charges are as follows:

 

Particle	Charge
Proton	+ 1 e
Neutron	0
Electron	- 1 e

The symbol e is often called the electronic charge.  Its value is 1.6 × 10^-19 C.

The protons and neutrons are the nucleons.

Atoms and Ions

Elements are often written like this:

A is the total number of nucleons.  This is called the mass number or the nucleon number.

Z is the total number of protons.  This is called the atomic number or the proton number.

The number of protons determines the element.  If we change the number of protons in the nucleus from 6 to 7, we change the element from carbon to nitrogen.  This will change the chemistry radically.

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

No of neutrons = mass number - atomic number

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

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.

Question 2  How many protons, neutrons, and electrons are there in an atom of Chlorine, mass number 35, atomic number 17?  ANSWER

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 3  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.

The Effect of Alpha and Beta Decay on Nuclei

Radioactive decay occurs in unstable nuclei.  The parent nucleus ejects a particle to form a new daughter nucleus.  The new nucleus is excited and loses energy by giving off a gamma ray.

Alpha Decay

When a nucleus decays by alpha decay, it ejects a helium nucleus (NOT atom).  The nucleus recoils, just like a canon firing a canon ball.

Question 4 What does the helium nucleus consist of?  ANSWER

The atomic number goes down by 2, because 2 protons are lost from the nucleus.  The mass number goes down by 4 because 4 nucleons are lost.

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

Beta Decay

When a nucleus decays by beta decay, a neutron turns into a proton.  A high speed electron is ejected from the nucleus along with a second tiny particle called an electron antineutrino.

The atomic number goes up by 1, so a new element is formed, but the mass number stays the same.  The electron comes out of the nucleus, NOT the electron shells.

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

Measuring Radiation

In the old days, radiation was detected by exposing a sheet of photographic film to the radioactive source.  Each decay caused the deposit of a grain of silver, and it was possible measure the density of the deposits when the film was developed.  This method is still used today with film badges that people wear if they are working with radioactive materials.

To get a real-time measurement, we measure the radiation from a radioactive sample using a radiation detector called a Geiger-Müller tube.  This is connected to a ratemeter.

The radioactive decay is measured by the number of counts per second.  A computer can act as a ratemeter and store the results.  It will also plot a graph.

When we take readings it is important that we measure the background count.  There is radioactivity all around us; it's a natural part of the environment.  So we find out what the background count is, then we take that away from the count we get with the source.

Question 7  A radiation detector detects a background rate of 2 counts per second.  When a source is exposed, it gives a reading of 50 counts per second.  What is the true rate of decay?  ANSWER

Models of the Atom

When the atom was first discovered, physicists gave a lot of thought as to what the atom consisted of.  J J Thompson proposed the plum pudding model, in which electrons were placed like cherries in a matrix of positive charge.  The neutron had not yet been discovered.

This was the accepted model.  Nobody had any reason to believe otherwise until Ernest Rutherford, A New Zealand physicist proved otherwise in the early 1900s.

Rutherford bombarded a thin layer of gold atoms with alpha particles.  He was using the alpha particles like bullets, expecting to see the atoms burst like watermelons.  He described his experiment as firing artillery shells at tissue paper.

To his surprise he found that some of the alpha particles bounced back in the direction they came from.  Other particles went straight though, while other particles were deflected.

This alpha scattering showed some amazing facts about the nucleus:

• The nucleus is very small;

• Most of the atom is empty space;

• The repulsion of the positively charged alpha particle showed that the nucleus is positively charged.

The alpha particles are NOT refracted.

This discovery led to the idea of the nuclear atom.  This was developed further by Neils Bohr, a Danish physicist (and goalkeeper of the Danish Olympic football team).  It is the model of the atom shown at the start of this topic.  The neutron was discovered twenty years later by an English physicist, Chadwick.

Since the nucleus is so small, the size of an atom is governed by the size of the electron shells.  Therefore big atoms and small atoms are all roughly the same size, about 10^-10 m in diameter.

Now try the Topic Quiz

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