Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 5: NUCLEAR PHYSICS
In this image, an alpha particle is emitted from a heavy nucleus. But what happens to the nucleus after this decay?
It is now lighter, by 2 protons and 2 neutrons. This means it no longer has the same atomic number or mass - it is a different element than before the decay.
The decay typically results in a loss of energy and/or mass, and this makes the remaining nucleus more stable.
Figure 1: Alpha decay
Public Domain-wikimedia
In this section we will look at each decay in turn, and work out what element remains.
Isotopes of an element may be radioactive due to an excess of neutrons in the nucleus and/or the nucleus being too heavy. The most common path for for nuclei to become more stable by losing mass and excess neutrons is through alpha decay.
The mass of an alpha particle is 4, and it has 2 protons (a charge of +2). Therefore, using standard notation as covered in section 5.1, we can show an alpha particle as:
| 4 | α |
| 2 |
Let's assume that the nucleus in figure 1 is the isotope uranium-238. Uranium has an atomic number of 92, so this is written as
| 238 | U |
| 92 |
So the nucleus remaining must have lost 2 protons, making it element number 90. It has also lost a mass of 4, so the mass number must be 234. Using a periodic table, the element with atomic number 90 is thorium (Th).
| 238 | U | → | 234 | Th | + | 4 | α |
| 92 | 90 | 2 |
After alpha decay, all nuclei will have lost a mass of 4 and a proton number of 2.
isotopes of an element may be radioactive due to an excess of neutrons in the nucleus and/or the nucleus being too heavy
Note: You will not be expected to use a periodic table to work out the element remaining. Often the element name is provided for you, or labelled element 'X'. or something similar.
Beta particles are very fast-moving electrons, emitted by the nucleus. As a beta particle has a charge of -1, it is opposite in charge to a proton, and therefore the proton number is given as -1. It has effectively no mass. the notation for a beta particle is therefore:
| 0 | β |
| -1 |
How can an electron come from the nucleus??
The answer is that under certain conditions, a neutron can do something very strange - it converts to a proton and electron. if we use standard notation for neutrons and protons, and then write this decay as an equation, we get:
| 1 | n | → | 1 | p | + | 0 | β |
| 0 | 1 | -1 |
You can see from the equation that this all adds up: The mass and proton numbers are equal on both sides of the equation.
If an isotope like carbon-14 (element number 6) undergoes beta decay, the equation is similar to the one above. The mass number stays the same, but the proton number has to increase by 1 to show that a neutron has turned into a proton and to balance the loss of negative charge carried away in the beta particle. The element with a proton number 1 higher than carbon is nitrogen (N). This gives the following final equation:
| 14 | C | → | 0 | β | + | 14 | N |
| 6 | -1 | 7 |
Beta decays are the hardest to work out and it can be confusing working with negative numbers, so check your answer carefully!
| 0 | γ |
| 0 |
This makes this decay very easy to show in an equation, for example, with radium-226:
| 226 | Ra | → | 0 | γ | + | 226 | Ra |
| 88 | 0 | 88 |
Note that there is no change to the radium.
Figure 2: Radioactive radium
By Mauswiesel CC BY-SA 3.0
Beryllium 13 is a radioactive isotope that decays by losing a neutron. Can you work out the decay equation for this?
A neutron has a mass of 1 and no charge, it is therefore written as:
| 1 | n |
| 0 |
Beryllium is atomic number 4, and the mass number is 13 from the question. The decay equation then becomes:
| 13 | Be | → | 1 | n | + | 12 | Be |
| 4 | 0 | 4 |
Notice that the atomic number is still the same, therefore it is still the same element. It has just lost a mass of 1 unit.
Questions:
1. An alpha particle is given the letter α .
| 239 | Pu | → | __ | U | + | __ | α |
| 94 | __ | __ |
a) Alpha particles have a mass of 4 and an atomic number of 2.
b) The missing numbers are shown here in red:
| 239 | Pu | → | 235 | U | + | 4 | α |
| 94 | 92 | 2 |
2. During beta decay, the mass of the nucleus remains constant, whilst the atomic number increases by 1.
| 90 | Sr | → | __ | β | + | __ | Y |
| 38 | __ | __ |
a) During beta decay, a neutron changes into a proton and an electron, causing an increase in the nuclear proton number of +1 but no change in mass. (The electron is ejected from the nucleus).
b) The missing numbers - shown in red - are:
| 90 | Sr | → | 0 | β | + | 90 | Y |
| 38 | -1 | 39 |
Extension.
This question is not on the syllabus, but you may well be able to do it easily:
3. Helium 5 is an extremely unstable isotope of helium. It decays by emitting a particle, labelled X as shown in the decay equation below:
| 5 | He | → | 1 | X | + | __ | Z |
| 2 | 0 | __ |
The remaining nucleus after the decay is a nucleus of element Z.
a) The particle X has a mass of 1 and no charge (no + or - proton number). It is therefore a neutron.
b) The missing numbers are:
| 5 | He | → | 1 | X | + | 4 | Z |
| 2 | 0 | 2 |
c) As the element Z has the same proton number of 2, it is still the element helium - just a different isotope! Remember you are not expected to remember the periodic table. The question says 'state..' so the answer must be obvious or a simple answer.
Try these quick, 10 minute questions on section 5.2 (nuclear decay and decay equations):
