Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 5: NUCLEAR PHYSICS
Figure 1 shows pieces of carbon, one of the best known elements on Earth. But what makes carbon have different properties than, say, iron?
The answer is that all substances are made of atoms, and the atomic structure of carbon is different from that of iron. In your chemistry classes you may well have learned some of this section already, as the topic spans both sciences. However, in this topic we are more interested in the structure of the nucleus than in the arrangement of electrons, which we will largely ignore.
Figure 1: Carbon pieces
All atoms are made of three sub-atomic particles:
Figure 2 shows a very simplified diagram of a carbon atom, and how these particles are arranged:
Figure 2: The structure of a carbon atom
Rather than drawing a carbon atom, we need a simple way of describing the nucleus. The way this is done is with the following notation:
You will need to learn the terms atomic (or proton) number, nucleon (or mass) number, and what they mean:
Questions:
1. The element sodium (Na) is written in notation as:
| 23 | Na |
| 11 |
State the number of protons and neutrons in this atom of sodium.
The bottom number represents the proton number, so there are 11 protons.
The top number of 23 gives the total number of protons and neutrons, so there are 23 - 11 = 12 neutrons.
2. Lead is a metal with the symbol Pb. A common atom of lead has 82 protons and 125 neutrons. Using standard notation (as shown in question 1), give the notation for this atom of lead.
| 207 | Pb |
| 82 |
Extension:
In some books and web pages showing atomic notation, the bottom number is missing completely. Why is this?
Carbon always has 6 protons (atomic number 6) and is element number 6 in the periodic table. For this reason, once you write the letter 'C' for carbon, you could miss out writing the '6' as it can be assumed. This is true for all atoms.
Nearly all of the carbon on Earth has atoms with 6 neutrons and 6 protons. this is often called "carbon-12", showing the mass. However, there is a small percentage of atoms with an extra neutron. This atom is still carbon, with atomic number 6, but now has a mass number of 13. Carbon-12 and carbon-13 are isotopes of carbon.
Isotopes have the same number of protons, but different numbers of neutrons.
The isotope Carbon-14 has 6 protons and 8 neutrons, and is a very rare atom. However, the extra neutrons in the nucleus makes it unstable. Why this happens is very complicated and is to do with the forces acting in the nucleus. Eventually, this atom will randomly split apart. We say it is radioactive because it will emit particles when it decays.
We will learn more about how atoms decay in the next section.
Questions:
3. The list below shows the notation for some atoms found in a sample of sea water. It includes 2 isotopes:
| 1 | H | 4 | He | 5 | Li | 2 | H | 9 | B | 9 | Be | |||||
| 1 | 2 | 3 | 1 | 5 | 4 |
a) isotopes have the same number of protons but different numbers of neutrons.
b) We are looking for 2 atoms with the same proton number (the bottom number) and hence the same symbol.
| The two isotopes are: | 1 | H | and | 2 | H |
| 1 | 1 |
You may have learned in chemistry that atoms are more stable when the outer shell of electrons is full. For this to happen, many atoms need to gain or lose electrons. Fluorine (element number 9) has 2 electrons in the first shell, and 7 in the second shell. It needs one more electron for the outer shell to become full. If it gains one electron, the atom is no longer neutral and is called an ion. It has an overall charge of -1.
Atoms can form form negative ions by gaining electrons, or form positive ions by losing electrons. This principle should be covered in more detail in your chemistry course.
The very first scientists to describe atoms were the ancient Greeks, including Democritus (about 400 BC). The word atom means 'uncuttable' in their language, and the Greeks came up with the idea that the qualities of an object depend on the type of atoms that are in it. This was only an idea - a hypothesis, with no evidence for a long time. It wasn't until a chemist called Dalton (working in the early 1800s) that the first evidence for atoms was put forward. His experiments along with other chemists at the time provided evidence for the relative sizes of different atoms, with hydrogen atoms being the lightest. His atomic model proposed that atoms of different elements could combine in simple ratios to form chemical compounds.
In 1897, a scientist called J.J. Thompson was experimenting with 'cathode rays' - rays created by charged metal plates in a vacuum tube. He found they had a mass MUCH smaller than atoms - about 1800 times smaller than the lightest atom , hydrogen. He also found they have a negative charge, and they became known as electrons. It was found that these electrons could also come from radioactive atoms: The first sub-atomic particles had been discovered.
Thompson tried to explain how atoms could be neutral with no overall charge, but contain negative electrons. His model became known as the 'plum pudding' model, with negatively charged electrons being distributed evenly throughout a positive 'pudding' that forms the rest of the atom.
Figure 3: 'The plum pudding' model of the atom
Note: You do not need to describe cathode rays and the plum pudding model of the atom for this syllabus, but it helps describe why the following alpha particle experiment was so ground breaking at the time, creating the model of the atom that we all use today.
In 1909, scientists at Manchester University in the UK were performing some experiments with alpha particles. These were known to be heavy, positively charged particles. (See the next section 5.2a for more about alpha particles). The experiment involved directing a beam of alpha particles from a radioactive source at an extremely thin sheet of metal (gold) foil. The aim was to see how the gold atoms affected the very dense alpha particles.
With the plum pudding model above, the team (Geiger, Marsden and the group leader, Rutherford) expected the dense alpha particles to go pretty much straight through the target, with only a small deflection observed.
However, the results were completely unexpected, as shown in figure 4 below.
Figure 4: Geiger and Marsden's alpha particle scattering experiment
As you can see from their observations above, it appeared that the alpha particle 'bullets' were bouncing back off the 'puddings'! Rutherford as leader of the team did a detailed analysis of the results and came to the conclusion that the alpha particles were being deflected, and in some cases repelled completely, by a dense, positive core inside the gold atoms. He rejected the plum pudding model, and put forward his own. His theory can be summarised as follows:
Rutherford went on to describe electrons orbiting this heavy central nucleus. His experimental evidence led to a complete change in the model for the atom, and this was a major breakthrough at the time.
Nuclear disintegrations release a huge amount of energy. Scientists have calculated that it must be this energy from radioactive substances (like uranium) deep in the Earth that keep the core hot, thus producing the activity we see in volcanoes. After many years of research in the 1940's, physicists finally worked out how to produce this energy on demand.
Nuclear fission is the process of a large nucleus splitting into smaller parts and releasing energy. This can happen naturally, or artificially. Natural, spontaneous fission is rare, and happens at a very slow rate. In a modern nuclear reactor, the element Uranium is usually used, as it has an extremely large and relatively unstable nucleus.
There is one rare isotope of uranium called uranium-235 that has a very special property: It is unstable, but a neutron that hits it at the right speed will cause it to disintegrate, typically into 2 fragments. The fragments are called daughter nuclei, and are also usually radioactive and unstable. Gamma rays are also released in this process.
However, when the uranium-235 disintegrates, it releases 2 or 3 neutrons, which in turn can hit other uranium-235 nuclei, as shown in figure 5.
As you can see from this diagram, the number of nuclei involved increases rapidly, and the energy released gets bigger and bigger at a huge rate.
This process of self-sustaining nuclear disintegrations is called a chain reaction:
Note: Chain reactions are not in this course, but you should be able to describe the fission process and the release of energy.
Figure 5: A nuclear fission reaction
Wikimedia Commons
Notice that in stage 2, a neutron hits a common U-238 nuclei, and nothing happens. It is only the rare uranium-235 isotope that produces a chain reaction.
The process of radioactive decay can be written as a nuclide equation, showing the starting nuclei and products of the fission reaction using the notation described at the start of this section. Here is one example:
| 235 | U + | 1 | n → | 141 | Ba + | 92 | Kr + 3 | 1 | n |
| 92 | 0 | 56 | 36 | 0 |
Note that a neutron has a nucleon number of 1, and a proton number of zero.
One way to simulate the stored energy being released in a chain reaction is with spring-loaded mouse traps. The video here is a great demonstration. The ping-pong balls are like the neutrons, and the mouse traps are like the uranium-235 nuclei:
YouTube video: Simulation of a chain reaction using mouse traps
Thanks to Pepsi (not endorsed) for this expensive science experiment / advert!
How is the energy in the mouse traps released?
We can see that the ping-pong balls move at high velocity, and the mouse traps also move. This is kinetic energy. During nuclear fission, the daughter nuclei and the neutrons move at high speed carrying a huge quantity of kinetic energy. This energy was originally stored in the nucleus of the uranium-235 as mass. During the fission reaction, a small quantity of mass is lost in the process and converted to energy in the form of radiation.
You will not be asked to calculate the mass lost or energy released, only describe the process of mass converting to energy during fission, as well as fusion (below).
When an unstable heavy nuclei like uranium decays, it slowly releases energy. In a chain reaction, it is forced to disintegrate by a collision. Both these processes involve the release of energy from the splitting of a heavy nuclei, and we call this fission. However, fusion is quite different.
Heavy nuclei are held together by a strong force like a nuclear glue. This force only acts over a short range. If you can get two light nuclei close together, then this force makes them snap together like strong magnets, and this releases energy. Just like fission, the energy comes from a small quantity of mass that is lost in the process, and converted to energy in the form of radiation.
This process - of light nuclei fusing together - is called nuclear fusion. It only works with light nuclei like hydrogen and helium, and only when they are put very close together. Unfortunately, these nuclei have protons in them that repel each other, so they really do not want to be close together! The electrostatic repulsion works over longer distances, so initially the protons repel unless they are forced very close together.
Fusion needs very high pressures and temperatures. This makes light nuclei move very fast and close together, causing some to fuse. This is the energy source that keeps the Sun and all stars producing heat and light! The temperature needed to do this for hydrogen in stars is millions of degrees Celsius, and even then the process is slow.
One day, physicists hope to be able to generate electricity using fusion - we could even use the hydrogen from water to do this which would be pollution free! Unfortunately, fusion will not work at low temperatures and pressures. The experimental reactors being built require dangerously high temperatures and the fuel inside will melt any container if it touches the sides. This is still a work in progress.....!
Figure 7: An experimental fusion reactor.
Oak Ridge Labs (CC by 2.0 licence)
Here is an example of nuclear fusion reaction written as a nuclide equation. In this example, two isotopes of hydrogen fuse to make a helium nucleus and a neutron:
| 2 | H + | 3 | H → | 4 | He + | 1 | n |
| 1 | 1 | 2 | 0 |
During both fission and fusion, a small quantity of mass is converted to energy. You may be interested that this conversion was predicted by Einstein using his famous formula E= mc2. However, how can both splitting nuclei (fission) and joining nuclei (fusion) both result in the release of energy?
The answer is that medium-sized nuclei - like iron - are the most stable and tightly bound of all nuclei. The strong nuclear force squeezes an iron nuclei inwards and this converts a small mass into energy. The closer a nuclei becomes to the size of an iron nuclei, the more mass can be converted to energy as the nucleus squeezes inwards, rather like releasing water from a sponge as it is squeezed. This is illustrated in figure 8 below:
Figure 8: Fission and fusion both result in a release of energy
Questions:
4. Nuclear fusion is a source of energy using atomic nuclei.
a) Fission is the splitting of heavy nuclei.
Fusion is the joining together of light nuclei.
b) Fusion occurs naturally in all stars (including the Sun!)
5. In one example of a fission reaction, a uranium-236 nucleus splits into xenon (Xe) and strontium (Sr) nuclei, as well as two additional neutrons. The nuclide reaction is shown here, with two numbers Y and Z missing:
| 236 | U → | 140 | Xe + | Z | Sr + 2 | 1 | n |
| 92 | Y | 38 | 0 |
Calculate the values of Y and Z.
The lower numbers are the atomic numbers, and should be equal on each side of the formula.
Therefore:
92 = Y + 38 + (2x0)
and so Y = 92 - 38
Y = 54
The upper numbers are the nucleon numbers and should also be equal on each side of the formula:
236 = 140 + Z + (2x1)
and so Z = 236 - 140 - 2
Z = 94
Now test your understanding on atomic structure using this 10 question quiz:
