Radioactivity

In the sections of chemistry concerned with bonding, you will see that chemists are interested in the electrons of an atom, Physicists, however, are more concerned with the contents of the nucleus.
Remember, before we proceed, that the number of neutrons in an atom can vary and we call the different atoms isotopes. It is the number of protons, or atomic number, that makes an atom a particular element. For example, no matter how many neutrons there are in an nucleus that contains 26 protons, it will always be an iron atom.

Sometimes, nuclei are unstable. They need to get rid of something to make them more stable. They can do this in the process called radioactive decay. They can emit an alpha particle, a beta particle, gamma rays or a mixture of all three until a stable nucleus remains. The new nucleus formed at each stage is called a daughter atom.



Alpha decay involved the release of 2 protons and 2 neutrons as a helium nucleus. It is the most ionising but as it is the largest, it is easily stopped by paper or just a few centimeters of air. Uses include smoke detectors. Here is an example of a balanced decay equation:
Notice how in the daughter atom, Th, the atomic mass has decreased by 4 and the atomic number has gone up by 2.

In Beta decay, a neutron splits into a proton and an electron. The proton stays in the nucleus and the electron is fired out of the nucleus at high speed. Beta particles can get through your skin, unlike alpha particles but are less ionising. They are used to check the thickness of aluminium foil when it is being made. They are smaller than alpha so can travel about a meter in air but are stopped by lead. Here is another example equation:


Note how the atomic mass stays the same but the atomic number goes up by 1
Gamma radiation is the simplest. Basically, the nucleus has too much energy and needs to let some out. It escapes in the form of a very high frequency, short wavelength electromagnetic wave called gamma radiation. It could potentially travel forever through space and takes up to 10m of concrete to absorb it making it weakly ionising but very penetrating. It is used to treat cancers and to sterilise hospital equipment.

These early experiments led to the discovery of our model of the atom. Originally, atoms were thought to be solid balls of positive mass with negative masses spread throughout them. There was thought to be very little space within. Rutherford, Geiger and Marsden set up an experiment to to fire alpha particles at a very thin sheet of gold foil. To their amazement, very few were detected to have bounced back as was predicted. They mainly passed straight through which told them that the atoms were made mainly of empty space.

There needs to be a way to measure and predict when a nucleus will decay. This is depicted mathematically as half-life. Half-Life is the time it takes for the radioactivity of a sample to halve. It can also be expressed as the time taken for half of the unstable nuclei in a sample to decay.

By reading off the graph, you simply see how long it takes to either halve the activity or how long it takes for half of the sample to decay.

This is the method used in radioactive dating. Scientists use known values for the half lives and estimates for the abundance of an isotope then calculate the number of half lives that have occurred since.

I must have had my first joke book when I was too young because it was years before I got the joke, "What do scientists get from the restaurant?" "Fission chips". Having had to wait years to understand the punch line, it lost most of its humour. Fission is the process of splitting atoms into smaller daughter atoms. Most nuclear power plants use Uranium-235 but some use Plutonium-239. As Uranium is mainly U-238,the fuel needs to have a higher concentration of U-235, around 3%. This is called enriched uranium. Splitting one atom releases a relatively small amount of energy so a chain reaction is required. An unstable nucleus absorbs a slow moving neutron and then splits into two. On doing this, it fires out more high speed neutrons and if at least one of these successfully splits another nucleus per spilt nucleus, it is a chain reaction. These neutrons need to be slowed down so water, heavy water or graphite is present to slow them down.

Nuclear fusion is quite the opposite but is one of the most sought after processes here on Earth to solve our energy crisis. This involves small nuclei being forced together under great pressure to make larger nuclei. This releases lots of energy as well but this is the process that keeps our world alive as it is what is occurring in the Sun. As well as great pressure, the nuclei need vast amounts of heat to overcome the repulsive forces between two positive nuclei.

Nuclear energy has many positives (no greenhouse emissions while it is running) but also negatives. After decommissioning nuclear fuel, the Uranium and Plutonium are removed by chemists but the radioactive waste has to be stored for a very very long time. For example, Chernobyl will be an unsafe place to live for at least 20,000 years. We are exposed to radiation all of the time, either cosmic radiation or from the radioactive decay in the rocks beneath our feet. It is only exposure time that makes them harmful. Radon gas is a particular concern, as it is the product of decay in areas like Cornwall, England in the underlying granite bedrocks. This gas is still radioactive and releases highly ionising alpha particles. It is also very dense so has been known to fill cellars.

Thanks for reading this far and good luck in your exams.

Key words and terms for this topic: nucleus, proton, neutron, electron, alpha radiation, beta radiation, gamma radiation, ion, isotope, atomic number, mass number, ionisation, half-life, tracer, radioactive dating.nuclear fission, chain reaction, nuclear fusion, gravitational attraction, star, protostar, main sequence star, red giant, white dwarf, black dwarf, supergiant, neutron star, black hole.
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