Atoms and Radioactivity

Atoms and radioactive relationships

The relationship between atoms and radioactivity is outlined in this section. Essentially radioactivity is a property of certain atoms whose nuclei are not stable. In order to understand the relationships between radioactive atoms and radioactivity, we need to know how to measure the levels of radiation being emitted by radioactive materials. Once we have a suitable measuring technique, establishing relationships and laws between radioactive elements and the radiation they emit becomes possible.

Measuring radioactivity

We can measure the activity of radioactive sources using a Geiger-Muller tube attached to a counter. Radioactivity is measured using count-rate, meaning the number of decays detected per second. The standard unit of activity is the becquerel (Bq). So, a source that has 10 decays per second would have a count rate of 10 Bq. When a radioactive isotope decays, it emits a radioactive product, commonly an alpha particle, beta particle, or gamma wave. Every time one of these products of radioactive decay enters the Geiger-muller tube, the counter clicks and the count rate is displayed to the operator.

Radioactive isotopes

Different radioactive nuclei will decay at different rates, even between different isotopes of the same element. One isotope might even be stable, while another isotope of the same element is radioactive. More massive elements tend to be more radioactive because their larger nuclei are more likely to have an unstable excess of internal energy.

Atoms and Radioactivity: Half-life

An important concept when understanding radioactive decay is a radioactive isotope’s half-life. The half-life of a radioactive isotope is defined as the time taken for half the isotopes within a sample to decay. Alternatively, the half-life can be described as the time taken for the count-rate of the sample to be reduced to half its original level. You might be wondering why the half-life is an important thing to learn. The answer is that radioactive decay is random.

Imagine you are studying a single unstable Bismuth-210 nucleus. It would be impossible to determine when it will decay due to the random nature of radioactive decay. However, if you had a 1 kg block of radioactive material containing approximately 10²⁵ Bismuth-210 atoms instead, then you could be nearly certain that some of the radioactive isotopes would decay. If it takes five days for the count rate of the sample to be halved, then you know that the half-life of Bismuth-210 is five days.

Decay of a radioactive substance, commons.wikimedia

The structure of an atom and radioactivity

It’s actually due to radioactivity that we even understand the underlying structure of the atom at all. After the discovery of the electron in 1897 by J. J. Thomson, the most popular theory of how an atom was structured was the plum pudding model or the Thomson Model. Thomson proposed that negatively charged ‘plums’ (electrons) were surrounded by a positively charged ‘pudding’.

Scattering of alpha particles if Plum Pudding model was correct compared to the real results, commons.wikimedia

In 1905, Ernst Rutherford tested the plum pudding model by directing a beam of alpha particles at a strip of gold foil. Alpha particles are a form of radiation with a large positive charge. He expected the alpha particles to pass through the gold with no deflection as the positively charged ‘pudding’ should be evenly spread out. However, a very small number of the alpha particles were deflected, sometimes being reflected completely.

He proposed that the atom actually consisted of a small, compact, and positively charged nucleus surrounded by a cloud of electrons, called the Rutherford model. The vast majority of the alpha particles passed through the atom without any deflection, proving how small the nucleus was compared to the atom as a whole.

The effects of radioactivity on an atom

A radioactive atom will be changed after undergoing radioactive decay, which can happen in several different ways. Radioactive decay can occur due to an unstable nucleus emitting radiation. The most common forms of decay are alpha particles, beta particles, gamma-rays, or neutron emissions. Each type of radiation has different properties and characteristics.

Atoms and Radioactivity: Alpha radiation

When the nucleus of an atom has too few neutrons compared to protons, it will emit an alpha particle ‘α’, which is made from two protons and two neutrons. This helps to restore the balance within the nucleus and reduce the ratio of protons to neutrons.

An Americium-241 nucleus decays into Neptunium-237 and emits an alpha particle, commons.wikimedia

An alpha particle${}_2{}^4\alpha$is exactly the same as a helium nucleus${}_2{}^{4}He$. Therefore, alpha decay will cause the nucleus of an atom to lose a mass number of 4 and a proton number of 2. This is helpful when using nuclear equations, as we are able to determine what element the nucleus will decay into.

Atoms and Radioactivity: Beta particle

Oppositely to alpha decay, if an unstable nucleus has too many neutrons compared to protons, it will emit a beta ‘β’ particle. A neutron within the nucleus will spontaneously turn into a proton, ejecting a high-velocity electron in the process. The beta particle is literally just one electron.

A Caesium-137 nucleus decays into Barium-137 and emits a beta particle, commons.wikimedia

Beta decay will cause an atom to change to a different element. Remember that a neutron has been converted into a proton. This will increase the proton number of the nucleus by one but keep the mass number unchanged, as an electron has virtually no mass. A beta particle can be written as${}_{-1}{}^0\beta$or${}_{-1}{}^{0}e$in the context of nuclear equations. The nuclear equation of beta decay of Caesium-137 into Barium-137 shown in the example above is${}_{55}{}^{137}Cs-{}_{-1}{}^{0}e={}_{56}{}^{137}Ba$.

Atoms and Radioactivity: Gamma-ray

After an instance of alpha or beta decay, an atomic nucleus will sometimes still have an excess of internal energy. The nucleus will emit this energy in the form of a gamma-ray ‘γ’, which is a high-energy electromagnetic wave.

Radioactive nucleus decaying by emitting a gamma-ray, commons.wikimedia

Unlike alpha or beta radiation, gamma-rays are waves and not particles. Therefore, during gamma-ray emission, the proton number and mass number remain completely unchanged. It is written as${}_0{}^0\gamma$in a nuclear equation. The nucleus loses some energy, but there is no change to the atomic structure.

Atoms and Radioactivity: Neutron emission

Some radioactive isotopes are capable of decay by emitting neutrons ‘η’ at high velocities. It is most commonly seen during nuclear fission (which is a form of radioactive decay) of high mass radioactive isotopes with a high neutron to proton ratio. Depending on the isotope that is undergoing decay, one or multiple neutrons can be emitted at once.

Neutron emission during the fission of an atomic nucleus, flickr

When a nucleus emits a neutron, its mass number decreases by 1, but its proton number remains the same. It is generally written as${}_0{}^1\eta$. An atom’s designated element depends only on the proton number and not the mass number. This means that neutron emission alone will never change the element of an atom, although it will change it to a different isotope.