Explainer: Nuclear ReactionsChemistry • First Year of Secondary School

In this explainer, we will learn how to write and interpret nuclear reactions.

In nuclear reactions, atomic nuclei change from one state to another. Nuclear reactions are responsible for making almost all of the large atoms in the universe and making solar energy in the Sun’s hot and high-pressure interior. Nuclear reactions can even be used to make electricity in nuclear power plants and diagnose diseases in hospital settings.

The change that occurs during a nuclear reaction is different from the change that occurs during a chemical reaction. Chemical reactions happen when electrons are exchanged between at least two interacting atoms. Even though chemical reactions produce different chemical compounds, the identity of the atoms remains the same. This means there are the same numbers and types of atoms in the reactants and products, they are just combined in different ways that we can see in the diagram below.

During a nuclear reaction, neutrons and protons can change and entire nuclei can combine or break apart. These changes often result in an atom of one element transforming into an atom of a completely different element. The transformation processes are usually accompanied by extremely large changes in energy that can be up to a million times greater than the amount of energy released during conventional chemical reactions.

Isotopes have similar chemical reactions because they have the same number of electrons, but they will have different nuclear reactions because they have different numbers of neutrons. One isotope can contain more nucleons than another isotope, and this will make it more or less likely to undergo a nuclear reaction. For example, carbon-14 is used for determining the age of carbon-based archeological samples because it slowly transforms into nitrogen-14, but carbon-12 will not making it useless for determining the age of anything that is discovered by archeologists.

These differences between chemical reactions and nuclear reactions can be summarized in the following table.

Nuclear reactions can be represented in a reaction equation style. To represent particles participating in the reaction, we will use nuclide notation , where is the symbol for the particle (such as the atomic symbol), is the mass number (sum of neutrons plus protons), and is the charge of the particle (number of protons for nuclei).

For example, the reaction that is used in carbon dating is shown below. In this reaction, carbon-14 transforms into nitrogen-14 and emits an electron:

For a nuclear reaction to be balanced, the total and the total must be the same on both sides of the reaction arrow. This is shown below:

In the above reaction, the total on both sides of the equation is 14 and the total on both sides of the equation is 6, meaning that the reaction is correctly balanced.

There are three different types of nuclear reactions: fission, fusion, and transmutation.

Fission occurs when a heavy nucleus is split into two or more smaller nuclei. Fission produces a lot of energy that is used to generate electricity inside nuclear power plants.

Fusion is when two or more lighter nuclei combine to form a heavier nucleus. The reaction that occurs in the Sun and other stars is a fusion reaction that begins with hydrogen nuclei fusing to create helium.

Transmutation involves the transformation of an atom of one element into an atom of a different element. There are two subtypes of transmutation: radioactive decay and bombardment.

Transmutation by radioactive decay is a process that happens in nature to isotopes that are unstable. An unstable or radioactive isotope will spontaneously emit particles or energy, called radiation, causing the isotope to transform into one that is more stable. There are several kinds of radioactive decay that will be discussed in more detail later.

Transmutation by bombardment also results in atoms of an element transforming into atoms of a different element. Bombardment is a nonspontaneous process where an atom is bombarded with smaller particles, such as neutrons or alpha particles, that combine with the atom to form a different, larger nucleus.

The first example of transmutation by bombardment occurred in 1919 when the scientist Ernest Rutherford bombarded atoms of nitrogen-14 with alpha particles. However, back then, apart from observing that protons were emitted, he did not know what the other product of the reaction was. A few years later, in 1925, fellow scientist Patrick Blackett was able to identify the residual nuclide as being oxygen-17. This process also emits protons as follows:

Elements that have an atomic number greater than 92, which are called the transuranium elements, are created by bombardment. Most of these elements do not exist in nature, and they are all radioactive.

We now know what nuclear reactions are, how they differ from chemical reactions, and the different types of nuclear reactions that can occur. However, there is one important topic we largely have not discussed yet. If carbon-14 is unstable and undergoes radioactive decay to nitrogen-14, then why is carbon-12 stable and does not decay?

The only difference between these isotopes is the number of neutrons: carbon-12 has six neutrons, while carbon-14 has eight; therefore, the answer must have something to do with the number of neutrons an atom has.

Nuclei are held together by the nuclear force: an extremely strong attractive force between protons and neutrons in the nucleus. The strong attraction of the nuclear force is why the positively charged protons in the nucleus do not repel each other.

Neutrons are important for nuclear stability because they provide more of the attractive nuclear force in the nucleus to overcome the electrostatic repulsion between protons. With the right number of neutrons, the nucleus will be held together by the nuclear force and the atom will be stable. If there are too many or too few neutrons in the nucleus, the atom will be unstable.

The ratio of neutrons to protons in the nucleus can be used to predict nuclear stability since atoms of an element with a specific neutron to proton ratio will be stable. This stable neutron to proton ratio can be visualized on a graph, shown below, that is commonly referred to as the “band of stability.” Each dot on the graph represents a stable isotope.

The most abundant stable isotopes of lighter elements, with atomic numbers of about 20 or less, have a neutron to proton ratio of about . However, the more protons there are in the nucleus, the more neutrons are needed for the nucleus to be stable. This is reflected by the neutron to proton ratio gradually increasing and approaching a ratio of for heavier elements. Any isotope not on the band of stability will be radioactive and will decay into an isotope that is on the band of stability over time.

There are different types of radioactive decay that isotopes can undergo to become stable. We will discuss several kinds of decay in detail: alpha decay, beta decay, positron emission, electron capture, and gamma emission. In each type of decay, radiation is emitted from the nucleus. This radiation can be a type of particle, energy, or a combination of the two.

When a nucleus undergoes alpha decay, an alpha particle is emitted. Alpha particles are nuclei with two protons and two neutrons or the ion. Emitting an alpha particle results in the nucleus losing two protons and two neutrons, decreasing by 4 and by 2.

The general reaction that occurs during the alpha decay of an atom can be shown as follows.

In decay, an electron (or beta particle) is emitted from the nucleus. This accompanies a neutron in the nucleus changing into a proton, increasing the number of protons in the nucleus by one. decay results in remaining unchanged because the total number of protons and neutrons stays the same.

The general reaction that occurs during the beta () decay of an atom can be shown as follows:

In positron emission, a positron is emitted from the nucleus. A positron is a particle with the same mass as an electron but with an opposite charge . During this process, a proton in the nucleus is converted into a neutron. Since the number of protons in the nucleus decreases by one, decreases by one, but is unchanged.

The general reaction that occurs when an atom undergoes positron emission can be shown as follows.

In electron capture, a low-energy electron in the atom is absorbed by the nucleus. This process converts a proton to a neutron and typically is accompanied by the emission of X rays. Electron capture has the same overall effect as positron emission, decreases by one and is unchanged.

The general reaction that occurs during electron capture can be shown as follows.

In gamma decay, only gamma rays or high-energy photons are emitted. During gamma decay, a nucleus that is in an excited state lowers its energy by emitting photons. Since photons do not have any mass or charge, this kind of decay does not change or .

The general reaction for gamma decay is as follows.

There are other types of radioactive decay, such as proton emission, neutron emission, and the emission of particles heavier than alpha particles, but they are far less common so we will not discuss them in detail here.

The following table summarizes the common types of radioactive decay.

Now that we know the different types of radioactive decay, we need to determine which isotopes will undergo which kind of decay. We can determine this with the neutron to proton ratio.

If the neutron to proton ratio is too high, placing the isotope above the belt of stability, the atom will have too many neutrons to be stable. Atoms with too many neutrons to be stable will undergo decay because decay converts a neutron into a proton, reducing the neutron to proton ratio.

An example of this is carbon-14, which has a neutron to proton ratio of about . This neutron to proton ratio is too low because atoms as light as carbon-14 should have a neutron to proton ratio closer to 1. When carbon-14 decays by beta emission (), it transforms into nitrogen-14 and its neutron to proton ratio is increased to 1:

If the neutron to proton ratio is too low, placing the isotope below the belt of stability, the isotope will become stable through either positron () emission or electron capture. This will increase the neutron to proton ratio since both processes convert a proton into a neutron. Positron () emission is more common in lighter elements, while electron capture is more common in heavier elements.

Carbon-11 has a neutron to proton ratio of 0.83, which is below the belt of stability. It undergoes positron emission, converting it into Boron-11 and increasing its neutron to proton ratio:

Alpha decay typically occurs for heavier elements that have a mass number greater than about 200. Uranium-238 is the most common isotope of uranium in nature. This heavy nuclide undergoes alpha decay, converting it into thorium-234:

This information is summarized in the graph below. Each region surrounding the belt of stability is color coded according to the type of decay atoms with that neutron to proton ratio will undergo to become stable.

Notice that gamma emission is not on the graph above. Gamma decay typically does not occur by itself. Instead, it typically accompanies other kinds of radioactive decay, particularly beta decay. Other high energy photons, like X rays, can be emitted during nuclear reactions as well.

Energy is often emitted during nuclear reactions because the mass during a nuclear reaction is not quite constant. There are actually tiny changes to the mass of the nuclides involved in the reaction. As Einstein’s famous equation tells us, changes in mass, like the changes in mass during a nuclear reaction, are accompanied by a proportional change in energy. This change in energy is emitted during a nuclear reaction as gamma rays or other photons.