Nuclear Reactions

Core Concepts

In this tutorial, you will learn all about nuclear reactions. This includes an introduction to nuclear reactions and a discussion about how to write their equations. We also consider the different types of nuclear reactions, which include nuclear fission, nuclear fusion, and radioactive decay.

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Introduction to Nuclear Reactions

What are nuclear reactions? Most of the chemistry we know deals with reactions between atoms and molecules. While the chemical properties of the compounds can change drastically, the identity of the atoms remains constant in these reactions.

Nuclear reactions, on the other hand, modify the nuclei of atoms; this can change an atom from one element into another, create different isotopes of an element, or even generate new elements that don’t exist in nature.

Many of the synthetic elements discovered in the late twentieth and early twenty-first centuries were created by nuclear reactions. Most of them are only stable for fractions of a second before they decay!

Writing Nuclear Reaction Equations

Writing nuclear reactions requires slightly different information than we are used to providing in ordinary chemical reaction equations. Sometimes we start and end with the same atoms here, so how do we show what is changing? We need to show the mass and atomic number of each nucleus or subatomic particle used in the equation. The way we do this is to write each particle with a superscript and subscript to the left of its symbol.

For example, below is the symbol for lead-207, an isotope of lead with 125 neutrons and 82 protons. The number of protons is equal to the nuclear charge and goes as a subscript before the elemental symbol. The total number of protons and neutrons goes just above that to produce the final written form.

Besides regular nuclei like this one, we can also have subatomic particles that have no elemental symbol (like electrons, positrons, neutrinos, and antineutrinos). Electrons are written as an e or the Greek letter beta with a charge of -1 and a mass of 0 (this is an approximation since it weighs much less than 1 amu or the mass of a proton). Positrons are the same, but we use a p (or the Greek letter beta, again) and a positive 1 for charge. Neutrinos are represented by the Greek letter mu with a zero for charge and mass. Antineutrinos simply have a bar over the symbol to differentiate them. Below you can see how to write many of the common particles.

Ways of representing various subatomic particles in a nuclear reaction. From left: two ways of writing a beta particle or electron, two ways of writing a beta plus particle or positron, neutrino, antineutrino, neutron, and proton. Notice that a proton can simply be written as a hydrogen nucleus.

Types of Nuclear Reactions

Fission Reactions

One of the most well-known categories of nuclear reactions is the fission reaction. Nuclear fission is when a nucleus is divided in half, generating two smaller nuclei. Some very heavy elements undergo fission spontaneously, while most require a nudge – something to spark the process. This nudge is usually provided by a neutron striking the nucleus.

Fusion Reactions

Another type of nuclear reaction is the fusion reaction. Effectively the reverse of the fission reaction, this type involves the combination of two nuclei to form a third, heavier nucleus.

Fusion is a common element of science fiction stories since it releases a large amount of energy. Currently, it is difficult to achieve fusion under useful conditions to generate energy, but developing better fusion reactors is a hot area of research!

Radioactive Decay

Radioactive nuclei “decay” or decompose into a different state over time. When they do this, they release particles into their environment, which can be potentially useful – or damaging. Some examples are the alpha particle, high-energy electron or beta particle, and the high-energy photon or gamma particle.

The rate of radioactive decay is measured using something called the “half-life.” The definition of half-life is the time it takes for half of the original atoms in a sample to decay, and it is represented by the symbol t_1/2. To understand what this means, let’s say you have a sample of sixteen atoms with a half-life of five minutes. After five minutes, on average, only eight of these atoms will remain. The other eight will have decayed into something else. After 10 minutes (two half-lives), an average of twelve atoms will have decayed – the eight from before, plus half of the remaining 8. After 15 minutes (three half-lives), an average of 14 atoms will have decayed, leaving just two. Eventually, after 20 minutes, only 1 atom will be left.

We can only say “on average” because we never know for sure when an individual atom will decay – only the rate at which they generally decay. If we ran the above experiment in real life, we would likely get different results. However, if we ran it many times, the results would get closer and closer to what we described.

Illustration of nuclear decay demonstrating the concept of half-life. 16 green circles at left. To the right, after five minutes have passed, are 8 green and 8 orange circles. After five more minutes have passed, there are 4 green and 12 orange. Finally, after 15 minutes total, there are 2 green and 14 orange. — Illustration of nuclear decay demonstrating the concept of half-life.

Alpha decay

Alpha decay is a type of nuclear fission. In this variety, one of the new nuclei is always the helium nucleus – 2 protons and 2 neutrons. This means that to figure out the other product, we can just subtract 2 protons and 4 mass units. For example, if uranium-238 undergoes alpha decay, it produces the alpha particle and an atom of thorium-234. Below you can see another uranium decay that starts with the common isotope U-235 and results in the formation of thorium-231. Shown below are two common methods of writing this reaction (one in which the alpha particle is written as a product explicitly and another where it is written over the reaction arrow as a shorthand).

Beta Decay

One type of radioactive decay results in the decomposition of a neutron into two parts: a high-energy electron ejected from the nucleus, known as a beta particle, and a proton, which remains in the nucleus, giving it one additional unit of positive charge. Another less commonly discussed subatomic particle is also generated – the antineutrino. Importantly, because the neutron splits into a positively and a negatively charged particle, the process conserves charge. Below you can see a beta decay starting with the thorium-231 from above and resulting in protactinium-231.

There is a variant of beta decay called beta plus decay, in which a positron is created instead of an electron, and a neutrino is created instead of an antineutrino.

Gamma Emission

Some types of nuclear reactions do not involve a change in the number of protons and neutrons in the nucleus. The most well-known, gamma emission, is a reaction that releases very high-energy photons (light) known as gamma radiation. This radiation is very dangerous to living organisms, and it’s the primary reason why radioactive materials are so difficult to handle safely.

The reason for the emission of gamma photons is the decay of a nucleus from a higher to a lower energy state. This is just like how visible light is emitted by electrons. The main difference is that the difference in energy between nuclear energy levels is much larger. As a result, the photons emitted are far higher in energy and shorter in wavelength. It’s their high energy that makes them dangerous to living things. They are a type of “ionizing” radiation, which can modify the important biomolecules that keep organisms functioning, like DNA.