Sometimes atoms aren’t happy just being themselves; they suddenly change into completely different atoms, without any warning. This mysterious transformation of one type of element into another is the basis of nuclear reactions, which cause one nucleus to change into a different nucleus. Just like chemical reactions cause compounds to turn into other compounds by swapping their electrons, nuclear reactions happen when the number of protons and neutrons in the nucleus of an atom change.
Some types of nuclear reactions can actually kick protons out of the nucleus, or convert them into neutrons. Since we know what to call an element by looking up its number on a periodic table and then reading off its name, when the atomic number (number of protons) changes, so does the name of the element. This makes nuclear reactions look somewhat like alchemy: an atom of potassium (atomic number 19) can suddenly and unexpectedly transform into an atom calcium (atomic number 20). The only sign that anything has changed is the release of radiation, which we’ll talk more about in a little bit.
Even more strangely, nuclear reactions often occur almost entirely randomly. If you have a single nucleus that you are certain will eventually decay into a different nucleus, you still have only a rough idea how long it will take for you to see it happen. You could be sitting watching the nucleus for anywhere between a few seconds to your entire lifetime, and at some point it would suddenly decay without any warning! However, depending on the type of nucleus, you can predict how long on average it would take to decay if you watched many nuclei at once. So while the average time to decay is a measurable number (for potassium it’s over a billion years), the exact time of the decay is entirely random.
There are three types of nuclear reaction, each of which cause the nucleus to shoot out a different, fast-moving particle (like a photon or electron). These released particles are a side effect of the element changing its atomic number or mass, and they are what scientists generally mean when they warn about nuclear radiation, since fast-moving particles can act like tiny bullets that poke holes in your body. However, much nuclear radiation is actually harmless, and it occasionally can be harnessed to provide new type of medical or diagnostic tools.
Why do nuclear reactions happen?
Not all elements undergo nuclear decay over timescales that we can observe. Some elements take millions of years to decay. In fact, most living things primarily consist of isotopes of carbon and nitrogen, which have such incredibly long lifetimes that they will essentially never decay within the lifespan of the organism. This is necessary because the biochemical function of each of these atoms is specifically tied to its atomic number: if a nervous receptor specifically seeks out and binds a carbon-based signalling molecule, then it won’t work if that carbon spontaneously changes into beryllium.
Different atoms of the same element can have different masses. For example, an atom of carbon (atomic number 6, so six protons) can have either 6 neutrons or 8 neutrons. The former case is more familiar from chemistry class, since a lot of the common light elements used in biology (like oxygen, carbon, and nitrogen) have the same number of protons as neutrons. But it turns out that the case of carbon having 6 protons and 8 neutrons, while not as stable as 6 and 6, is stable enough that it can actually occur in nature in observable amounts. Because the 8 neutron nucleus and the 6 neutron nucleus are technically both carbon, we call them different isotopes of carbon.
Since protons and neutrons have roughly the same mass, the more common version of carbon is called carbon-12 (6 protons + 6 neutrons). The heavier isotope is called carbon-14 (6 protons + 8 neutrons). But when you look up the mass of carbon on the periodic table, it says that the mass is 12.011 atomic mass units (amu). This is because if you went out and weighed a huge batch of carbon atoms, most of the atoms you would find would weigh exactly 12 amu. But within that huge batch you’d occasionally find a carbon-14 nucleus, which would skew the average of your measurements to a value slightly higher than 12.
For reasons that are deeply related to the fundamental forces that act in the nucleus, the tendency of a substance to undergo nuclear decay is related to both the atomic number and the atomic mass of an element. This means that two different isotopes of the same element will have different tendencies to undergo nuclear decay. In the case of carbon, the isotope carbon-14 wants to decay into nitrogen while carbon-12 (which is most of the carbon in your body) would remain stable.
As a result, knowing which isotope is present in a sample of element not only tells us the sample’s stability, but also the type of decay it will undergo.
What are the types of nuclear reaction?
Cartoon showing alpha decay.
During alpha decay, a nucleus actually breaks up into two chunks: a pair of protons bound to a pair of neutrons (a collection of four particles which is essentially a helium nucleus, and is called an alpha particle), and another piece constituting the original nucleus minus this chunk. So we can actually write down a chemical reaction equation for alpha decay:
Ra → Rn + He2+
The radium nucleus (Ra, atomic number 88) breaks up into the helium nucleus (He2+, the little chunk) and a daughter nucleus that corresponds to the element radon (Rn, atomic number 86). The medical risks associated with radiation usually involve the fast speeds at which the products of nuclear reactions move.Think of the alpha particle released by this reaction as a tiny bullet, which can puncture soft tissues like the lining of the stomach and lungs. Fortunately, alpha decay tends to release large, slow-moving decay products, and so it’s easy to shield against this type of radiation.
The reaction shown above illustrates another, indirect method by which alpha decay can pose a hazard. Radium, the element on the left hand side of the reaction arrow, can be found deep underground as a solid rock mixed in with granite. However, when it undergoes alpha decay it turns into radon, which naturally prefers to be a gas. The radon then seeps out of the ground and into the basements of people’s homes, where it can enter their lungs and then decay again, releasing more alpha particles (or other types of radiation) directly into the unprotected tissues. This method of radon exposure represents a major lung cancer risk factor in many parts of the world.
Cartoon showing beta decay.
In beta decay, one of the neutrons in the nucleus suddenly changes into a proton, causing an increase in the atomic number of an element. Recall the name of an element is determined by its atomic number. Carbon is carbon because it has an atomic number of 6, while nitrogen is nitrogen because it has atomic number 7. That means that a reaction that changes the number of protons in the nucleus changes what element we actually consider the nucleus to be. This makes beta decay a great example of how nuclear reactions can eerily transform one substance into another.
The product potassium chloride is commonly sold as a salt-substitute in grocery stores. This product contains trace amounts of potassium-40 (K), which tends to undergo beta decay into calcium-40 (Ca). Symbolically, this reaction looks like:
K→ Ca + e− + v
In addition to changing its atomic number, the nucleus creates and releases an electron (e-) from the atom that serves to counterbalance the positive charge it gained by transforming a neutron to a proton. The other released particle v is a mysterious particle called a neutrino, which has no charge and barely any mass. The emitted, free electrons are the “radiation” associated with beta decay
This means that if you were to go to the grocery store and buy a jar of potassium-40 isotopes (which are prone to beta decay) and then leave it sitting on your countertop for a couple of years, you would end up having less potassium than you started out with (calcium would take its place). This process happens incredibly slowly and in miniscule numbers for the potassium chloride available in the grocery store, and so the actual health risk posed by this radiation is nil.
A related type of beta decay actually decreases the atomic number of the nucleus when a proton becomes a neutron. Due to charge conservation, this type of beta decay involves the release of a charged particle called a “positron” that looks and acts like an electron but has a positive charge. Because this particle’s interactions with other tissues are easily identifiable, some medical imaging techniques involve purposefully injecting a patient with an element that beta decays into positrons, and then monitoring where the positrons are emitted. When beta decay creates a positron it’s called beta-plus decay, and when it creates an electron it’s called beta-minus decay.
Cartoon showing gamma decay.
During gamma decay the nucleus emits radiation without actually changing its composition: We start with a nucleus with 12 protons and 12 neutrons, and we end up with a nucleus with 12 protons and 12 neutrons… but somehow radiation gets released along the way!
The nucleus is made out of a glued-together arrangement of protons and neutrons, but there are multiple possible ways that these protons and neutrons can be arranged. Some of these arrangements have a lower total energy, and so a nucleus in which the protons are initially close together may shift to the lower energy configuration after some time.
Recall that the electrons orbiting the nucleus have energy levels, and that each time an electron moves from a high energy level to a low energy level it emits a photon. The same thing happens in the nucleus: when it rearranges into a lower energy state, it shoots out a high-energy photon known as a gamma ray.
Gamma rays are very high energy and are one of the most dangerous sources of radiation because photons can pass through most common shielding materials and cause DNA damage in living tissues. But gamma radiation also has practical uses; for example, the element technetium emits relatively low-energy gamma decays that can be detected using a specialized scanner, and so it has found use as a tracer element for imaging the inside of patients’ bodies.
Consider the following… radiation exposure during long-term space travel
Radiation sometimes comes up the news in the context of the risks associated with long-distance space travel. It turns out that dying stars and all the other strange stuff in space generates a lot of exotic and strange types of radiation---even beyond the types described above. These unusual, high-energy particles are collectively called cosmic radiation, and in many parts of space they mostly consist of extremely high energy photons, or gamma radiation. Recall that gamma radiation is particularly dangerous because photons can pass through most barriers and thus travel deep into living tissues, where they can cause internal damage (and eventually cancer).
In a spaceship, astronauts don’t have the full benefit of Earth’s thick atmosphere to protect them from cosmic rays. This means that any long-term space travel initiative (such as a mission to mars) will have to invest considerable resources in properly shielding the living areas of the spaceship with materials that are impervious to cosmic radiation. The development of materials that can shield astronauts is an active area of research!