Alpha Decay
Decay in chemistry refers to changes in the nuclei of certain atoms.
Why Some Nuclei Decay
Radioactive elements and isotopes have unstable nuclei. To become more stable, the nuclei undergo radioactive decay. In radioactive decay, the nuclei give off, or emit, radiation in the form of energy and often particles as well. There are several types of radioactive decay, including alpha, beta, and gamma decay. Energy is emitted in all three types of decay, but only alpha and beta decay also emit particles.
What Is Alpha Decay?
Alpha decay occurs when a nucleus is unstable because it has too many protons. The Figure below shows what happens during alpha decay. The nucleus emits an alpha particle and energy. An alpha particle consists of two protons and two neutrons, which is actually a helium nucleus. Losing the protons and neutrons makes the nucleus more stable.
[Figure 2]
Equations for Alpha Decay
Radioactive nuclei and particles are represented by nuclear symbols that indicate their numbers of protons and neutrons. For example, an alpha particle (helium nucleus) is represented by the symbol , where He is the chemical symbol for helium, the subscript 2 is the number of protons, and the superscript 4 is the mass number (2 protons + 2 neutrons).
Nuclear symbols are used to write nuclear equations for radioactive decay. Let’s consider an example. Uranium-238 undergoes alpha decay to become thorium-234. (The numbers following the chemical names refer to the number of protons plus neutrons.) In this reaction, uranium-238 loses two protons and two neutrons to become the element thorium-234. The reaction can be represented by this nuclear equation:
If you count the number of protons (subscripts) as well as the number of protons plus neutrons (superscripts), you’ll see that the total numbers are the same on both sides of the arrow. This means that the equation is balanced. The thorium-234 produced in this reaction is also unstable, so it will undergo radioactive decay as well. The alpha particle (helium nucleus)
that is produced in the reaction can join with two free electrons to form the element helium. This is how most of Earth’s helium formed.
How Dangerous Is Alpha Decay?
All types of radioactive decay pose risks to living things, but alpha decay is the least dangerous. That’s because alpha particles are relatively heavy, so they can travel only a few centimeters through the air. They also are not very penetrating. For example, they can’t pass through a sheet of paper or thin layer of clothing. They may burn the skin, but they can’t penetrate to the tissues underneath the skin. However, if alpha particles are emitted inside the body, they can do more damage. One way this can happen is by inhaling cigarette smoke. People who smoke actually inhale the radioactive element polonium-210. It undergoes alpha decay in the lungs. Over time, exposure to alpha particles may cause lung cancer.
