Some isotopes are radioactive (unstable), and tend to break apart or transform spontaneously in a process called radioactive decay. Some elements (e.g. uranium, radium, and radon) have no stable isotopes: all of their isotopes are radioactive.
There are many types of radioactive decay. The two most common types are alpha decay and beta minus decay, but beta plus decay and spontaneous fission will also be important in the discussion that follows. Here is a description of each:
Alpha decay (α decay) occurs when a large nucleus breaks apart into a smaller nucleus and an alpha particle, which (as it turns out) consists of two protons and two neutrons. The alpha particle flies out of the nucleus and away from the atom, which now has two fewer protons and two fewer neutrons. Thus, the atomic number of the atom decreases by 2, and its mass number decreases by 4.
What type of atom is left when an atom of radium-226 undergoes alpha decay? Well, the result will no longer be radium (atomic number 88), since the atom now has two fewer protons in its nucleus. Instead, it will be an atom of radon (atom number 86). And since the mass number was 226 prior to emitting the alpha particle, the mass number afterwards will be 222. So the isotope of the resulting radon atom is radon-222.
If that radon-222 atom undergoes alpha decay, the result will be polonium-218 (atomic number 84, mass number 218).
Beta minus decay (β– decay), sometimes referred to simply as “beta decay,” occurs when a neutron spontaneously transforms into a proton, and an electron is emitted. This electron isn’t one of those that had been orbiting the nucleus of the atom. In fact, according to the presently accepted theory of particle physics (which we’ll discuss later in this chapter), the emitted electron didn’t exist at all prior to the decay! It is a brand new electron created “from scratch,” so to speak, from energy that is released when the neutron transforms into a proton. (As we’ll see in chapters 6 and 7, energy can be converted into matter and vice versa.) Since beta minus decay turns a neutron into a proton, the atomic number of the resulting atom is one more than that of the original atom, and the mass number stays the same.
Hydrogen-2 (known as deuterium, or “heavy hydrogen”) has one proton and one neutron in its nucleus. When the neutron decays into a proton, the atom is no longer hydrogen (atom number 1) but helium (atomic number 2). So the result is an atom of helium-2, which has two protons and no neutrons in its nucleus.
Beta plus decay (β+ decay) occurs when a proton transforms into a neutron, and a particle called a positron is emitted. (We’ll learn more about positrons later in this chapter.) This type of radioactive decay is relatively uncommon, but it does happen with a few isotopes. Since beta plus decay turns a proton into a neutron, the atomic number of the resulting atom is one less than that of the original atom, and the mass number stays the same. A similar process called electron capture occurs when a proton transforms into a neutron by absorbing an electron, rather than by emitting a positron.
Magnesium-23 is a radioactive isotope of magnesium (atomic number 12). When magnesium-23 undergoes beta plus decay, one of its 12 protons turns into a neutron, so the resulting atom is sodium-23.
Spontaneous fission occurs only with some very large, unstable nuclei like uranium-235. Although uranium-235 usually undergoes alpha decay, it can also break apart in other ways. Occasionally a uranium-235 atom will disintegrate into multiple pieces, including a few neutrons that are not attached to any protons. These free neutrons may strike other atoms and cause nuclear reactions, as I’ll explain later in this chapter.
When a radioactive atom decays, the result is often another radioactive atom, which decays into yet another radioactive atom, and so on, until at last the process ends with a stable atom. A series of decays like that is called a decay chain. At right is a diagram of a decay chain beginning with uranium-238 and ending with lead-206. Notice that some of the isotopes in this chain can decay in more than one way. For example, polonium-218 usually undergoes alpha decay into lead-214, but it can also undergo beta minus decay into astatine-218.
Radioactive decay is a spontaneous, “chancy” process. According to the current theories of particle physics, nothing causes a radioactive atom to decay at a particular time. It just happens at random, or by chance. Nevertheless, there is a certain probability that a radioactive atom will decay over any given period of time. For example, an atom of tritium (hydrogen-3) has a 50% chance of decaying during any 12-year period. So, if you have a large collection of tritium atoms, then 12 years from now you’ll have about half as many, because approximately 50% of the tritium will have decayed by then. And in 12 more years, about half of the remaining atoms will decay, so in 24 years you’ll have about a ¼ as many tritium atoms as you have today. And 36 years from now you’ll have ⅛ as many, and so on.
The average time it takes for half of a large collection of atoms (of any given isotope) to decay is called the half-life of that isotope. The half-life of tritium (hydrogen-3) is 12 years, as explained above. The half-lives of different isotopes vary dramatically. For example, uranium-238 has a half-life of about 4.5 billion years, lithium-8 has a half-life of about 1 second, and hydrogen-4 has a half-life less than a billionth of a trillionth (1/1022) of a second.
You may be wondering why physicists care about the time it takes for half of the atoms to decay, rather than the time it takes for all of them to decay. The reason is that the time it takes for all of them to decay depends on how many atoms there were to begin with. But the half-life is more or less the same regardless of how many atoms there are.