Radioactivity

Radioactive Decay

Radioactive decay happens when an unstable nucleus gives off energy as radiation. Alpha, beta, and gamma decay are the three main types of radioactive decay.

Radioactive decay is the process by which an unstable nucleus loses energy by emitting radiation. This process allows a nucleus to reach a more stable state. The radiation emitted by an unstable nucleus may be a particle or energy in the form of electromagnetic radiation.

Certain isotopes are more stable than others. Their stability is determined by the ratio of the number of neutrons to the number of protons in the nucleus. If the ratios of neutrons to protons for all elements are plotted as a graph, a nuclear zone of stability can be identified. The zone of stability in the graph shows that lighter isotopes (atomic mass number under 20) are most stable when the neutron-to-proton ratio is 1:1. For these elements, the repulsive force within the nucleus is not strong enough to overcome the attractive force that holds the nucleus together. The nucleus is unstable if the neutron-to-proton ratio is greater than 1.5. This instability is typical for very heavy elements. The nuclear zone of stability can be used to predict how unstable isotopes will decay.

Zone of Stability

Stable isotopes form a zone, or band, at the center of the graph. The N:Z ratio shows the ratio of the number of neutrons (N) to the number of protons (Z). The closer the N:Z ratio is to one, the more stable the isotope is. Isotopes that have a greater number of neutrons can become more stable through beta decay, which changes a neutron to a proton. Isotopes that have a greater number of protons can become more stable through alpha decay, which decreases the number of protons and neutrons, or through positron emission or electron capture, both of which change a proton to a neutron.
A nuclide is a nucleus with specific proton and neutron numbers. The parent nuclide is the nucleus in radioactive decay that exists before the emission of radiation. The daughter nuclide is the nucleus in radioactive decay that remains after the emission of radiation. A radioisotope, or radioactive isotope, is an isotope with an unstable nucleus that experiences radioactive decay. An isotope is one of two or more atoms of an element that have the same number of protons but different numbers of neutrons. A radioisotope has a different number of neutrons from other, more stable, isotopes of the same element.

There are three main types of radioactive decay:

  • Alpha decay (α\alpha decay) is radioactive decay that gives off an α\rm\alpha particle. An α\rm\alpha particle is composed of two protons and two neutrons. It is essentially a helium nucleus.
  • Beta decay (β\beta decay) is radioactive decay that gives off a β\rm\beta particle. A β\rm\beta particle is a high-energy electron emitted from the nucleus rather than the electron cloud. In the nucleus, neutrons can decay into protons by emitting a β\rm\beta particle.
  • Gamma decay (γ\gamma decay) is radioactive decay that gives off a gamma (γ\rm\gamma) ray. A γ\rm\gamma ray is electromagnetic radiation with the shortest wavelength (λ\mathbf\lambda), meaning it has the highest energy of all light.

There are other, less common types of radioactive decay. These include the emission of a positron (a nuclear particle with the mass of an electron but a charge of +1) and electron capture, in which the nucleus of an atom captures one of the atomic electrons and then emits a γ\rm\gamma ray as well as an almost massless subatomic particle called a neutrino.

In α\rm\alpha and β\rm\beta decay, the nature of the daughter nuclide is fundamentally changed compared to the parent nuclide. A chemical equation can be written to show the transmutation (conversion of one type of nucleus into another) that occurs with radioactive decay. By convention, the notation for a radioactive element is to write the mass number in superscript to the left of the element's atomic symbol. For example, a carbon-14 isotope can be shown as 14C{}^{14}\rm{C}. Alternatively the atomic symbol followed by a dash and the mass number can be used (e.g., C-14). Note that carbon-14 is more unstable than carbon-12 and is therefore more likely to undergo radioactive decay. A carbon-12 isotope is more stable and is much less likely to undergo radioactive decay.

For α\rm\alpha decay, the general equation is ZAXZ2A4Y+24He{}_ Z^ A{\rm {X}}\rightarrow{}_{Z-2}^{A-4}{\rm {Y}}+{}_2^4{\rm{He}}. The daughter nuclide is the element composed of two fewer protons than the parent nuclide—for example, thorium (Th) has two fewer protons than uranium (U). Thus, when 238U{}^{238}\rm{U} undergoes α\rm\alpha decay, the equation shows that the mass number is decreased by 4 and the atomic number is decreased by 2.
92238U90234Th+24He{}_{\;92}^{238}\rm {U}\rightarrow{}_{\;90}^{234}{\rm{Th}}+{}_2^4{\rm{He}}
For β\rm\beta decay, the general equation is ZAXZ+1AY+10β{}_Z^A{\rm {X}}\rightarrow{}_{Z+1}^{\;\;\;\;A}{\rm {Y}}+{}_{-1}^{\;\,\,0}\rm\beta. Alternatively, the β\rm\beta particle, which is a high-energy electron, may be written as 10e{}_{-1}^{\,\,\;0}\rm {e}, the term for an electron. The daughter nuclide is the element composed of one more proton than the parent nuclide—for example, sulfur (S) has one more proton than phosphorus (P). Thus, when 32P{}^{32}\rm{P} undergoes β\rm\beta decay, the equation shows that the mass number is unchanged and the atomic number is increased by 1.
1532P1632S+10β{}_{15}^{32}\rm {P}\rightarrow{}_{16}^{32}\rm {S}+{}_{-1}^{\;\,\,0}\rm\beta
A subcategory of β\rm\beta decay is positron emission, radioactive decay that emits a positron. During positron emission, one proton in the nucleus transforms into a neutron. A positron is a particle with the same mass as an electron. The positron carries a positive charge with a magnitude equal to the negative charge on an electron. A positron is represented as +10e{}_{+1}^{\,\,\;0}\rm{e}. The general formula for positron emission is ZAX+10e+Z1AY{}_Z^A{\rm {X}}{\rightarrow{}_{+1}^{\;\,\,0}{\rm{e}}+{}_{Z-1}^{\;\;\,\,\,A}\rm {Y}}. Positron emission changes the atomic number, making the product a different element, but the mass number stays the same.

A nuclear reaction that is related to positron emission is electron capture, in which the nucleus captures a high-energy electron. This transforms one proton into a neutron. The general formula for electron capture is ZAX+10eZ1AY{}_Z^A{\rm {X}}{+{}_{-1}^{\;\,\,0}{\rm{e}}\rightarrow{}_{Z-1}^{\;\;\;\,A}\rm {Y}}. In electron capture, the atomic number and the identity of the element changes, but the mass number stays the same. Electron capture and positron emission are two ways through which a proton transforms to a neutron in the nucleus. Electron capture is technically not radioactive decay, as there is no particle emission.

For γ\rm\gamma decay, no general equation exists. This is because the makeup of the nucleus is not fundamentally changed—the emission is a high-energy photon, not a subatomic particle composed of protons and neutrons. However, γ\rm\gamma emission often accompanies other types of radiation. When a γ\rm\gamma ray is emitted during radioactive decay, it is added to the products as γ\rm\gamma or 00γ{}_0^0\gamma. For example, when 238U{}^{238}\rm{U} decays to 234Th{}^{234}{\rm{Th}}, two γ\rm\gamma rays of different energies are emitted. They can be added to the α\rm\alpha decay equation. Notice that the sum of the mass numbers of thorium and helium is equal to the mass number of uranium, and the sum of the atomic numbers of thorium and helium is equal to the atomic number of uranium. The γ\rm\gamma decay does not contribute to mass number or atomic number changes.
92238U90234Th+24He+2γ{}_{\,\,92}^{238}{\rm{U}}\rightarrow{}_{\,\,90}^{234}{\rm{Th}}+{}_2^4{\rm{He}}+2\gamma
When writing nuclear decay equations, keep in mind that the total mass numbers (the top numbers) of the products and reactants must be equal, as must the total atomic numbers (the bottom numbers). The charge of atoms is not considered because the electrons do not take part in the nuclear reaction.

Types of Radioactive Decay

Type Nuclear Equation Representation Change in Mass Number and Atomic Number
Alpha (α\mathbf\alpha) decay ZAX24He+Z2A4Y{}_Z^A{\rm {X}}{\rightarrow{}_2^4{\rm{He}}+{}_{Z-2}^{A-4}}\rm {Y}
Mass number: –4
Atomic number: –2
Beta (β\mathbf\beta) decay ZAX10β+Z+1AY{}_Z^A{\rm {X}}{\rightarrow{}_{-1}^{\;\,\,0}\beta+{}_{Z+1}^{\;\;\;\,A}\rm {Y}}
Mass number: No change
Atomic number: +1
Gamma (γ\mathbf\gamma) decay ZAX00γ+ZAY{}_Z^A{\rm {X}}{\rightarrow{}_0^0{\rm\gamma}+{}_Z^A\rm {Y}}
Mass number: No change
Atomic number: No change
Positron emission ZAX+10e+Z1AY{}_Z^A{\rm {X}}{\rightarrow{}_{+1}^{\;\,\,0}{\rm{e}}+{}_{Z-1}^{\;\;\;\,A}\rm {Y}}
Mass number: No change
Atomic number: –1
Electron capture ZAX+10eZ1AY{}_Z^A{\rm {X}}+{}_{-1}^{\;\,\,0}{\rm{e}}\rightarrow{}_{Z-1}^{\;\;\;\,A}\rm {Y}
Mass number: No change
Atomic number: –1

The three major types of radioactive decay are alpha (α\rm\alpha) decay, beta (β\rm\beta) decay, and gamma (γ\rm\gamma) decay. Less common are positron emission and electron capture.

Some unstable elements decay into other unstable elements, which in turn decay into other elements. This forms a decay series, a cascading series of radioactively decaying elements and their daughter nuclides. Uranium-238 is the first element in a well-known series, the uranium decay series. This series begins with 238U{}^{238}\rm{U} and goes through decay at least 14 times before finally reaching lead-206 (206Pb{}^{206}{\rm{Pb}}), a stable element.

Uranium Decay Series

The uranium decay series shows how unstable elements can decay into other unstable elements. This cascade of radioactive decay continues until a stable element is reached-in this case 206Pb{}^{206}{\rm{Pb}}.
An important consideration when studying radioactive isotopes is half-life (t1/2), which is the time it takes for half the nuclei of a sample of a radioactive element to decay. Half-lives can range from a tiny fraction of a second to many trillions of years. Half-life does not imply that half of the original sample disappears; rather, it indicates how long it takes for half of the original, unstable sample to decay into a daughter nuclide. For example, the half-life of iodine-131 is about eight days. It undergoes beta decay to produce xenon-131. Thus, a 100.0 g sample of 131I{}^{131}\rm{I} will become 50.0 g 131I{}^{131}\rm{I} and 50.0 g 131Xe{}^{131}{\rm{Xe}} after eight days. Importantly, if the daughter nuclide is itself unstable, it will have its own half-life, and thus the decay product will be a mixture of the unstable daughter nuclide and the products of its decay. A useful equation exists for calculations involving half-lives.
N(t)=N02(tt1/2)N(t)=N_02^{-\big(\frac t{t_{1/2}}\big)}
N(t) is the amount of sample at time t, N0 is the original amount of sample, t is the amount of time elapsed, and t1/2 is the half-life. For example, this equation can be used to calculate how much 131I{}^{131}\rm{I} will remain from an initial mass of 100.0 g after 32.0 days, given that its half-life is 8.06 days.
N(t)=N02(tt1/2)=(100.0g)2(32.0days8.06days)=6.38g\begin{aligned}N(t)&=N_02^{-\big(\frac t{t_{1/2}}\big)}\\&=(100.0\;\rm {g})2^{-\big(\frac{32.0\;{\rm{days}}}{8.06\;{\rm{days}}}\big)}\\&=6.38\;\rm{g}\end{aligned}
Half-lives can be used to determine the age of a sample containing a radioisotope. The process of using a radioisotope to determine the age of a sample of material is called radiometric dating. The most common form of radiometric dating is radiocarbon dating, also called carbon dating, which is the process in which the isotope used to determine the age of a sample is carbon-14 (14C{}^{14}\rm{C}). This isotope forms naturally in the atmosphere as cosmic rays combine with atmospheric nitrogen. The newly created 14C{}^{14}\rm{C} combines with atmospheric oxygen to form radioactive carbon dioxide, which is taken up by plants and algae during photosynthesis. Animals then eat these plants and algae, incorporating the 14C{}^{14}\rm{C} into their own tissues. Thus, living organisms maintain a fairly constant amount of 14C{}^{14}\rm{C} in their bodies until they die. Upon death, their uptake of 14C{}^{14}\rm{C} ceases, leaving a fixed amount in their remains. The half-life of 14C{}^{14}\rm{C} is known to be 5,730 years, which means it can be used to date organisms that have been dead for thousands of years. Radiocarbon dating is accurate to about 50,000 years ago. Some methods, such as using large sample sizes or long measurement times, may allow the accuracy to be extended to up to 75,000 years ago.

Importantly, radiocarbon dating can only be used to determine the age of once-living things. Nonliving things do not take in atmospheric carbon and thus cannot be dated in this manner, although rock strata may be dated based on the ages of dead organisms embedded in them. For dating nonliving matter, other radiometric dating methods are used. One such method is uranium-lead (U-Pb) dating. The mineral zircon incorporates uranium and thorium, but not lead, into its crystal structure. Analysis of the amount of lead in these crystals gives information that can be used to date the rocks in which they are found. U-Pb dating is precise to about 2 million years and can be used for rocks as old as 4.5 billion years. The method was initially created in order to accurately determine the age of Earth. Other radiometric dating methods, such as samarium-neodymium, potassium-argon, and rubidium-strontium, are also used for similar purposes.