Describe alpha, beta, and gamma decay, half-life, and the processes of fission and fusion, and explain why nuclear changes release large amounts of energy (MA STE HS-PS1-8(MA), nuclear processes).

What this topic is asking

Nuclear chemistry is about changes in the nucleus, not the electrons, so it behaves very differently from ordinary chemistry. In the Massachusetts framework the nuclear standard HS-PS1-8(MA) is officially placed under Introductory Physics, but because it follows directly from atomic structure, chemistry courses almost always teach it, and this library covers it here. You need to describe the three kinds of radioactive decay, balance simple nuclear equations, use half-life, and explain why fission and fusion release such enormous energy.

Why nuclear changes differ from chemical changes

A chemical reaction rearranges electrons and leaves every nucleus intact, so atoms keep their identity. A nuclear change alters the nucleus itself, so one element can turn into another (transmutation). Nuclear changes also release roughly a million times more energy per atom than chemical changes, which is why nuclear processes power stars and reactors. The driving force is nuclear stability: nuclei with an unfavorable ratio of neutrons to protons are unstable and decay toward a more stable arrangement.

The three types of decay

Alpha particles are the largest and least penetrating (stopped by paper or skin), beta particles are more penetrating (stopped by aluminum), and gamma rays are the most penetrating (needing thick lead or concrete). This penetration order matters for shielding and safety.

Balancing nuclear equations

A nuclear equation is balanced when two totals match on both sides:

• the sum of the mass numbers (the top numbers) is equal, and
• the sum of the atomic numbers (the bottom numbers) is equal.

This is the nuclear version of conservation. For example, in the alpha decay $^{226}_{88}\text{Ra} \rightarrow\ ^{222}_{86}\text{Rn} +\ ^{4}_{2}\text{He}$, the masses balance ($226 = 222 + 4$) and the atomic numbers balance ($88 = 86 + 2$). To find an unknown product, make each total match and read off the missing mass number and atomic number, then use the periodic table to identify the element.

Half-life

Half-life is a fixed property of an isotope, from fractions of a second to billions of years. Because the decay is exponential, you halve the amount once per half-life. Carbon-14, with a half-life of about 5730 years, is used to date once-living material, and long-lived isotopes are used to date rocks.

Fission and fusion

Fission splits a large, heavy nucleus (such as uranium-235) into smaller nuclei, releasing energy and more neutrons; those neutrons can trigger a chain reaction, the basis of nuclear power and weapons. Fusion joins small, light nuclei (such as hydrogen isotopes) into a larger one, releasing even more energy per gram; it powers the Sun. Both release energy because a small amount of mass is converted into energy according to Einstein's relation $E = mc^2$, where the enormous value of $c^2$ means a tiny mass yields a vast energy.

Try this

Q1. Polonium-210 emits an alpha particle. State the mass number and atomic number of the product (polonium is element 84). [2]

• Cue. Mass number $210 - 4 = 206$; atomic number $84 - 2 = 82$ (lead).

Q2. A sample has a half-life of 4 hours. What fraction remains after 12 hours? [1]

• Cue. 12 hours is three half-lives, so $\tfrac{1}{2} \times \tfrac{1}{2} \times \tfrac{1}{2} = \tfrac{1}{8}$ remains.