What you'll be able to do: Identify alpha, beta, positron and gamma decay and balance any nuclear equation by conserving mass number and charge.
Nuclear reactions change the nucleus itself, not just the electrons around it. Once you know what each decay particle does to the mass number and the atomic number, every nuclear equation becomes a two-line bookkeeping problem.
These are recommended, not required. You can start this lesson at any time.
A nuclide is written with its mass number (protons + neutrons) on the upper left and its atomic number (protons) on the lower left of the element symbol. In plain text this lesson writes it as element-massnumber, for example ²³⁸U. The atomic number is fixed by the element symbol, so it is the mass number that tells you which isotope you are dealing with.
²³⁸₉₂U has 92 protons and 146 neutrons.
Alpha (α) is a ⁴He nucleus: mass 4, charge +2. Beta-minus (β⁻) (β⁻) is an electron ejected from the nucleus: mass 0, charge -1. A positron (β⁺) is the antimatter twin of the electron: mass 0, charge +1. A neutron has mass 1, charge 0. Gamma (γ) is a high-energy photon: mass 0, charge 0.
Alpha decay: mass number falls by 4, atomic number falls by 2. Beta-minus (β⁻) decay: mass number unchanged, atomic number rises by 1. Positron emission or electron capture: mass number unchanged, atomic number falls by 1. Gamma emission: neither number changes, only energy is released. Learning these four rows is most of the topic.
Add the mass numbers on each side and set them equal; do the same with the atomic numbers. Two small equations give you the missing particle unambiguously. Once you have its atomic number, the periodic table gives you the element symbol.
sum of mass numbers (left) = sum of mass numbers (right); sum of atomic numbers (left) = sum of atomic numbers (right)
Stable light nuclei sit near a neutron-to-proton ratio of about 1:1; heavier stable nuclei run richer in neutrons. A nuclide with too many neutrons converts one into a proton by beta-minus (β⁻) decay. A nuclide with too few neutrons uses positron emission or electron capture. Nuclides heavier than bismuth-209 are too large to be stable at all and shed mass by alpha decay.
A helium-4 nucleus: mass number 4, charge +2.
An electron emitted when a neutron becomes a proton: mass number 0, charge -1.
A particle with the electron's mass and the opposite charge: mass number 0, charge +1.
Mass number and atomic number are each conserved; ordinary chemical mass is not.
Too many neutrons favours beta-minus (β⁻); too few favours positron emission or electron capture; too heavy favours alpha.
X(A, Z) → Y(A − 4, Z − 2) + ⁴₂He
X(A, Z) → Y(A, Z + 1) + ⁰₋₁e
X(A, Z) → Y(A, Z − 1) + ⁰₊₁e
Uranium-238 undergoes alpha decay. Write the balanced nuclear equation and name the daughter nuclide.
Try it first: Before reading on, subtract the alpha particle's mass number and atomic number from uranium's and see which element you land on.
0 of 4 steps revealed.
carbon-14 decays by beta-minus (β⁻) emission. Write the balanced equation.
0 of 4 steps revealed.
Fill in the missing particle: ⁷₄Be + ⁰₋₁e → ? (electron capture)
0 of 3 steps revealed.
Why it's wrong: The nucleon count does not change in beta decay, only the identity of one nucleon; the emitted electron has mass number 0.
Check instead: Always write the beta particle as mass 0, charge -1, then confirm the mass numbers already balance.
Why it's wrong: Beta-minus (β⁻) raises the atomic number, so the daughter is one place to the right, not the left.
Check instead: Work out the daughter's atomic number numerically first, then look up that number on the table.
Why it's wrong: A gamma ray carries no mass and no charge, so both numbers are unchanged.
Check instead: Treat gamma emission as the nucleus dropping to a lower energy state, nothing more.
No practice questions are available for this topic yet. You can still practice the whole unit.
Radioactive nuclei become more stable by emitting particles or energy. Alpha decay ejects a helium-4 nucleus, lowering the mass number by 4 and the atomic number by 2. Beta-minus (β⁻) decay converts a neutron into a proton and ejects an electron, raising the atomic number by 1 at constant mass number. Positron emission and electron capture do the opposite, lowering the atomic number by 1. Gamma emission releases energy only and changes neither number. Every nuclear equation must balance twice: the sum of mass numbers on the left equals the sum on the right, and the same is true of the atomic numbers.
This lesson is original Chem Help content. No external sources were adapted.