Which of the following equations represents beta decay?

Analyze the question: Beta decay is a nuclear change in which a neutron in the nucleus changes into a proton and an electron is emitted. The emitted particle is a beta particle, written as ⁰₋₁e.

Concept:

• The mass number (top) stays the same.
• The atomic number (bottom) increases by 1, because a neutron becomes a proton.
• A beta particle (⁰₋₁e) appears on the product side.

Checking the options (the printed formulas have typos, so I read them as the intended equations):

• A. ¹⁴₇N + ⁴₂He → ¹⁷₈O + ¹₁H. This is artificial transmutation, where nitrogen is bombarded with alpha particles. Nothing is emitted spontaneously, so it is not beta decay.
• B. ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. The mass number drops by 4 and the atomic number by 2. This is alpha decay.
• C. ²³⁸₉₂U → ²³⁹₉₃Np + ⁰₋₁e. The atomic number rises from 92 to 93, and a beta particle (⁰₋₁e) is emitted. This is the beta-decay pattern.
• D. ¹³₇N + ⁴₂He → ¹⁶₈O + ¹₁H. This is another bombardment reaction (transmutation), not decay.

A note on option C: As printed, the mass numbers do not balance (238 on the left, 239 on the right). This looks like a typo in the question. A balanced version would be ²³⁹₉₂U → ²³⁹₉₃Np + ⁰₋₁e, which is real beta decay. Also, "8238" in options B and C is just a misprint of 238. Even with the typo, C is the only option that shows the beta particle and the increase in atomic number by 1, so it is the intended answer.

Answer: C

Exam tip: Look at the emitted particle.

• ⁴₂He means alpha decay (mass number −4, atomic number −2).
• ⁰₋₁e means beta decay (mass number unchanged, atomic number +1).
• If a small particle is hitting a nucleus on the left side, it is transmutation, not decay.