A radioactive nucleus that undergoes alpha emission emits a helium nucleus, ⁴₂He. The remaining nucleus has two fewer protons and two fewer neutrons: the atomic number declines by two and the mass number by four.

For example: ²³⁸₉₂U undergoes alpha decay: its product is ²³⁴₉₀Th

²³⁸₉₂U -> ²³⁴₉₀Th + ⁴₂He

Example: ²³⁹₉₄Pu, the isotope of plutonium used in atomic bombs, undergoes alpha decay. What is the product?

Solution: Since a helium nucleus is emitted, the atomic number (94) declines by 2, the mass number (239) declines by 4. The new atomic number, 92, indicates that the decay creates an atom of uranium.

²³⁹₉₄Pu -> ²³⁵₉₂U + ⁴₂He

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