Balanced equations for radioactive decay

A decay equation shows the nucleus before and after emission. It is balanced when the total mass number on the left is equal to the total mass number on the right, and the total atomic number on the left is equal to the total atomic number on the right. An alpha particle is written as a helium nucleus, ⁴₂He. A beta particle is written as an electron, ⁰₋₁e, with a mass number of 0 and an atomic number of −1.

Alpha decay of uranium-238: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. The mass numbers balance because 238 = 234 + 4, and the atomic numbers balance because 92 = 90 + 2.

Beta decay of carbon-14: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. The mass numbers balance because 14 = 14 + 0, and the atomic numbers balance because 6 = 7 + (−1). The new nucleus has atomic number 7, so it is nitrogen.