质量数Mass number

核素符号 ᴬ_ZX 中，左下角 Z 为质子数（原子序数），左上角 A 为质量数。中子数 N = A − Z。例如 ²³₁₁Na 含 11 个质子、12 个中子、11 个电子；²³₁₁Na⁺ 有 10 个电子；³²₁₆S²⁻ 含 16 个质子、16 个中子、18 个电子。

In the symbol ᴬ_ZX, the lower left Z is the proton (atomic) number and the upper left A the mass number, so the neutron number is N = A − Z. ²³₁₁Na has 11 protons, 12 neutrons and 11 electrons; ²³₁₁Na⁺ has 10 electrons; ³²₁₆S²⁻ has 16 protons, 16 neutrons and 18 electrons.

质子和中子的相对质量都约为 1，电子质量可忽略，所以某同位素的相对同位素质量近似等于其质量数，但并不完全相等（¹²C 定义为 12 除外），原因是核子质量略有差别以及结合能造成的质量亏损。同一元素的不同同位素质量数不同。

Protons and neutrons each have a relative mass of about 1 and electrons are negligible, so the relative isotopic mass of an isotope is close to its mass number, but not exactly equal (except for ¹²C, defined as 12), because nucleon masses differ slightly and binding energy causes a mass defect. Different isotopes of an element have different mass numbers.

核反应前后质量数之和与质子数之和都守恒，可用来配平核反应方程式。例如 α 衰变 ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He，β⁻ 衰变 ¹⁴₆C → ¹⁴₇N + ⁰₋₁e。

In nuclear reactions the totals of mass numbers and of proton numbers are both conserved, which is used to balance nuclear equations: alpha decay ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He, beta-minus decay ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.