同位素Isotope

同位素的电子数和电子排布相同，因此化学性质几乎相同。它们的质量不同，所以密度、扩散速率、熔沸点等物理性质略有差别，例如重水 D₂O 的沸点为 101.4 °C。质量差异也使含重同位素的键反应稍慢，称为动力学同位素效应。

Isotopes have the same number and arrangement of electrons, so their chemical properties are almost identical. Their masses differ, so physical properties such as density, rate of diffusion and melting and boiling points differ slightly; heavy water D₂O, for example, boils at 101.4 °C. The mass difference also makes bonds to heavier isotopes react slightly more slowly, the kinetic isotope effect.

元素的相对原子质量是其各种天然同位素相对同位素质量按丰度的加权平均值，这就是它往往不是整数的原因。例如氯由约 75.8% 的 ³⁵Cl 和 24.2% 的 ³⁷Cl 组成，Ar(Cl) ≈ 35.45。同位素的质量和丰度用质谱仪测定。

The relative atomic mass of an element is the abundance-weighted mean of the relative isotopic masses of its naturally occurring isotopes, which is why it is often not a whole number. Chlorine, for example, is about 75.8% ³⁵Cl and 24.2% ³⁷Cl, giving Ar(Cl) ≈ 35.45. Isotopic masses and abundances are measured with a mass spectrometer.

有的同位素原子核稳定，有的会自发衰变，称为放射性同位素。¹⁴C 的半衰期约 5730 年，用于测定考古样品的年代；¹³¹I 用于甲状腺疾病的诊断和治疗；⁶⁰Co 用作 γ 射线源。稳定同位素如 ²H、¹³C、¹⁵N、¹⁸O 常用作示踪原子，研究反应机理。

Some isotopes have stable nuclei, while others decay spontaneously and are called radioactive isotopes. ¹⁴C, half-life about 5730 years, is used to date archaeological samples; ¹³¹I is used to diagnose and treat thyroid disease; ⁶⁰Co serves as a gamma-ray source. Stable isotopes such as ²H, ¹³C, ¹⁵N and ¹⁸O are used as tracers to study reaction mechanisms.