定义Definition
原子半径是衡量原子大小的量。由于电子云没有明确边界，原子半径由实验测得的核间距推算：共价半径为同种原子共价单键键长的一半，金属半径为金属晶体中相邻原子核间距的一半，范德华半径为不成键原子最近接触距离的一半。
Atomic radius measures the size of an atom. Because the electron cloud has no sharp edge, it is derived from measured internuclear distances: the covalent radius is half the single-bond length between identical atoms, the metallic radius half the distance between neighbouring nuclei in a metal, and the van der Waals radius half the closest distance between non-bonded atoms.
周期性规律Periodic trends
同周期从左到右，核电荷增加而电子层数不变，有效核电荷增大，原子半径减小（Na 186 pm → Cl 99 pm）。同族从上到下，电子层增多，原子半径增大（F 71 pm、Cl 99 pm、Br 114 pm、I 133 pm）。过渡元素的原子半径变化较小。
Across a period, nuclear charge rises with the same number of shells, the effective nuclear charge increases and atoms shrink (Na 186 pm → Cl 99 pm). Down a group extra shells make atoms larger (F 71 pm, Cl 99 pm, Br 114 pm, I 133 pm). Transition-metal radii change only slightly.
影响Consequences
原子半径影响电离能、电负性、金属性以及键长和键能：半径越大，最外层电子越容易失去，金属性越强；卤素从上到下半径增大，与碳形成的 C–X 键越长越弱。
Atomic radius affects ionisation energy, electronegativity, metallic character, bond length and bond strength: larger atoms lose outer electrons more easily and are more metallic; down the halogens the C–X bond becomes longer and weaker.
年表Timeline
- 1920
布拉格由晶体结构数据整理原子半径
W. L. Bragg compiles atomic radii from crystal structures
- 1964
邦迪发表范德华半径表
Bondi publishes his table of van der Waals radii