定义Definition
元素周期律指元素的性质随原子序数的递增而呈周期性变化的规律。其本质是核外电子排布的周期性重复：最外层电子构型相似的元素性质相似。元素周期表是周期律的表现形式。
The periodic law states that the properties of the elements vary periodically with increasing atomic number. Its basis is the periodic repetition of electron configurations: elements with similar outer-shell configurations have similar properties. The periodic table is the expression of this law.
主要规律Main trends
同周期从左到右：原子半径减小，第一电离能和电负性总体增大，金属性减弱、非金属性增强，氧化物由碱性经两性变为酸性。同主族从上到下：原子半径增大，电离能和电负性减小，金属性增强。
Across a period from left to right, atomic radius decreases, first ionisation energy and electronegativity generally increase, metallic character falls and non-metallic character rises, and oxides change from basic through amphoteric to acidic. Down a group, atomic radius increases, ionisation energy and electronegativity decrease and metallic character increases.
发现过程Discovery
1869 年门捷列夫按原子量排列元素，为未知元素留出空位并预言了它们的性质，后来镓、钪、锗的发现证实了预言。1913 年莫塞莱证明原子序数才是排序的依据，解决了碲与碘等元素顺序颠倒的问题。
In 1869 Mendeleev arranged the elements by atomic weight, left gaps for unknown elements and predicted their properties; the discovery of gallium, scandium and germanium confirmed them. In 1913 Moseley showed that atomic number is the true basis of the order, resolving reversals such as tellurium and iodine.
年表Timeline
- 1829
德贝莱纳提出三元素组
Döbereiner proposes his triads
- 1865
纽兰兹提出八音律
Newlands proposes the law of octaves
- 1869
门捷列夫发表元素周期表
Mendeleev publishes his periodic table
- 1913
莫塞莱确立原子序数为排序依据
Moseley establishes atomic number as the basis of the order