元素周期表Periodic table

元素性质随原子序数呈周期性变化，根本原因是核外电子排布的周期性。周期表分为 s、p、d、f 四个区，分别对应最后填充的轨道类型。同周期从左到右原子半径减小、电负性和电离能增大；同主族从上到下金属性增强。

Properties vary periodically with atomic number because electron configurations repeat periodically. The table divides into s, p, d and f blocks according to the type of orbital being filled. Across a period atomic radius decreases while electronegativity and ionisation energy increase; down a main group metallic character increases.

1817 年德贝赖纳提出“三元素组”；1864 年纽兰兹提出“八音律”。1869 年门捷列夫按相对原子质量排列已知的 63 种元素，并大胆留下空位，预言了“类铝”“类硅”等未知元素的性质；1870 年迈耶尔也独立发表了类似的表。随后镓（1875）、钪（1879）、锗（1886）的发现与预言惊人吻合，使周期律得到公认。

Döbereiner proposed triads in 1817 and Newlands his law of octaves in 1864. In 1869 Mendeleev arranged the 63 known elements by atomic weight, boldly leaving gaps and predicting the properties of eka-aluminium, eka-silicon and others; Meyer independently published a similar table in 1870. The discoveries of gallium (1875), scandium (1879) and germanium (1886) matched the predictions closely and won the periodic law acceptance.

1894 年起稀有气体陆续发现，增加了零族（第 18 族）。1913 年莫斯莱用 X 射线光谱证明元素顺序由原子序数（核电荷）而非原子量决定，解决了 Ar–K、Co–Ni 等顺序反常。1945 年西博格提出锕系概念，把锕系元素单独列在表下。2016 年随着 Nh、Mc、Ts、Og 命名，第 7 周期全部填满。

The noble gases, found from 1894, added group 0 (group 18). In 1913 Moseley's X-ray spectra showed that order is set by atomic number (nuclear charge), not atomic weight, resolving the Ar–K and Co–Ni anomalies. In 1945 Seaborg proposed the actinide series, placed below the main table. With the naming of Nh, Mc, Ts and Og in 2016, period 7 was complete.