卤素Halogen

从上到下分子中电子增多，色散力增强，熔沸点升高，颜色加深：F₂ 为淡黄色气体，Cl₂ 为黄绿色气体，Br₂ 为红棕色液体，I₂ 为紫黑色固体（蒸气紫色）。

Down the group molecules have more electrons, dispersion forces strengthen, and melting and boiling points rise while colours deepen: F₂ is a pale yellow gas, Cl₂ a yellow-green gas, Br₂ a red-brown liquid and I₂ a grey-black solid with violet vapour.

从上到下原子半径增大，得电子能力减弱，单质氧化性减弱：F₂ > Cl₂ > Br₂ > I₂。较活泼的卤素能把较不活泼卤素从其卤化物中置换出来，如 Cl₂ + 2KBr → 2KCl + Br₂。卤化氢的热稳定性从 HF 到 HI 依次减弱（H–X 键能减小），而 HCl、HBr、HI 在水中都是强酸，HF 是弱酸。

Down the group the atoms get larger and attract electrons less, so the elements become weaker oxidisers: F₂ > Cl₂ > Br₂ > I₂. A more reactive halogen displaces a less reactive one from its halide, e.g. Cl₂ + 2KBr → 2KCl + Br₂. The thermal stability of the hydrogen halides falls from HF to HI as the H–X bond weakens, while HCl, HBr and HI are strong acids in water and HF is weak.

在稀硝酸酸化的溶液中加入 AgNO₃：Cl⁻ 生成白色 AgCl，溶于稀氨水；Br⁻ 生成奶油色 AgBr，溶于浓氨水；I⁻ 生成黄色 AgI，不溶于氨水。

Add AgNO₃ to the solution acidified with dilute nitric acid: Cl⁻ gives white AgCl, soluble in dilute ammonia; Br⁻ gives cream AgBr, soluble in concentrated ammonia; I⁻ gives yellow AgI, insoluble in ammonia.