ハロゲンと水酸ハロゲン

Reactive non-metals

• Each halogen gains an electron to form a $1-$ ion — so it is an oxidising agent 氧化剂.
• Its oxidising power decreases down the group.
• The hydrogen halides 卤化氢 get less stable down the group.

Oxidising power and displacement

• A halogen acts as an oxidising agent (it gains one electron). This power falls down the group (larger atoms attract the extra electron less).
• A more reactive halogen displaces 置换 a less reactive one from its salt:
  $$\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$$

Halogen oxidising power falls and halide reducing power rises down the group

Reactions with hydrogen

• Each halogen forms a hydrogen halide: $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$.
• The reaction gets less vigorous 剧烈 down the group (chlorine explosive in light; iodine slow and partial).

The silver halide test: AgCl is white, AgBr cream and AgI yellow

Thermal stability 热稳定性 of HX

• The thermal stability of the hydrogen halides decreases down the group.
• The H–X bond gets weaker as the halogen gets larger, so HI breaks apart on gentle heating while HCl is very stable.

You've got it