Chemistry

In reaction, each halogen gains an extra electron to attain its octet (8 electrons in the outermost shell).
在反應中，每個鹵原子獲得一個額外電子，以完成其八隅體（最外層有 8 粒電子）。

When going down Group VII, the size of halogen atom is gradually increasing due to the increase in number of shells. A smaller halogen atom attracts the extra electron more strongly because of the shorter distance between the extra electron and the nucleus, and this leads to a more stable product and a faster reaction. Therefore, the halogens near the top of the group are more reactive.
沿第 VII 族向下，由於電子層數的增加，鹵原子的體積遞增。較細小的鹵原子，因為額外電子與原子核間的距離較短，所以對額外電子的吸引力較強，導致更穩定的生成物以及較快的反應。故此，接近鹵族頂部的鹵素，活性較高。

Electronegativity decrease down the halogens.

2013-11-21 22:44:24 補充：
Electronegativity of halogens
F (3.98) Cl (3.16) Br (2.96) I (2.66) At (2.2)

electronegativity 是元素(element)對電子(electron)的吸引力大小的比較。electronegativity愈大，對電子的吸引力愈強。