why the reactivity of halogen element is decrease?

鹵素 (halogen) 原子的最外層電子數是7，它們趨向得一粒電子達到穩定貴族氣體 (noble gas) 的電子排列。位置越高的元素，越容易得到電子，因為電子殼越少，原子核和最外層電子殼距離越短，對來自外界電子的吸引力越大。

我的英文不太好，參考一下就算...

The number of outermost shell electrons in the atom of halogen is 7. They tend to gain 1 electron to obtain the stable electronic arrangement as noble gas. The halogen at high position, they are easy to gain electron. It is because they have less electron shell, the distance between the nucleus and the outermost shell is short, the attractive force between the nucleus and the electron form surroundings is strong.

卥素最外層電子數目係7粒
佢想變個穩定的話,佢最外層電子數目要係8粒,以達至貴氣體最外層電子數目(最穩定)
卥素就要吸1粒電子,以變成8粒最外層電子數目
而每增加一層電子殼,原子核同最外層電子殼ge距離增加
所以原子核對最外層電子ge吸力就下降,
由於卥素這一族,越下ge 元素,原子ge體積就越大,電子殼越多,所以就不斷增加吸多粒電子ge難度


The number of outermost shell electrons of halogen is 7.
Since the electric configuration of noble gas is the most stable(8 outermost shell electrons , except for helium),so halogen will gain an electron to become stable.
When halogen element down in group,the size of halogen atom increase and the distacne between outermost shell and nuclues increase,the attraction between the outermost shell electron and nuclues will decrease.It is more difficut to gain electron in low attention....