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    Why does the reactivity of a halogen decrease as the number of shells increases, but the reactivity of an alkali metal increases when the same thing happens?
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    One is trying to gain electrons, the other is trying to lose electrons.

    Increasing shell number makes it harder to attract more electrons, it also makes it harder to keep hold of them.
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    (Original post by flibber)
    Why does the reactivity of a halogen decrease as the number of shells increases, but the reactivity of an alkali metal increases when the same thing happens?
    Since you posted this under the Secondary School tag, I can't really explain it in terms of A-Level

    As Pigster says Halogens and Alkali metals (both as you go down the group) have more energy shells

    Halogens try to gain a full config by adding another electron however Alkali metals try losing one electron, understood so far?

    It's harder for Halogens (as you go down the group) to gain that extra electron because factors such as distance between the outermost electron and the nucleus (the attraction holds the electron in position however it becomes harder if the distance is wider) which is why the reactivity of Group 7 elements decrease as you go down the group

    Its easier for Alkali metals (as you go down the group) to lose an electron because the distance becomes wider (between the outermost electron and the nucleus) so they can remove it quicker since the attraction isn't weaker so the reactivity increases as you go down the group of Group 1
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    Another thing to note is that the halogens become less reactive down a group because the electronegativity decreases. Fluorine is so electronegative that it can rip electrons off pretty much anything, making it very reactive
 
 
 
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