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I was wondering, as you go down a group, the atoms get bigger... This means that the electrons in the outer shell are further away from the nucleus, and so the attraction decreases and it is easier for the atom to lose electron, thus becoming more reactive, as is the case for group 1 (the alkali metals).

However, as you go down group 7 (the halogens), reactivity actually decreases, and the reason given is that it is harder for it to gain electrons, which contradicts the previous statement? Any ideas?

I'm guessing that because halogens only need 1 more electron to complete their shell; they need to be small (to have a strong attraction between the nucleus and the electrons) and vise versa for the alkali metals.

P.S. Please keep answers brief and generalise as much as possible. The answer need only be correct to a GCSE standard, thanks.

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  • $\begingroup$ " The answer need only be correct to a GCSE standard, thanks." -- Is that another way of saying "the answer doesn't need to be correct"? :P $\endgroup$ – M.A.R. ಠ_ಠ Nov 6 '15 at 18:55
  • $\begingroup$ Lol maybe, it's all a matter of perspective.... I just put that in there because whenever I ask question, I get this degree standard explanations that are simply unecessary. $\endgroup$ – Vedbot Nov 7 '15 at 15:22
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Yes, you are correct in your approach.

Larger alkali metals means that there is less Z-effective or effective nuclear charge between the central nuclei and the valence electrons due to a screening/shielding effect. This allows them to be more reactive as they can lose electrons easily.

In halogens, to react more, they will need to be able to accept more electrons. This means that if the atom is smaller, the Z-effective is large, the nuclear attraction is high and it is more reactive. For a large atom, it is tougher to accept electrons due to reduced nuclear charge because of increased screening/ shielding effect.

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  • $\begingroup$ Ahh I see, thanks. So what you're saying is that the effect of the size of the atom (the nuclear attraction) changes depending on whether you need more or less electrons. $\endgroup$ – Vedbot Nov 7 '15 at 15:25
  • $\begingroup$ Kind of, but not exactly. If the nuclear charge increases, it becomes harder for the atom to lose the valence electrons. Reactivity depends on how easy or difficult it is for an atom to lose the valence electrons. $\endgroup$ – Malhar Khushu Nov 7 '15 at 15:39
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The answer to your question is quite simple.The alkali metals of group 1 grow larger in size downwards and since the effective nuclear charge remains the same as we go down group 1 the number of shells also increases and so it is easy to lose an electron and therefore as we move downwards the reactivity increases.

However in group 17 the halogens have to gain an electron. So when the size increases and the number of shells increases the attraction of electrons decreases as we move away from the nucleus and so it is difficult to gain an electron as we move downwards.

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