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Periodic table trends?

Why do the melting/boiling points of the alkali metals decrease down the group and yet the melting/boiling points of the halogens increase down the group? What is the difference between the two groups that cause this variation?
The trend of the alkali metals is due to a weakening of their metallic bonds. As the atoms get bigger, the nuclei of the ions get further away from their free electrons, so the attraction is less.

The trend of halogens is due to an increase of van der Waals forces, caused by electrons shifting to one side.

This isn't required for GCSE science though, I believe.
Original post by epaul
Why do the melting/boiling points of the alkali metals decrease down the group and yet the melting/boiling points of the halogens increase down the group? What is the difference between the two groups that cause this variation?

Alkali metals (Group 1) have 1 electron in their outer shell, and chemical reactions involve losing their outer electron to become stable. As you go down the group, the atoms get bigger, and the negative electron on the outer shell gets easier to lose as (a) it gets further from the positive nucleus to which it is attracted and (b) there is more shielding between it and the nucleus with the negative electrons on the other shells.

For halogens (Group 7) all of the above is the exact opposite. They have 7 electrons and need to gain 1 to become stable. As the outer shell gets further from the attraction to the positive nucleus, this gets more difficult, so the smaller atoms are more reactive.

This is what we learn and I can only assume that more reactive elements are necessarily more volatile and require less energy to change state, so they have lower melting/boiling points. But it might be a good idea to not take this as fact :wink:
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Original post by Dragolien
The trend of the alkali metals is due to a weakening of their metallic bonds. As the atoms get bigger, the nuclei of the ions get further away from their free electrons, so the attraction is less.

The trend of halogens is due to an increase of van der Waals forces, caused by electrons shifting to one side.

This isn't required for GCSE science though, I believe.


I'm not really sure to be honest; some say it is required for GCSE and other say it's not. But hey, I'm interested anyway. Thank you 😊
Original post by epaul
I'm not really sure to be honest; some say it is required for GCSE and other say it's not. But hey, I'm interested anyway. Thank you 😊


You don't need to know about van der Waals forces (at least not for AQA), but you do need to know that for halogens, the attraction forces between electrons and the nucleus get smaller as the atom gets bigger so it is less able to take an electron to react. You also need to know that the inner electrons block some of the attractive force - for AQA C3 it's enough to say that there's a shielding effect.

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