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We know that the atomic radii increases down the group. So, less energy is required to pull out the outermost electron as we go down the group, hence, gradually melting point decreases down the group. The topmost element has large melting point because of its small atomic radii and so more energy is required to pull out its outermost electron.

But, it is observed that the melting point slightly increases in case of the bottom-most element of group as compared to the previous element. Why?

Examples:

  1. Group 13 - indium - $\pu{156.6^\circ C}$ but thallium - $\pu{304^\circ C}$
  2. Group 14 - tin - $\pu{231.92^\circ C}$ but lead - $\pu{327.46^\circ C}$
  3. Group 17 - iodine - $\pu{113.5^\circ C}$ but astatine - $\pu{302^\circ C}$
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    $\begingroup$ I am curious about the melting point of astatine. The Wikipedia article lists it as the value you present, but in the body of the article it goes on to say: The bulk properties of astatine are not known with any certainty. Research is limited by its short half-life, which prevents the creation of weighable quantities. A visible piece of astatine would immediately vaporize itself because of the heat generated by its intense radioactivity. It seems to me that this problem precludes determining its melting point. $\endgroup$
    – Ben Norris
    Jul 10, 2015 at 10:44
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    $\begingroup$ @BenNorris That could be a very good question $\endgroup$
    – user15489
    Jul 10, 2015 at 11:56
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    $\begingroup$ @santiago - I found my answer in the same Wikipedia article: Most of the physical properties of astatine have been estimated (by interpolation or extrapolation) ... The melting and boiling points of astatine are also expected to follow the trend seen in the halogen series, increasing with atomic number. $\endgroup$
    – Ben Norris
    Jul 15, 2015 at 0:31

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There's a counter effect in play here: van der Waals forces. As you become larger and larger, induced dipole-induced dipole interactions become stronger and stronger.

periodic table

You'll notice that, in halogens (where abstracting an electron is a fairly tall order), the trend is that, the bigger your atom, the higher your boiling point, regardless of where you are in the group. The boron group loses the conflict of ease of abstraction versus van der Waals forces at around the gallium/indium leap, and for the carbon group, the conflict is lost to van der Waals forces only between tin and lead.

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    $\begingroup$ I agree with your explanation for the halogens but I'm less convinced by the other two. The boiling points of group 13 and 14 elements decrease down the group, which is opposite to the trend suggested by van der Waals interactions. I suspect that the increase in melting point results from the change in crystal structure and the increasing metallic character of the elements as you go down the group. $\endgroup$
    – bon
    Jul 13, 2015 at 14:13

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