# Trend in the melting point down the group

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 - $\pu{302^\circ C}$
• 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. – Ben Norris Jul 10 '15 at 10:44
• @BenNorris That could be a very good question – user15489 Jul 10 '15 at 11:56
• @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. – Ben Norris Jul 15 '15 at 0:31