F.6 CHEM (bonding)

2009-03-21 6:04 am

State and explain how the melting point of the elements/compounds
change
a) in GroupVII from fluorine gas and iodine gas

Undoubtedly, the no. of electron shell increases down the group.

However, is the melting point decrease down the group because the
outermost shell electrons experience less nuclear attraction (stronger
screening effect) or the melting point increase down the group
because more electrons are present and thus are more diffuse and
polarizable, resulting in stronger intantaneos dipole-induced dipole
interaction?

b) in hydrides of Group VI from water(H2O) toH2Te ?

回答 (1)

老爺子

2009-03-21 7:06 am

✔ 最佳答案

a)
Going down Group VII, the melting point decreases down the group. This is mainly due to increase in molecular size down Group VII. The molecules of Group VII elements are non-polar, and thus the molecules are held together by instantaneous dipole - induced dipole attractions (one form of van der Waals' attraction). Electron clouds in larger molecules are distorted more readily, and this would lead to greater instantaneous dipole - induced dipole attractions. Consequently, greater molecular size would lead to higher melting point.

b)
Among the hydrides of Group VI elements, water has abnormally high melting point. This is because water molecules are held together by intermolecular hydrogen bonds which are the strongest intermolecular attractions, while others are held together by van der Waals' forces. The melting point is increasing from H2S, H2Se and then H2Te. Down Group VI, the molecular size increases from H2S to H2Te, and thus the van der Waals' forces increase from H2S to H2Te.
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