[Junkfunkydude]

I am completely stumped as to how to what forces inside an atom control the heat of fusion and heat of vaporization. Could anyone explain it to me?



My chemistry teacher assigned a lab that involves writing a detailed description of forces within the atom that cause 12 different periodic trends of main-group elements. Of those 12, I have no problem explaining 7 of them. The rest are a little bit more difficult for me.

Here is what is left:
Melting point
Boiling Point
Heat of vaporization
Specific heat
Heat of fusion

I have noticed that they all have either a high or a low point on group 14, so there must be some interconnecting factor. I have been crawling all over Wikipedia looking for an answer, but have arrived at none. Could someone please give me a nudge in the right direction?

[constant thinker]

Think about electron configuration and delocalization. Those 2 things may help you out.

[Junkfunkydude]

Thanks for the nudge. I got to thinking, and I started wondering weather these properties are measured with single atoms, or diatomic molecules. So i guess I have a new question now.

[enahs]

Umm, those ones you have less are questionable. I would not consider the pattern of M.P. and B.P. to be any kind of real trend, at least when you only account for the Main Group elements.

[Junkfunkydude]

Well, the assignment started out with making 3 dimensional graphs. We each took a well plate with wells about the size of a straw in diameter. We each then made a graph in the shape of the periodic table (skipping the transition metals since this is only main group). We stuck different length straws in the plate for different values. It really worked well, and there were trends in all of the 12 categories. Now I would just like to know when things like melting point and boiling point are measured, are they done by single atom, or are they measured in diatomic molecules such as O² and N²?

[enahs]

Ahh, you did not say 3D trends, that opens up a whole new avenue

Yes, they are for the diatomic form. Those are the elemental forms. You do not find a single O atom in nature, or a single Cl atom in nature. You in fact, can not even really to any extent force them that way, they are to highly reactive.

[Junkfunkydude]

As much as I hate to admit it, I am alas throughly stumped with heat of fusion and vaporization. In all groups, the values increase as you move down a group accept for groups 2, 13, and 14. What on earth causes this?

[Yggdrasil]

Heat of vaporization and heat of fusion are related to the strength of intermolecular forces.  Within a group, the elements have similar properties, including the types of intermolecular forces holding the atoms/molecules together.  However, as you move down a group, the size of the atoms/molecules increases, which increases the strength of the dispersion forces between the atoms/molecules.

For groups 13 and 14, the reason for the break in the trend is due to a change in the nature of the intermolecular forces (at the top of the group the elements are nonmetals whereas at the bottom the elements are metalic).  I don't know why group 2 would disobey this trend, however.