|MadSci Network: Chemistry|
The process of fusion/melting of a solid has to do with the interactions, attractive and repulsive, between the units that compose the corresponding solid.
If the attractive forces are very strong compared to the repulsive ones then we will need to add an extra energy to the solid to break those attractions. This means that we will need a higher temperature to melt the solid, or it will solidify again at a higher temperature.
If the repulsive forces are instead the most important ones then we will not need such a high temperature to melt the solid, or we will need a lower temperature to solidify it.
The attraction and repulsion forces always have to do with the interactions of the outer shell of electrons of atoms forming molecules. The attractions can be coulostatic, as molecular or atomic fragments charged differently attract each other and viceversa. If the electrons are shared, this is, the electrons can go from one atomic center to other without actually feeling being a property of any of them in particular, the atoms will be kept together in the meantime.
In the case of oxygen and fluorine (and nitrogen will be added to the group for the sake of clarity), they show a common structure, A2, the elements arranging in molecules composed by two atoms. The solid we obtain when we cool down these substances is formed by these molecular units, not by isolated atoms. This molecular structure is very stable that is another way of saying that the molecules do not show much attraction between them.
As we know, the most stable configuration for an atom is having its outer shell with the same number of electrons of the closer noble gas (typically eight, an octet). Molecules are formed by atoms so that, either by giving or taking electrons or by sharing them, they can all have that octet complete. In the case of nitrogen, oxygen and fluorine, they share a different number of electrons, but they all have eight electrons in that outer shell.
N2 shares 6 electrons between the two nitrogen atoms, giving out a triple bond (2x3=6) and leaving each atom with another couple of non-bonding electrons.
O2 shares 4 electrons between the two oxygen atoms, giving out a double bond (2x2=4) and leaving each atom with 2 couples, 4 non-bonding electrons.
F2 shares 2 electrons between the two fluorine atoms, giving out a single bond (2x1=1) and leaving each atom with 3 couples, 6 non-bonding electrons.
Mol. bond order mp (K) A-A distance (pm) Fusion enthalpy (kJ/mol) N2 3 63.7 109 0.72 O2 2 54.4 122 0.44 F2 1 53.5 143 0.51(Data obtained from GREENWOOD,N "Chemistry of Elements". 2nd Ed. Reed Educational and Professional Publishing Ltd. 1997)
Summarizing, apart from the fact of having a bigger mass, we have to look for the differences in the electronic outer shell of the molecules studied. We can see that N2 only has 4 non-bonding electrons, while O2 has 8 and F2 has 12. This makes the accumulation of negative charge in that outer shell much higher in the case of fluorine molecule and that will produce a bigger repulsion between molecules, bigger than the attraction provided by the increase of mass, so needing a lower temperature to solidify it, or we can say, it will melt at a lower temperature.
And just in case it was too easy, the rate of increase of charge has also to do with the rate of growth of the atomic radius: a bigger atomic number means a bigger number of positive charges and therefore the same number of electrons will add to the atom. But a bigger positive charge means a bigger attraction on those added electrons, so the size of the atom will not increase as much as could be expected.
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