London forces.
All of them are covalent compounds – I know
you may find strange for metals to make covalent bonds, but you know all rule
has its exceptions. Take a look at their electronegativity, and calculate the
difference:
EnGe – EnH < 2
EnSn – EnH < 2
So all these differences are below 2. Usually,
when the difference is lower than 2, they are covalent. Besides, if they are
gasses, there’s no way they can be ionic!
Now that we know they are all covalent, let’s
take a look at its geometry. C, Si, Ge, and Sn, are all within group IVA in a
traditional periodic table, which means they all have 4 outer electrons. So
they need 4 bonds to complete their octete – that’s why they bond with 4 H.
Remember H has 1 electron and is stable with 2 electrons, so it also needs 1
bond.
So all of them have 4 bonds, one with each H.
And no free electrons. That means they are tetraedric. A tetraedric molecule where
all bonds are equal, is always non polar – because of simetry.
So
perfectly simetric non polar molecules have only London forces. London forces
depend on the number of electrons the molecule has. Sn has more electrons than
Ge, Ge has more electrons than Si, and Si has more electrons than C. This time
I am talking about the TOTAL number of electrons, not just the outer electrons.
Sn has 50 electrons, Ge has 32 electrons, Si has 14 electrons and C has 6
electrons. So C is the smaller one. The relation between the number of electrons
and their ability to generate London forces is called polarisability.
So CH4 is the less polarizable, which means
it has less London forces, so it’s easier to separate one molecule of CH4 from
another molecule of CH4, which means it is easier to make it boil – so the
boiling point is lower.
