# Thread: Why is CF4 a gas and CBr4 solid at roomtemperature??

1. ## Why is CF4 a gas and CBr4 solid at roomtemperature??

I was doing homework, but then I got wrong on an assignment..

Here is assignment: at room temperature, which substance of these; CF4, CCl4and CBr4 is liquid, gas and solid. Correct answer: CF4 (g), CCl4 (l) CBr4 (s).

OK, electro-negativity difference between Carbon and Fluoride (1.5) is FAR greater than Carbon and Brom (0.3)..

Both are polar substances, but CF4 is far more polar than CBr4, due to the difference in electro negativity.

Ergo, the bindings between CF4 molecules is stronger than those between CBr4 molecules, and thus the CF4's melting and evaporation point should be higher (it takes more energy to break the bindings between the CF4 molecules) than CBr4's....

But I am wrong. Please tell me what factors I have left out...

2. Actually, none of those substances are polar. The centre of charge of each electronegative element coincides with the centre of carbon.

What you need to consider is the Van der Waal's forces (or induced dipoles).

Following this, we see that
$CF_4$ should have the lowest boiling point, followed by $CCl_4$ and $CBr_4$.

Hence, $CF_4$ should be gas, $CCl_4$ then liquid and then $CBr_4$ solid.

However, don't forget that F is one of the three most electronegative elements and as such, can form hydrogen bonds which are about 10 times as strong as VDW forces which makes it a liquid. Thus, carbn tetrachloride becomes the gas.

3. ah yes! like with CO2. Darn I should've thought about that1.

I have no idea what Van der Waal's force is. We haven't been thought about that (I don't live in USA, I'm 17 and just started on chemistry class), so I will check it out. So thanks man, nice help!

Edit: I found the law at wikipedia but I really can't understand it.. Here they teach "natural sciences" until we're 17, and don't start teaching chemistry/physics/biology for those that want it.. So I hope you could explain to me a bit simpler?

4. I assume that the heavier the molecule is, the harder it melts/evaporates?

5. Not quite. You might want to google "Van der Waal's". The more the number of lectrons between the molecules, the stronger the forces of attraction.

6. Ah, I found out that Van Der Waal's forces are the same as "weak bindings" which they call them in my country. OK. But, what you mean above is that temporary polarizations are more common among molecules that have more electrons, right?

Like, sometimes there will be unbalances between molecules, so one has too much electrons on one side and becomes slightly polarized. This in turn polarizes the neighbouring molecule and so on - until we end up with weak polar bindings? Sorry if my lack of understanding is frustrating..

7. edit: never mind, I found a part in my book where this is explained (the more electrons a molecule has, the higher the evaporation/melting point). I must have been half-asleep when I read that part! thanks for the help, anyhow.