For example, at room temperature H2O (water) is a liquid, CH4 (methane) is a gas, NaCl (salt) is a solid, why?
What are the properties of a molecule that determines its state?
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Aah, this is due to what is called “Hydrogen liaisons” or “Van der Vaals liaisons”. (Let’s stick to hydrogen).
TL:DR at the bottom, but I tried to keep it understandable below.H2O and CH4 are perfect examples.There are a few notions to unpack:
– 1 First: an atom is made of a *core* of neutrons and protons. Orbiting this core are as many electrons as there are protons.
– 2 Second: in both example you have a central atom, Oxygen (O) or Carbon (C), surrounded by Hydrogen (H) atoms. Each hydrogen atom has a *covalent* liaison with the center atom. It’s liaison made by two electrons, one from each atom, that are now common to both. It’s extremely solid, hard to break. That makes a molecule.
– 3 Third: the core of those two center atoms are more massive than the Hydrogen. O has 8 protons and 8 neutrons, C has 6 protons and 6 neutrons. H only has… 1 proton.
– 4 Fourth: because of this, electrons in a O-H liaison and in a C-H liaison are closer to O and C than they are to H. This means that you have in average a more *negative* electric charge around O and C and a more *positive* charge arouns each H. (Electrons are charged negatively).
– 5 Fifth: Due to reasons, O is better at attracting those electrons than C.- 6 Sixth: Structure ! It gets more complicated here:So, electrons always go in pairs, and an atom has “layers” of “boxes” in which you can put 2 electrons. And only the “outer” layer is used to create liaisons.For reasons, the outer layer has 4 boxes. The one from O has 6 electrons while C has 4.
Now, the way electrons fill those boxes is they find an empty one. If there are none, then they go pair up with an electron already in a box.
So C has 4 boxes with 1 electron each. That’s why it can have 4 H around: C has 4 lonely electrons waiting to pair up.
O has two full boxes and two boxes with one electron. That’s why it can only have 2 H around.
BUT ! Electrons are charged, and similar charges push against each other until equilibrium (like magnet).
CH4 has 4 *identical* liaisons. They push each other the same way to CH4 is perfectly simmetrical in 3D (as seen here: [https://csg-prd.tinkercad.com/things/aTu6bqeu0ac/t725.png?rev=120&s=&v=0](https://csg-prd.tinkercad.com/things/aTu6bqeu0ac/t725.png?rev=120&s=&v=0))
The O in H2O has two full boxes and two liaisons. Take the above CH4 molecule. Replace two H atoms with full boxes and you now have this [https://1.bp.blogspot.com/-OlnUXIHKtDg/Teh5ekJUTjI/AAAAAAAAAD8/UIgoC2JK2PI/s1600/770px-Water-2D-flat.png](https://1.bp.blogspot.com/-OlnUXIHKtDg/Teh5ekJUTjI/AAAAAAAAAD8/UIgoC2JK2PI/s1600/770px-Water-2D-flat.png).
This is 2D, but the placement is very much 3D. However, since it’s only 3 atoms, you can consider H2O as a “2D” particule.
Notice that now you have 4 electrons on one side of H2O and 2 Hydrogen atoms on the other. And remember point 4: *The two H in H2O have a slightly positive charge*
So now H2O is negative on one side and positive on the other side. What happens when you put two H2O next to each-other ? The positive side of one attracts the negative side of the other. And now those molecule will be harder to separate. It’s why it’s a liquid. We call this a *hydrogen liaison* (because it’s made with hydrogen atoms).
CH4 now: It’s all symmetrical. Negative charges *all around*. CH4 molecules cannot go together. They are free to roam around and that’s why it’s a gas.
Now for NaCl it’s because it’s a crystal. Strong liaisons all around. Very hard to break.
**TL;DR: H2O molecules are electrically positive on one side and negative on the other side. Those two sides attract the opposite side of another H2O molecule and they stick more to each other. CH4 doesn’t have this property and thus its molecules can move more freely.**
(If you feel I wasn’t clear on some points, feel free to ask).
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