As in: Helium is a gas but Lithium is a metal, even though they’re right next to each other on the Periodic table.
Or: Carbon is a gas but Silicon is a mineral, even though their chemical behavior is exceedingly similar due to their similar position (on different rows) of the table?
In: Chemistry
There’s a number of factors that determine the state of an element at room temperature.
By far the most important is the crystalline structure of the given element (or compound). Specifically, the force which keeps the cristal together, which is dictated by the valence electeons of an element, or the structure of a compound. Basically, the stronger the bond, the higher the melting point. Metals for example are bounded by the metallic bond, which is a very strong interaction, and thus, most metals are solid. Water uses Hidrogen bonds, which are much much weaker, so it is only a liquid. Helium is a gas, because only Van der Waals forces act between the atoms, which are very very weak. There are also others I have not mentioned (ionic and covalent bonds, dipole-dipole interactions, etc.)
The molar mass also plays a big role in the state. For example, Bromine is a liquid and not a gas, despite the fact that its crystal is held together by Van der Waals forces, just like Chlorine. That’s because Bromine is heavier, and has a bigger electron cloud, which can moph more than that of Chlorine. If You look just one period lower, You’ll find that Iodine is solid, for the same reason. Heavier atoms with larger radii.
When You are looking at larger molecules (tipically in organic chemistry) the actual shape of the molecule makes a difference too. It’s a bit more technical, but imagine You have two different molecules, made from only carbon and hidrogen both with roughly the same number of each. Only difference is that one is a long line, the other is a ball. Now, the line molecules would have more contact with eachother, thus the interaction would be stronger. This would make something like isobuthane have a lower melting point than butane, even though they are both made out of 4 carbon and 10 hidrogen atoms, just steuctured differently.
I think I listed the most relevant factors, if I left something out, or made a miatake, feel free to correct Me.
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