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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Basically the interactions between the individual molecules, notably how attracted they are to each other.
The molecules are always moving. The temperature of stuff is related to the the energy of the molecules’ movement.
If the attraction is so strong that the molecules are stuck in place and can only vibrate, that’s a solid.
If the attraction is strong enough that the molecules are stuck together but can move around each other, that’s a liquid: the molecules can move around so the material can take the shape of its container–but because they’re all stuck together, the volume is effectively constant.
If the attraction is too weak to prevent the molecules from wanting to fly everywhere, that’s a gas: it will expand to fit its container.
The inter-molecular interactions are determined largely by the way/shape the negative electric charge (of the electrons) get distributed around the positive electric charges (of the protons in the atoms’ nuclei).
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