Thread: What determines the state of matter?

I was curious (because I didn't know) what is it about an element that determines whether it's a solid, liquid or gas?

For example, why is Mercury the only metal that's a liquid at room temperature whereas all of the others have such high melting points? Why is having 80 protons "special" in this way while 79 and 81 don't even come close?

Why are most of the low numbers gaseous yet a few aren't?

At first glance, there seems to be no apparent logical connection between the atomic number and the solidity of an element, or is there one but I'm just ignorant of it?

I do not see a why question. Things of the periodic table are found to be as we find them.
That the density of and the temperature of seems to dictate the state of matter as found here..
The abundance of Hydrogen across the Universe and that it is a very simple atom.
Looking for a why and the workings of the Big Bang and the Nova events of giant short life stars.
Searching for the high number elements of the table, are the reasons why ever found ?
I can only offer complexity and pressure of fusion.. I do not see a answer is available.. other.

Originally Posted by Daecon

I was curious (because I didn't know) what is it about an element that determines whether it's a solid, liquid or gas?

For example, why is Mercury the only metal that's a liquid at room temperature whereas all of the others have such high melting points? Why is having 80 protons "special" in this way while 79 and 81 don't even come close?

Why are most of the low numbers gaseous yet a few aren't?

At first glance, there seems to be no apparent logical connection between the atomic number and the solidity of an element, or is there one but I'm just ignorant of it?

Larger elements tend to be solids or liquids at room temperatures rather than gases. The larger the electromagnetic field, the great probability for random distribution of the electrons. This causes higher dispersion forces at any one time.

Metals have really high intermolecular forces, electron pooling, and stable crystalline structure which doesn't allow them to be gases very easily, couple this with a large ratio of dispersion force at any moment.

For matter to be in a gas state, the molecules or atoms need to have high enough thermal energy to break free from all intermolecular forces and vaporize (energy of vaporization).

Originally Posted by Daecon

I was curious (because I didn't know) what is it about an element that determines whether it's a solid, liquid or gas?

For example, why is Mercury the only metal that's a liquid at room temperature whereas all of the others have such high melting points? Why is having 80 protons "special" in this way while 79 and 81 don't even come close?

Why are most of the low numbers gaseous yet a few aren't?

At first glance, there seems to be no apparent logical connection between the atomic number and the solidity of an element, or is there one but I'm just ignorant of it?

There is an explanation but it is only qualitative and does not predict everything we find in practice.

The first thing I would do is to recognise the importance of the metal/non-metal diagonal in the Periodic Table. Elements that are gases at RTP are non-metals that form small numbers of covalent bonds, leading to light molecules with weak intermolecular attraction between them.

Metals are formed by elements that readily release one or more outer electrons (have low 1st ionisation energies). These tend to form metallic, extended lattice-type, structures and thus do not lend themselves to being gases. Non-metals do not readily release electrons, hence the preference for covalent bonding. As you go down the Periodic Table, atoms get heavier, have higher nuclear charge, thus have more electrons, and they get bigger. As a result, the outermost electrons tend to be bound more and more weakly, being further from the nucleus, so the prevalence of metals increases.

As to melting points, a low MP signifies that the barrier to atoms or molecules moving from their location in the solid is low. It is a bit of a nightmare trying to account for the trends in MP across the Periodic Table. Among the s block elements, MPs tend to decrease with increasing atomic weight, due to weaker bonding of the ions in the "sea" of electrons in the metal. In the p block the dominant influence seems to be the move to greater metallic character as one descends to greater atomic weight. In the d block, all hell breaks loose. However on the right hand extremity, Zn Cd and Hg have all got full d subshells and in the case of Hg, uniquely, the filled d subshell comes after a filled 4f subshell (cf. "Lanthanide Contraction"). The f subshell screens the overlying shells poorly from nuclear charge, so the effective nuclear charge binding the outer electrons of Hg is higher than for Cd and Zn. Consequently it bonds particularly weakly.

This is admittedly a hand-waving explanation, and I was intrigued to discover the attached article which suggests there may be even more subtle effects at work in the case of Hg: Relativity behind mercury's liquidity | Chemistry World

All in all, attempting to account for the melting point of the chemical elements is a subject for a longish undergraduate essay. Not trivial at all, and I may be challenged on some of what I have said by other readers.

Yeah, and most of those gases that you see at room temperature, have spherical geometries. Which doesn't give them much opportunity for interaction and dispersion attraction force. These atoms will not form nice long ranging molecular patterns like the metals. Most gases are in a stable diatomic formation. The noble gases which are atomic atoms and do not form molecules at all do to their inertness, neither reacting or even interacting with much.

I'm not sure what you are trying to say here, what do you mean by "spherical geometries" (you might want to look up atomic orbital shapes, only s orbitals are spherically symmetrical and most gases at STP are in the p-block, p orbitals are not spherically symmetrical). Also Noble gases can form compounds too: Noble gas compound - Wikipedia, the free encyclopedia

Originally Posted by PhDemon

I'm not sure what you are trying to say here, what do you mean by "spherical geometries" (you might want to look up atomic orbital shapes, only s orbitals are spherically symmetrical and most gases at STP are in the p-block, p orbitals are not spherically symmetrical). Also Noble gases can form compounds too: Noble gas compound - Wikipedia, the free encyclopedia

What I mean is VSEPR theory "spherical geometry" shape, that is, gases don't tend to form long ranging molecules like organic compounds or metals. Therefore they have more of a compact spherical shape, with less opportunity for dispersive attractions.

I didn't mean spherical like electron orbital spherical, yeah S orbital is not what I mean. I meant to say more compact and not long and rangy.

02 linear geometry, H2, diatomic linear geometry not much dispersion force in those either.

Lol You know more than me, the only reason I'm responding back on a chemistry forum is to defend myself.

Noble gases do make compounds for example XeF6, XeF4 or XeO4, as I have learned.. But since noble gases are so stable we are all taught that its very rare or unlikely due to it stability.

I still can't make out what you are trying to say. How does I₂ which is a solid fit into your thinking?
Response to your edit: OK that makes what you are trying to say a bit clearer although I'm still not convinced "spherical geometry" in VSEPR has anything to do with the state of matter. Whether something is a solid liquid or gas is down to the intermolecular forces (of which dispersion forces are only one type, and that is related to polarisability not geometry), small molecules are not large enough to have appreciable induced dipole-dipole interactions which is why they tend to be gases. If other intermolecular forces are present, for example hydrogen bonds (which are permanent dipole-dipole interactions), these also have an effect that is why water -- with a molecular weight of 18 is a liquid whereas ozone with the same geometry and a larger molecular weight (48) is a gas.

Originally Posted by AndresKiani

Yeah, and most of those gases that you see at room temperature, have spherical geometries. Which doesn't give them much opportunity for interaction and dispersion attraction force. These atoms will not form nice long ranging molecular patterns like the metals. Most gases are in a stable diatomic formation. The noble gases which are atomic atoms and do not form molecules at all do to their inertness, neither reacting or even interacting with much.

Eh? Diatomic molecules such as H2, O2 N2, CO, NO are dumbells, and CO2 is bar shaped. Only the noble gases are spherical, being monatomic gases. But your point about dispersion forces having less opportunity to act in small molecules seems fair enough.

Addendum: Or, I should have said, in small atoms, i.e. atoms with lower polarisability than large ones.

That's exactly what I'm trying to say.. PhDemon scared the shit out of me with his response lol. I feel like its my Chem. Professor grading my short answer response on final exam lol.

Diatomic gases forming linear geometries with very little to no existent opportunity of dispersion attraction. Noble gases have more of spherical compact shape "VSEPR theory". Thus, less intermolecular stability or attraction, more than likely a gas at room temperature.

I wasn't trying to be scary, just trying to get you to explain what you meant more clearly

Originally Posted by AndresKiani

That's exactly what I'm trying to say.. PhDemon scared the shit out of me with his response lol. I feel like its my Chem. Professor grading my short answer response on final exam lol.

Diatomic gases forming linear geometries with very little to no existent opportunity of dispersion attraction. Noble gases have more of spherical compact shape "VSEPR theory". Thus, less intermolecular stability or attraction, more than likely a gas at room temperature.

Well PhDemon had the, ahem, privilege of being taught by one of the more terrifying Oxford dons (who by the way I found to be an absolutely inspirational lecturer on quantum theory - but then I didn't have tutorials with him) and I expect it has made him the rounded human being we see before us today.

But it was your remark about the gases being mostly spherical that seemed odd. Now you've clarified you really mean either spherical or too small to have much polarisability, it makes more sense.

But VSEPR is something else, isn't it? As I recall, that is all about the idea that the angles of bonds and "lone pairs" are such as to minimise electron pair repulsions. So nothing to do with dispersion forces, which are attractive forces due to induced transitory dipoles and so on. Or have I missed something?

Originally Posted by exchemist

Well PhDemon had the, ahem, privilege of being taught by one of the more terrifying Oxford dons (who by the way I found to be an absolutely inspirational lecturer on quantum theory - but then I didn't have tutorials with him) and I expect it has made him the rounded human being we see before us today.

I'm getting better *twitch*

Originally Posted by exchemist

Originally Posted by AndresKiani

That's exactly what I'm trying to say.. PhDemon scared the shit out of me with his response lol. I feel like its my Chem. Professor grading my short answer response on final exam lol.

Diatomic gases forming linear geometries with very little to no existent opportunity of dispersion attraction. Noble gases have more of spherical compact shape "VSEPR theory". Thus, less intermolecular stability or attraction, more than likely a gas at room temperature.

Well PhDemon had the, ahem, privilege of being taught by one of the more terrifying Oxford dons (who by the way I found to be an absolutely inspirational lecturer on quantum theory - but then I didn't have tutorials with him) and I expect it has made him the rounded human being we see before us today.

But it was your remark about the gases being mostly spherical that seemed odd. Now you've clarified you really mean either spherical or too small to have much polarisability, it makes more sense.

But VSEPR is something else, isn't it? As I recall, that is all about the idea that the angles of bonds and "lone pairs" are such as to minimise electron pair repulsions. So nothing to do with dispersion forces, which are attractive forces due to induced transitory dipoles and so on. Or have I missed something?

According to what I have learned, let me pull my old Chemistry book I don't want to bullshit on here with PhDemon looking over. Lol

In one of the examples on pg.413 "Principles of Chemistry" Nivaldo J. Tro,

Dispersion Force Examples


... he compares n-Pentane molar mass = 72.15 g/mol with a boiling point of 36.1 degrees Celsius to Neopentane molar mass = 72.15 g/mol however with boiling point is 9.5 degrees Celsius

Technically speaking, n-Pentane and NeoPentane should idealistically have roughly the same boiling point due to their identical molar mass, following the concept that the higher the molar mass the lower the boiling point.

However, and I quote, "Because the two molecules have different shapes. The n-pentane molecules are long and can interact with one another along their entire length, thus a higher possibility for dispersion force interaction. In contrast, the bulky round shape of the NeoPentane, as predicted using the VSEPR theory, results in a smaller area for dispersion force interaction."

Originally Posted by AndresKiani

Originally Posted by exchemist

Originally Posted by AndresKiani

That's exactly what I'm trying to say.. PhDemon scared the shit out of me with his response lol. I feel like its my Chem. Professor grading my short answer response on final exam lol.

Diatomic gases forming linear geometries with very little to no existent opportunity of dispersion attraction. Noble gases have more of spherical compact shape "VSEPR theory". Thus, less intermolecular stability or attraction, more than likely a gas at room temperature.

Well PhDemon had the, ahem, privilege of being taught by one of the more terrifying Oxford dons (who by the way I found to be an absolutely inspirational lecturer on quantum theory - but then I didn't have tutorials with him) and I expect it has made him the rounded human being we see before us today.

But it was your remark about the gases being mostly spherical that seemed odd. Now you've clarified you really mean either spherical or too small to have much polarisability, it makes more sense.

But VSEPR is something else, isn't it? As I recall, that is all about the idea that the angles of bonds and "lone pairs" are such as to minimise electron pair repulsions. So nothing to do with dispersion forces, which are attractive forces due to induced transitory dipoles and so on. Or have I missed something?

According to what I have learned, let me pull my old Chemistry book I don't want to bullshit on here with PhDemon looking over. Lol

In one of the examples on pg.413 "Principles of Chemistry" Nivaldo J. Tro,

Dispersion Force Examples


... he compares n-Pentane molar mass = 72.15 g/mol with a boiling point of 36.1 degrees Celsius to Neopentane molar mass = 72.15 g/mol however with boiling point is 9.5 degrees Celsius

Technically speaking, n-Pentane and NeoPentane should idealistically have roughly the same boiling point due to their identical molar mass, following the concept that the higher the molar mass the lower the boiling point.

However, and I quote, "Because the two molecules have different shapes. The n-pentane molecules are long and can interact with one another along their entire length, thus a higher possibility for dispersion force interaction. In contrast, the bulky round shape of the NeoPentane, as predicted using the VSEPR theory, results in a smaller area for dispersion force interaction."

Ah OK I see what you meant now: VSEPR predicts molecular shape and shape predicts scope for dispersion forces to act. Yes, that makes perfect sense, sorry to have been thick.

But…"the higher the molar mass the lower the BP?" Is that right?

Yes, because the higher the molar mass, means there is a larger electromagnetic field, more electrons. Therefore, the higher the amount of electrons, the higher the amount of randomness and random localization of electrons.. Thus, the greater probability that there will be a slight charge at any moment in time, meaning the greater probability for a dispersive attraction between molecules. Leading to a higher bp, due to the fact that higher thermal energy is required to break those intermolecular bonds to get them into the gas state.

The larger the molar mass the less room the wisdom teeth have to grow in.

Originally Posted by AndresKiani

Yes, because the higher the molar mass, means there is a larger electromagnetic field, more electrons. Therefore, the higher the amount of electrons, the higher the amount of randomness and random localization of electrons.. Thus, the greater probability that there will be a slight charge at any moment in time, meaning the greater probability for a dispersive attraction between molecules. Leading to a higher bp, due to the fact that higher thermal energy is required to break those intermolecular bonds to get them into the gas state.

Exactly. But you said lower, not higher.

Originally Posted by shlunka

The larger the molar mass the less room the wisdom teeth have to grow in.

Arf arf.