posted
I just had a question about moleculular bonding that's probably not even worth bringing up because of its stupidity, but here it goes anyway.
[begin stupid question] From what I know, density = mass/volume. Gases are the least dense out of solids, liquids, and gases.
I was thinking about each individual gas molecule, for example, in H2. I know that H is the lightest element, but it also has one of the smallest atomic radii. So when and H bonds with another H, wouldn't the volume of the molecule be very small compared to other compounds, such as metals? I know that there is only 1 proton, electron, and neutron in a H atom, but when you divide by a small number (volume), shouldn't the density be higher, or within some close range to the metals? This is obviously not the case in nature, as H2 is gas, and much less dense than the metals.
From what I understand, each successive period in the periodic table adds another valence shell to the atoms, thus greatly increasing the atomic radii. Since both the mass and the radii increase, shouldn't the mass/volume effects be somewhat cancelled out?
posted
Gases are mostly empty space. The diffuse molecules zip about. The density of the actual molecules is almost (but not quite, hence the difference in behavior of real gases versus ideal gases) negligible.
posted
Well, if you ignore the differences for change in state, and for the openness of the lattice structure in different solids, I'm still curious why atoms with small atomic number seem to be so much less dense than those with large. Most of the mass of an atom is in the nucleus, right? But aren't the electron shells like HUGE compared to the size of the nucleus? And since the overall size of the atom depends on the size of the outermost shell, wouldn't the densities balance out somewhat?
Does this have to do with the relative strength of the strong vs. the electromagnetic force or something? It seems like there should be some observation that makes better sense of the atomic density of various elements than I can see.
I know there are lots of chemistry wizards on hatrack. Will somebody please make more sense of this than I can?
I think it might be because the electron shells are not really shells but energy levels. Because of this increasing levels do NOT result in a proportionate increase in the size of the atom - wheras more neutrons and protons do have the effect of a proportionate increase in atomic mass.
This is a combination of first year chemistry and third year physics being reluctantly dredged from the bottom of my mind however (and through a strawberry and rockmelon daquiri!) - I'm sure Rivka will have a more cogent answer.
posted
When you're talking about gases, even the large electron shells have no impact on overall size of the gas, at least not at the pressure level's we'll ever experience (and live...) The small elements are very light, or non-dense gases because they fill the same space as any other gas (it was discovered that any gas will take an identical amount of atoms or molecules to fill a space at equal volume as any other, the bigger ones just go slower) so the lightness of the small elements means that the overall weight, and therefor density, is less.
Basically, there's the same number 'n' molecules in the same volume 'v', no matter the weight, so one with a smaller weight, let's say Hydrogen's 2 AUs compared to Oxygen's 16 would be:
Density of H2 = 2*n/V Denisty of O2 = 16*n/V
Obviously H2 wont be as dense as O2, and that's that.
posted
The whole idea of atomic radii, at least in when using the electron shell as the boundary, is a bad idea. This is because that between the electron shell and the nucleus is a heck of a lot of empty space. Especcially since the shell is the area around a nuclues tht electron(s) can exist, not that they exist at every point on it at any given time. Atomic nucei are REALLY tiny. So even in your context of atoms, atoms themselves are mostly empty space, and thus not very dense.
No matter what scale, the volume is guaranteed to be orders of agnitutde greater than the mass, I would bet.
posted
Well the electron exclusion rule, stating how many electrons can exist in each energy shell techincally does, but this is the spacec limiter for Nuetron stars, you wont find a lot of gases here that are that dense (i.e. none even close). Atomic weight ... well it's number of electrons that effect the overall compression size, which is related to atomic weight, but they aren't the same thing.
quote: The whole idea of atomic radii, at least in when using the electron shell as the boundary, is a bad idea. This is because that between the electron shell and the nucleus is a heck of a lot of empty space. Especcially since the shell is the area around a nuclues tht electron(s) can exist, not that they exist at every point on it at any given time. Atomic nucei are REALLY tiny. So even in your context of atoms, atoms themselves are mostly empty space, and thus not very dense.
quote: I think it might be because the electron shells are not really shells but energy levels.
If this was the case, then why is there an easily defineable trend in ionization energies? Whenever you go down a row in the periodic table, the energy of ionization decreases significantly the electrons located in the outer shell are farther away from the positively charged nucleus. This, at least to me, tells me that the atomic radius is a decent approximation for the boundary of an atom.
And others have provided data for gases as a whole, but I was interested in each individual molecule of the gas.
For example, H in its natural state is H2. I'm not sure how to determine the radius of this, but if each share one of their electrons, then radius of the molecule should be about 2 times the radius of each H?
Hydrogen's covalent radius is 29. Iodine's covalent radius is 139.5.
From those images of each molecule, it seems to me like the radius shoud make a difference in the density, but I guess it doesn't. Posts: 2756 | Registered: Jul 2002
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posted
Don't forget that the density is highly dependent on how the molecules pack together. Let's look 1 mole of hydrogen gas (H2). At 1 atmosphere, this will take up 22.4L (with some decent assumptions for a single, simple pure substance). We can pour energy into the gas by compressing it (assuming that we did this isothermally, we could compress it until it became a liquid but lets not compress it quite that much). At which time, the density of the same 1 mole of hydrogen will be much higher than it was when we started. But we haven't changed the amount of hydrogen, or even the phase its in, only the pressure.
Further more, in the first post you talk about calculating the density of single hydrogen molecule. That doesn't work. It doens't mean anything. Density is a property of a collection of atoms or molecules at a given temperature and pressure, not of a single one. Super critical fluids, for example, have a density near that of liquids, but they flow like a gas, due to pressure/temperature effects.
There some interesting examples concerning density though, by your more subatomic particles -> greater mass idea the density of benzene should be higher than hexane, right? Lets look it up.
Benzene (C6H6): 1.228 g/cm^3 Hexane (C6H14): 0.660 g/cm^3 (Values from the 85th Ed. CRC Handbook, web edition)
Whats this, more atoms -> lower density?
In this particular case the aromatic ring makes benzene be almost perfectly flat, while the hexane is free to rotate around all those carbon-carbon bond, and bristling with hydrgens, so its big in three dimensions and floppy. This difference in three-dimensional structure allows benzene to pack really tight, but the hexane is looser. (Yes, this is somewhat a contrived case, hexane vs. heptane will be closer, but the point was to show an example that didn't fit.)
edited: added that density is dependent on both pressure and temperature.
quote: Further more, in the first post you talk about calculating the density of single hydrogen molecule. That doesn't work. It doens't mean anything. Density is a property of a collection of atoms or molecules at a given temperature and pressure, not of a single one. Super critical fluids, for example, have a density near that of liquids, but they flow like a gas, due to pressure/temperature effects.
Hydrogen is naturally diatomic?
quote:more subatomic particles -> greater mass idea
That's not my idea - I don't even have an idea. I said that density is dependent on mass AND volume. So while Hexane (C6H14): 0.660 g/cm^3 has a greater mass, it also has a larger volume ('so its big in three dimensions and floppy'), which confirms with my question. How come an H2, or any other diatomic gas, per say, have such a low density when the volume is so low?
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posted
Because the average distance between molecules is much bigger than the average size of molecules, for any moecule you care to name. In fact, that's basically what defines a gas. So the size of the molecule is irrelevant.
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posted
Because the molecules don't bond to each other. They float about and only interact very loosely with each other.
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posted
You are looking at the wrong level, I think. An individual molecule is not a gas; gas / liquid / solid is an emergent property belonging to ensembles of several billion (at least) molecules.
The density of an individual molecule is not very well defined; but if you want to take some kind of half-assed average, then yes, the hydrogen molecule probably is more dense than hexane. (I say probably because it would be a damn complicated integral, and the average distances between atomic nuclei in a complicated molecule is a tough problem in quantum mechanics anyway.)
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posted
I believe the confusion lies in comparing density at one level or organization (the molecule or atom) with that at another level of organization (a gas in a container).
The "density" of an atom or molecule is kind of a strange concept, but it would make a certain amount of sense that an atom with a heavier nucleus would have a "smaller" and thus more dense inner shell of electrons than an atom with a lighter nucleus.
However, conceptually this is probably meaningless as pointed out above. Electrons exist in quantum states and the whole idea of electron "shells" is perhaps a bit of a misrepresentation.
Now, when you look at behavior of various gases in a specific volume, what you have is the mass of the gas divided by the volume of the container. It's looking at groups of molecules.
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posted
Let's start with a picture. Note that down each column, atomic size does increase -- substantially. That's because there's an additional energy level each time. However, across each row, even though there are more electrons being added, the size DECREASES. That's because there are also more protons being added, and the attraction between the electrons and protons gets stronger the more there are of each.
As an analogy, each new column is adding a floor to a house; each step along a row is squeezing that house down (without decreasing the number of floors, just squeezing all of 'em).
When atoms bond covalently, (as in diatomic hydrogen gas), they actually get a bit smaller -- due to the (effectively) increased electrons. Ions, OTOH take up either much MORE space than their parent atoms (cations, which have given up both electrons (decreased attraction) AND an entire energy level; or take up much LESS space (anions, which have gained an additional electron, increasing the attraction).
Add in the packing issues HollowEarth mentioned, and you can see why the densities of solids (and liquids) occupy such a wide range -- even among related compounds.
When you get to gases, though, none of that matters much. There is SOOOOO much more space between molecules than is occupied by molecules!
Also, what has hydrogen's diatomic nature to do with anything? The fact that it is made of molecules, rather than individual atoms simply makes almost no difference when it's a gas.
quote: Does the atomic weight of the molecule affect the extent to which the gas can be compressed?
IIRC, very very little. The strength of the relevant intermolecular forces (hydrogen bonds, van der Waals forces, etc.) makes some difference -- especially at high pressure and low temperature. But if you compress it enough that the molecular weights matter, you're not looking at a gas any more.
Posts: 32919 | Registered: Mar 2003
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posted
The preceding table I generated using the atomic radii from the earlier link that kaioshin00 posted, along with the atomic weights from my chart of the nuclides. This thing I call atomic density is just an invented measure of roughly how dense various atoms are, eliminating for the time being the considerations of changes of state or the exact molecular configurations.
It makes sense to me, given the periodic chart. On any given row of the periodic chart, atomic density increases to the right, due to the increase in nucleons or mass, at roughly the same volume for a given electron shell (though, as rivka's picture shows, the volume decreases some due to higher attraction from the additional protons in the nucleus). Each time you drop down a row another electron shell is added on, so you get suddenly much more volume, decreasing the atomic density. But elements in the same column increase in density still, moving straight down the chart, because even though each new shell makes the atom have much higher volume, the atomic weight increases even more.
So that explains what's going on when you just look at individual atoms. The whole picture including changes in state and molecular bonding, I guess, is a lot more complicated, as rivka explained.
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