Density? I'm dense.

January 21, 2008 8:28 PM

Is there a chemical explanation for the density of an element or is it simply an innate physical characteristic?

I can't seem to find an explanation of density that relates to something on the atomic level of an element. It doesn't seem to follow with an increase in atomic mass, number, radius, etc (Noble gases generally refute these possibilities). Is this something that is just inherent to each element?

Hopefully some of you are more hardcore science savvy than I am. Thanks in advanced.

posted by beta male to science & nature (14 comments total)

The discipline that explains these types of questions is called 'physical chemistry,' so whether it's chemical or physical sort of depends on your perspective, I guess.

Gases do not have a fixed density - they are compressible - so I'm not sure why you mention the noble gases, unless you're referring to their liquid states.

Van der Waals radii are important, but just as important is the ability of a pure element's atoms to pack closely, often in a crystalline lattice. This depends not just on mass or atomic number; it involves the number and configuration of the atom's orbitals (s, p, d, and f) as well. Different elements have different numbers of nucleons and different configurations of the nuclear strong and weak forces as well, which influence the size of the atom.

Someone will come along and post an answer that makes this one look confusing and elementary, I'm sure.
posted by ikkyu2 at 8:34 PM on January 21, 2008

It'll depend on how the element bonds to itself. Noble gases don't, so the density will be ruled by the ideal gas law, taking the mass of the individual atoms into account. Some form simple molecules, like hydrogen which forms H₂; again, the ideal gas law, but the mass of each molecule is what's relevant. Some, like metals, form regular crystalline structures; the density of the structure depends on the lattice spacing and the exact type of the lattice (face-centered-cubic, body-centered-cubic, etc.), and that depends on the element as well as a whole lot of environmental factors --- sometimes several different phases will be stable at a given temperature and pressure.

The short answer is "it's chemistry", with all of the electron-shell stuff that implies.
posted by hattifattener at 8:34 PM on January 21, 2008

I'm pretty sure all physical characteristics result from the size and configuration of the molecules involved.
posted by gjc at 8:36 PM on January 21, 2008

Yes, density has very little to do with atmoic mass, or number. Radius, to some extent.

Density has to do with how compactly fitted each of the protons and neutrons are. There are mathematical models that try to explain this.

Density is typically for solids, although there also density measurements for the same material in liquid or gaseous (at defined temperatures and pressures) states. iirc, gases at defined (ie., the same) molarity (total number of molecules), pressure, and temperature will have densities consistent with their atomic mass.

For the record, Osmium and Iridium are 'tied' for densest, the differences are small enough to tax even the most sensitive of instruments (although new instruments keep proclaiming a new winner only to be refuted by the error inherent in the machine that was solved in the next machine, &c).
posted by porpoise at 8:55 PM on January 21, 2008

Thanks again everyone, this was a lot of help.
posted by beta male at 9:02 PM on January 21, 2008

The density of copper is 8.96 grams per cubic centimeter at room temperature.

That's a product of two numbers: how many copper atoms can be fit into that volume, multiplied by the amount that each atom weighs on average (taking into account the isotope mix).

The former is a function of the crystalline structure, which is primarily controlled by the electron pattern in the outer shell. The latter is a function of the atomic weight, the number of protons and neutrons in the nucleus (on average).
posted by Steven C. Den Beste at 9:05 PM on January 21, 2008

Density has to do with how compactly fitted each of the protons and neutrons are.

Um, no. Macro-scale density doesn't have anything to do with the size of the nuclei (where the protons and neutrons are). It's entirely determined by the mass of the nuclei (the atomic mass) and how the atoms are arranged in space (which, being chemistry, is mostly determined by the interactions between their outermost electron shells). Nuclei are much, much smaller than even the innermost electron orbital, and so for everyday chemistry, nuclei are effectively point-masses with charge.

Deriving a solid's crystal structure, and hence its density, from first principles is a hard problem — a brief google suggests it's still an active area of research, and usually involves lots of computer time. I assume that some substances are easier than others, though.
posted by hattifattener at 9:19 PM on January 21, 2008 [1 favorite has favorites]

You asked specifically about elements, so I'll echo what Haff said: bigger nuclei, more mass. More mass, that element is more "dense" or has more weight for a given size. To match size we could stick to elements with the same size electron shells, which I think is all the elements in the same row in the periodic table. If we consider atoms in general, weight and therefore density increases the higher atomic number. So, uranium is much heavier and/or denser than an equal size chunk or aluminum or even iron, which are all metals so are probably of similar molecular densities
posted by skybolt at 9:42 PM on January 21, 2008

If you haven't really studied the periodic table, now is a good time. The organization of the elements, besides being arranged sequentially by atomic number, is basically that elements whose outer electron shells are similar are grouped together.

One would expect then that a density map, overlaid on the periodic table, would show tight grouping. And it does! I actually saw this on my Apple periodic table widget. I was going to take a screenshot but its the default picture on the widget page.

You can see almost immediately that despite what some have said above, density has very little relation to the mass of the atom itself! It is more tightly related to the atomic configuration.
posted by vacapinta at 9:47 PM on January 21, 2008

Also this table of atomic electron configurations lets you see more clearly how the structure of the outer shell creates elements with similar properties. The color of the element corresponds to its place in the periodic table.
posted by vacapinta at 9:54 PM on January 21, 2008

Right! Sorry, not how tightly the nucleus of the atom is packed (protons & neutrons) but how tightly packed the atoms are, which are dependent on the way the protons & neutrons are arranged, which is dependent on the numbers of each.
posted by porpoise at 11:22 PM on January 21, 2008

'To match size we could stick to elements with the same size electron shells, which I think is all the elements in the same row in the periodic table.'
No, going across the row changes the way the atom bonds with other atoms, which affects if and how it forms a crystalline structure.

'Sorry, not how tightly the nucleus of the atom is packed (protons & neutrons) but how tightly packed the atoms are, which are dependent on the way the protons & neutrons are arranged'
I think you're phrasing this very confusingly. It isn't how individual protons and neutrons are arranged, but how the nuclei are arranged, which is a result of how the electrons are arranged as those are what tie the atoms into whatever structure they are in. How the electrons are arranged is a function of the number of electrons, which is a function of the number of protons per nucleus. The arrangement of the neutrons is effectively irrelevant, as long as they're arranged such that the nucleus doesn't fall apart on the same kind of timescale it took the questioner to post his question here.

In fact it's not even a pure function of the element, as hattifattener says. Elements can have different allotropes, meaning they have more than one way of forming a structure (obvious well known example is carbon, with diamond and graphite), meaning it's not even a function of element and current environmental conditions but past conditions as well.
posted by edd at 7:47 AM on January 22, 2008

Right! Sorry, not how tightly the nucleus of the atom is packed (protons & neutrons) but how tightly packed the atoms are, which are dependent on the way the protons & neutrons are arranged, which is dependent on the numbers of each.

No, the arrangement of protons and neutrons in the nucleus has nothing to do with the arrangement of atoms in a crystal. It is the interactions of the electron shells that determine how the elements interact with each other (and there can be multiple possible configurations, eg graphite and diamond are both pure carbon, but have very different crystalline structures and therefore different densities). The electron behavior is determined by the number of protons and nothing else.

As far as this discussion is concerned, the nucleus can be considered a point in space. All that matters is:

the mass (# protons plus # neutrons) which determines total weight

and

the charge (number of protons) which determines how the atoms arrange themselves.
posted by jpdoane at 8:51 AM on January 22, 2008

All that matters is...

There are other factors that affect density as well, for example, temperature and pressure. What I meant was, all that matters wrt the nucleus is...
posted by jpdoane at 9:08 AM on January 22, 2008