Thanks to all those who answered my last question about “heavy metals”; now I have another. I have heard that the heaviest (ie densest) metal out there is osmium, which is element number 76 on the Periodic Table. Why wouldn’t the honour go to something with a greater atomic number (and therefore atomic mass) than that: maybe one of those transuranic mothers? Why isn’t there a simple relationship between atomic mass and density?
Mainly because density is in large part due to electron structure, not nuclear structure. However, there is a loose cyclic relationship, as you can see by the graph here. Density is also temperature-dependant, and since different materials have different coefficients of thermal expansion, any realtionships are further obscured.
To put it in simpler (e.g. somewhat inaccurate “high school chemistry” terms), higher atomic number elements tend to have more inner electrons which partially shield the charge of the nucleus. Therefore the outer orbitals are disproportionately larger than the inner ones, they are not evenly spaced. Even if they were evenly spaced, the volume would increase with the cube of the radius, so the density would still tend to go down.
That’s completely inaccurate on a quantum level, and I’m ashamed of myself for spreading such a horrific misrepresentation of the meaning of “electron orbitals” and “shells”, but I hope it helps you imagine how the outermost orbital, which defines the packing of an element, can be disproportionately larger (more volume/mass).
Of course, the preceding answer was more accurate. I just wanted to paint a picture, and I didn’t think we’d be likely to get you to study the quantum structure of an atom
There are three things which will determine the density of a metal (or most other substances): First, how heavy the nuclei are. This will increase more or less with increasing atomic number (it’s possible for an element to have higher atomic number, but enough less neutrons that it has a lower atomic mass). Secondly, the average distance between atoms will vary. Roughly speaking, as you move down a column in the periodic table, the distance between atoms will increase, but as you move across the rows, it’ll decrease. Thirdly, different substances have different crystalline structures, and some are more efficient than others at fitting more atoms into a smaller space. The most efficient (that is, densest) crystal structures are the Face-Centered Cubic and the Hexagonal Closest Packing (equal in density, and somewhat similar in structure), and the least dense crystalline structure is the diamond structure (not often seen in metals). Simple cubic and body-centered cubic fall in between these (diamond - simple cubic - BCC - HEX/FCC)
So Osmium has the right combination of small interatomic distances, heavy nuclei, and a HEX or FCC crystalline structure.