jgourlay - 28-4-2009 at 12:42
So I'm staring at the periodic table last night when it hits me like a ton of bricks. You have gasses over at the right side with atomic weights
higher than metals on the left side.
How is it that such heavy atoms (on the right side) can be gaseous at room temp when lighter atoms are not?
Pomzazed - 28-4-2009 at 13:02
"Density" ?
panziandi - 28-4-2009 at 13:12
Atomic make up.
Take for example radon. It is very dense but is still a gas: It has a full outter electron shell. It can not form chemical bonds with other randon
atoms, thus it exists on its own as individual atoms.
Nitrogen, oxygen, fluorine etc all exist as diatomic molecules and the intermolecular interactions are not great enough to allow them to be solid at
room temperature.
Metals and giant molecular structures such as seen with carbon, boron etc have many more interactions which allows them to be solid.
The periodic table should be looked at in more than just one dimension.
jgourlay - 28-4-2009 at 13:12
Yeah, see, I always thought that density was related to atomic weight. And on one level it makes sense. You have low density sodium with a low aw
and high density plutonium with a high aw.
They you have xenon (high aw).
Lambda-Eyde - 28-4-2009 at 13:13
Atomic weight does not reflect the density of the element, just the weight of the atoms.
Although I admit it's rather strange comparing e.g. lithium to radon. 
Edit: people are quick to reply, I see!
[Edited on 28-4-2009 by Lambda-Eyde]
jgourlay - 28-4-2009 at 13:15
Thanks Guys! Love this board!
DJF90 - 28-4-2009 at 13:25
I think its all about the structures that the elements adopt. Over on the far right you have gases, because these elements exist as diatomic molecules
("noble gases" are monoatomic) with few intermolecular forces (van der waal's/london's forces).
Then into the nonmetals you have small molecules, like P4 and S8. Again these only have van der waal's/london's forces, but the molecules are larger
giving a bigger area for the forces to act upon.
Then we get to the metals. The metallic bonding consists of metal cations surronded by a "sea of electrons". This is very strong bonding and all atoms
participate. Realistically not all metal atoms are existant as cations; I seem to remember being told by a physicist that the real number of ionised
atoms in a metallic structure is a very small percentage.
Carbon is strange in that it forms giant structures whilst being a non-metal. In diamond all the atoms are connected by strong covalent bonds which
gives diamond an abnormally high melting/boiling point. In graphite, there are large layers of fused hexagons that are weakly bonded to eachother, but
the bonding is still strong enough to also give graphite an abnormally high melting/boiling point.
Sorry if I've missed anything out, I'm supposed to be doing some electrochemistry work thats due in tonight 
jgourlay - 28-4-2009 at 13:39
DJF90: Now that's something. I didn't realize carbon had a liquid phase!
DJF90 - 28-4-2009 at 14:06
Well I dont believe it does at atmospheric pressure (I think it sublimes). Although at increased pressure it should have.
[Edited on 28-4-2009 by DJF90]
Sauron - 29-4-2009 at 00:34
Chemistry is all about physical and chemical properties dictated by atomic and molecular orbitsld snd their interactions. In other words where the
electrons are. Atomic weight is mostly about thw mass of the nucleus (protons and neutrons - nucleons) and while these are not totally irrelevant to
chemistry they are sort of an absentee landlord, The nucleus defines the identity of the element, and the number of protons equals the number of
electrons populating the orbitals. All else has to do with the distribution of the electrons in those orbitals and the interaction between atomic
orbitals or like or unlike atoms to form molecules.
A consequence is that a great deal of chemical behavior can be modelled with ever increasing precision by the ab inition and semiempirical ESS
programs (GAMESS General Atomic & Molecular Electronic Shell Structure), MOPAC (Molecular Orbital Package) and its sister AMPAC, Gaussian, Spartan
and many others. Just follow the acronyms and they tell uou what the playing field is.
In physics, the entire subdiscipline and technology electronics depends on the behavior of electrons at the no mans land between metalloids and metals
where things get very strange indeed. (This is a huge oversimplification because I am pretending that electronics did not exist before the invention
of the transistor, which is not at all true. Vacuum tube demons will haunt my dreams tonight.)
Get yourself a really good PT and start tracking things like electronehativity (this increases L to R and Toop yo Bottom) amd electrical conductivity.
Then get a periodic table of the ISOTOPES and see how matter is really organized. Look at the abundance and diversity of the natural isotopes (stable
and otherwise) and then the man made ones and see what playings with the number of neutrons in a nucleus achieves.
jgourlay - 29-4-2009 at 08:22
Sauron, thanks for rundown!