Why do molecules have the densities they have? Is it teh relative strength of the bonds between the atoms? The stronger the bond (ex: C-C bond) the more dense the molecule?

 

~EE

It might be its density at a predefined temperature.

 

wanted to follow this

Density is mass per volume - so it depends on how heavy the matter is over what space it occupies, so 2 things are important - 1)The molecular weight of the atoms making up the molecules in the substance. 2)How densely packed they are.

 

Molecules exhibit several types of packing, like hexagonal close packing, cubic or body centered cubic - look up molecular packing. The closer the molecules pack together influences the density - for example, hexagonally close packed molecules give higher densities because they are more densely packed, although it doesn't always go that the most dense substances have hexagonal packing, Lead for example is cubic or body centered cubic but is a very high density because the atoms are of a high molecular weight.... if it packed hexagonally it would be even denser.

Density is mass per volume - so it depends on how heavy the matter is over what space it occupies, so 2 things are important - 1)The molecular weight of the atoms making up the molecules in the substance. 2)How densely packed they are.

 

Molecules exhibit several types of packing, like hexagonal close packing, cubic or body centered cubic - look up molecular packing. The closer the molecules pack together influences the density - for example, hexagonally close packed molecules give higher densities because they are more densely packed, although it doesn't always go that the most dense substances have hexagonal packing, Lead for example is cubic or body centered cubic but is a very high density because the atoms are of a high molecular weight.... if it packed hexagonally it would be even denser.

So, like graphene's structure: layered 2 dimensional hexagon?

Density is mass per volume - so it depends on how heavy the matter is over what space it occupies, so 2 things are important - 1)The molecular weight of the atoms making up the molecules in the substance. 2)How densely packed they are.

 

Molecules exhibit several types of packing, like hexagonal close packing, cubic or body centered cubic - look up molecular packing. The closer the molecules pack together influences the density - for example, hexagonally close packed molecules give higher densities because they are more densely packed, although it doesn't always go that the most dense substances have hexagonal packing, Lead for example is cubic or body centered cubic but is a very high density because the atoms are of a high molecular weight.... if it packed hexagonally it would be even denser.

Nicely explained!

So, like graphene's structure: layered 2 dimensional hexagon?

The carbon molecules in graphene and graphite are looped into rings and make a sort of honeycomb structure, so they aren't hexagonally close packed. Even though their C-C bond lengths are shorter than that of diamond, for instance, the carbon atoms in diamond are tetrahedrally arranged so there are more of them per unit area/volume and thus diamond is more dense as a material. Hexagonally close packed structures are where the atoms are arranged like balls in a box all tightly packed together - some metals have their atoms arranged like this and tend to be denser than metals with atomic stacking with the atoms arranged in a cubic structure. Although of course the atomic mass of the atoms in the material are probably a much larger factor, especially if the atoms are from lower down on the periodic table.

The carbon molecules in graphene and graphite are looped into rings and make a sort of honeycomb structure, so they aren't hexagonally close packed. Even though their C-C bond lengths are shorter than that of diamond, for instance, the carbon atoms in diamond are tetrahedrally arranged so there are more of them per unit area/volume and thus diamond is more dense as a material. Hexagonally close packed structures are where the atoms are arranged like balls in a box all tightly packed together - some metals have their atoms arranged like this and tend to be denser than metals with atomic stacking with the atoms arranged in a cubic structure. Although of course the atomic mass of the atoms in the material are probably a much larger factor, especially if the atoms are from lower down on the periodic table.

Yes, I see now, thanks.

Results from:

- The density of the constituent elements, for instance MoS₂ and WC are dense;

- Each and every subdetail of how the molecules can pack into a liquid or solid, and this is unpredictable to Mankind in 2015. Every density prediction method or software fails grossly, telling for instance 1.9 for observed 1.5.

- Bcc, Fcc, Hc are candidates for metals. Organic compounds have much more choice. When crystallized - if possible - they eventually adopt a known crystal form (among 20? 50?) but the crystal's primitive cell can contain many molecules, opening the number of possibilities.

 

This, and the related melting point, is a field needing research, and little is being done. Heavy computation may let a software work some day, that's one hope.