density

It does vary but it is negligible because a solid by definition holds the majority of its atoms and molecules together in a relatively fixed formation in space. Any expansion due to heating is minor compared to that of liquids and that of liquids is minor compared to gasses whose particles move constantly relative to each other causing them to fill any container.

Okay Fres but maybe Sam and Your Average Clods well (me) might wish to know how

Here's the answer for an average 14 yr old.
.
http://www.bbc.co.uk/bitesize/ks3/science/chemical_material_behaviour/particle_model/revision/2/

Intermolecular bonds are essentially electro-dynamic.

Yes Fres tho must confess , Yes, I can't guess electric, galvanic, static, gravitational

..unless voltaic, dynamic, hermenuetic.....

http://onelook.com/?w=Electro-dynamical&ls=a

https://www.google.com/?gws_rd=ssl#q=electric%2C+galvanic%2C+static%2C+gravitational

Just pullin' your leg Fres, and thanks for the link. I am however an average 84-year-old