Why is diamond a solid, whereas carbon dioxide, which is a heavier molecule, is a gas?
Because diamond is a non-molecular material, with no molecular boundaries.
Carbon dioxide is a molecular species, and occurs as discrete molecules of
On the other hand, diamond is a non-molecular solid. Its particles are held together by (very!) strong
The important criterion is NOT whether the bonds are covalent. And in fact covalent bonds are strong; the covalent bonds in carbon dioxide, and carbon monoxide ARE IN FACT STRONGER than the
What does this argument suggest with respect to the properties of ionic solids, such as
See here for a consideration of metallic bonding.