Re: helium

Kathryn,

In determining values like the melting and boiling points of a material, the mass of the element is not as important as its electronic structure. This determines how the atom will interact with other atoms. Note that carbon is frozen solid at room temperature even though it is lighter than the gaseous elements nitrogen, oxygen, and neon.

Helium is one of the noble gases, along with neon, argon, krypton, xenon, and radon. The structure of these atoms is such that they don't like to bond with other atoms, even others of the same type. In quantum mechanical terms, they have a closed outer shell of electrons. There are no gaps in their structure where the electrons from other atoms easily fit in, and no loosely-bound electrons available to share with another atom in a bond. This is why they are monatomic (single-atom) gases, and liquify at very low temperatures. Radon is a gas at room temperature even though it is heavier than lead!

Hydrogen and the other elements are electronically "incomplete" atoms, with gaps in their outer shells, and so will react with other atoms. Hydrogen, for example, has an outer shell that can be filled with two electrons, but the element only has one. Therefore, hydrogen atoms pair up and "share" their electrons, forming a chemical bond. The resulting molecule is relatively stable, and does not interact very strongly with other hydrogen molecules. A similar process forms the bond in other diatomic (two-atom) gases such as nitrogen and oxygen. By "relatively stable," I mean relative to a single atom. Oxygen, which shares two electrons, forms a more stable molecule than hydrogen, and nitrogen, which shares three electrons, is almost inert (non-reacting) as a gas.

Because the hydrogen molecule is made up of two pieces that can vibrate and rotate and are imperfectly sharing their electrons, there is still a stronger interaction between hydrogen molecules than there is between the ultra-stable helium atoms with their nicely closed two-electron outer shell. This interaction causes the hydrogen molecules to collect into a liquid at a higher temperature than helium, and hydrogen will freeze even though helium stays liquid all the way down to absolute zero unless you put it under pressure.

Neat stuff. Good question!

Stay curious.