Explain why the atoms in neon gas do not join up in the same way that hydrogen atoms do?

Because they already have a stable .
Hydrogen atoms bond together to form hydrogen molecules because they gain a stable by doing so.
A hydrogen atom, which only has one electron, will bond to another hydrogen atom, which also has one electron, to form diatomic hydrogen, or ##H_2##.
The two electrons are shared equally between the bonded hydrogen atoms, which allows them to obtain a stable called duet.
Neon atoms, on the other hand, already have a stable electron configuration, so they will not gain any more stability from sharing their electrons with other neon atoms.
Neon is a noble gas, which means that it has very stable electron configuration – 8 electrons in its outermost shell. This is known as a .
Atoms that already have a complete octet (or duet, as is the case of helium) are very stable, so they have no “desire” to react with other similar atoms because they cannot gain more stability from doing that.
On the other hand, atoms that don’t have a complete octet, or duet as is the case with hydrogen, will bond with similar atoms to form homonuclear molecues (molecules that are made up of only one type of atom), which is what helps them become very stable.