I don’t know a lot about chemisty (cue the music) so this may be stupid, but…
Lithium is element #3 on the periodic table. Its atomic weight is about 7. It’s a very light element; by comparison, nitrogen atoms are twice as heavy.
So why is lithium a solid at room temperature? It kind of looks like a silvery metal, at least until it tarnishes. But nitrogen’s a invisible gas; it’s floating all around you right now.
Why is the lighter element a solid, but the heavier a gas?
It’s because of IMFs. Lithium, at room temprature, is a lattice - it’s internal molecules attract into a lattice shape that requires a great amount of energy to seperate the molecules.
Nitrogen, on the other hand, is diatomic; meaning it attracts each other into molecules with two nitrogen ions. These molecules don’t readily attract or bond to each other, meaning they are neither close enough to be liquid nor close enough to be solid overall.
(IANAC, IANS, IHNIWIAS, etc - but I believe I have the jist of it.)
This is going to be a somewhat hand-wavy, simplified-version-of-the-truth answer, but I’ll give it my best…
The atomic numbers down the right side of the periodic table (2, 10, 18, 36, 54, 86) are the amount of electrons an element needs to be stable. Hydrogen, with its 1 electron, can share that 1 electron with another hydrogen atom, and both will have stable 2. Helium alread has stable 2, so it can exist as a single atom. But that molecule of hydrogen doesn’t “stick” to other molecules of hydrogen at room temperature, so it’s a gas. Likewise, the atom of helium doesn’t “stick” to other atoms of helium at room temperature (you have to get it REALLY cold before it does, very close to absolute zero), so, again, a gas. The reason for the lack of “sticking,” btw, is most easily explained by the lack of any ability to separate the charges between the two atoms of hydrogen or the 1 atom of helium; there’s nothing to attract a “happy,” 2-electron hydrogen atom in a molecule of hydrogen to another 2-electron hydrogen atom in another molecule of hydrogen.
In order for lithium to get a stable number of electrons, it can lose 1 electron (gain a +1 charge), or, in theory, gain 7 (gain a -7 charge). It isn’t possible to pack 7 of those +1 lithium ions around a theoretical -7 lithium ion (plus the charge density is too high compared to the lowly 3-proton nucleus, anyway), so forming some sort of lithium “molecule” doesn’t work. Instead, a bunch of lithium atoms get together, and essentially each get to temporarily give up their extra electron, sharing it with all of the other lithium atoms in the solid.
Nitrogen, with its 7 electrons, only needs 3 extras to hit the stable 10. It can get this by sharing 3 electrons with another nitrogen atom, so each atom in N2 has the stable 10 electrons. But, again, this molecule of N2 doesn’t “stick” very well to other molecules of N2, so it’s a gas at room temperature.
I don’t think anything is wrong enough there to make anyone have to throw the explanation out the window, but it’s possible someone will come along with a clearer explanation that doesn’t leave quite so much out 
On preview… ZebraShaSha’s lattice explanation is good to include, but there’s no such thing as a lithium “molecule,” per se. “Molecule” generally refers to atoms connected by covalent bonds, and the bonds in lithium are the oft-overlooked “metallic” bond, which is something like covalent but not quite the same thing.
and the combination of lithium and nitrogen, lithium nitride is a solid
A material is a solid if it has enough attraction between the atoms. This is true for most elements as they share electrons, either in directional bonds like carbon, or nondirectional bonds like lithium. Nitrogen also shares electrons, but does it in pairs of N2 - there is little interaction between the separate N2 molecules.
Mercury vaporizes very easily, too, at room temperature. I don’t have my periodic table handy (and I call myself a nerd!), but if you check, you’ll see it’s very heavy compared to either lithium or nitrogen. My point being that atomic mass has very little to do with whether an element is a solid, liquid or gas at room temperature.
We now return you to your regularly scheduled explanation of what it DOES have to do with.
Just for completeness, Mercury has an atomic number of 80, and an atomic mass of 200.59.
And while I’m here… two atoms are walking down the street. One of them trips and falls, then cries out, “Oh no! I’ve lost an electron!” His friend says “Are you sure?” to which the first atom replies “Yes, I’m positive!”
To expand more on what’s already been said: atomic weight has little to do with whether a substance will be a solid, gas or liquid. Take water, for example; it can be a solid, liquid, or gas in circumstances common enough to create in a kitchen, but its molecular weight is the same no matter what state it’s in. What you’re really asking is how can an element with a lower atomic weight have a lower freezing point, which is a matter of how much repulsion and attraction exists between the atoms.
To expand on this point, isn’t it true that the defining characteristic of metallic structure is that the nuclei, more or less, assume permanent positions in the crystal while the electrons roam freely throughout the structure? Though I don’t understand how it works, I’ve read that this structure is responsible for metallic opacity and luster.
Also, since the Li atoms themselves are smaller, they can pack more tightly, which will promote bonding as a solid, where atoms are closer together.
I think that shielding may have something to do with it too. Can anyone verify?
It’s not as permanent as most lattices, which is why metals tend to be malleable and not brittle like crystalline solids.
Also, it’s only the valence electrons that float freely. The electrons in complete shells stay with their respective neuclei.
I understand why Lithium is a solid, but what about non-metallic solids like Boron and Carbon? What holds them together?