Unfortunately the answer is "just because." Most elements are metals, which have extended "metallic bonding" frameworks and therefore have higher melting points than substances composed of separate molecules. A few metals (mercury and gallium) are liquids at "normal" temperatures and pressures, but this is really unusual--and in fact, if you drop the temperature much both of them will become solids. That's why mercury thermometers aren't good for temperatures more than about -10° or -15°; mercury freezes at -39°C, which is -38°F. This temperature is common enough; you get it on a cold winter day in Fargo, North Dakota, where I went to college. And gallium freezes if you set your thermostat a little bit low.

On the other hand, many elements that are composed of separate molecules (for example, N₂ or Cl₂ or Ar) are gases under "normal" conditions. Their molecules just aren't heavy enough to liquify or solidify. Worse yet, molecules made of all the same kind of atom don't stick together very well, so they have very narrow ranges in which they are liquids. That's why bromine (Br₂) is the only non-metallic, liquid element. (Radon is almost a liquid, and it is expected that Element 118, if it's ever isolated in bulk quantities, will be a liquid under "normal" conditions.)

The only molecular elements that are solids are white phosphorus (P₄) and iodine (I₂). All other non-metallic elements are solids because they are "all one molecule"--they are composed of very large chains or networks, with each atom bonded to two or more neighbors. Boron, carbon, silicon, red phosphorus, arsenic, sulfur and selenium are all examples.

Or, to sum up: most elements are solids under earthly conditions because they are metals or extended network "molecules," and there isn't enough energy at 25°C to knock them apart. The few elements that are not solids are composed of small molecules, with only one or two atoms each.

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