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Carbon with 6 protons, the first of its group, is a solid, while the very next elements, nitrogen and oxygen, are gaseous in their elemental form.
Why carbon is a solid? If it's not the molar mass that defines carbon phase (carbon's molar mass is lesser)
In pnictogen and chalcogen group only the second element, phosphorus and sulfur, is solid.
Carbon forms large molecules such as the ring structure of graphite, or tetrahedrons of diamond. Boron, like carbon, forms relatively large covalently-bonded molecules.
Nitrogen, oxygen and fluorine are diatomic, so molecules are less massive. The inert gases He and Ne are monatomic and therefore have lower b.p. yet.
In a similar fashion, metallic bonds hold together Li, Be and other metals.
This is an extended comment. First notice that in any period, as we head towards noble gas configuration, we eventually become a gas. As we go down the periodic table, that transition is delayed, so by period 5 we transition from solid (I) to gas (Xe) in one step. In period 1, both elements (H, He) are gases at STP. In period 3, we get to gas at STP once we hit Cl, and the next element is a noble gas (Ar). Period 2 is the "most interesting" in that we get a solid to gas transition from C to O. That this transition eventually occurs in any period is not that interesting. But it is somewhat interesting why the C to O transition occurs some distance from the noble gas configuration (Ne). Given all of the properties I have discussed so far, it may just be a basic periodic property, but I am hopeful that a better answer is out there!
