Why elemental carbon is solid? 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.