Diatomic elements are chemical elements whose stable form at STP consists of diatomic molecules, molecules of two covalently bonded atoms of that specific element.
Most of all gases, except the nobels, are diatomic elements e.g. H
2 O
2 F
2 Cl
2 N
2 and some others like Br
2 and I
2 not being gases; all of them have covalent bondings.
To my best knowledge there is only
one TRI-atomic element: ozone (O
3)
Elements with 4, 6, or 8 atoms also exist:
P4 (white tetrahedral phosphor)
C6 (graphite, gray hexagonal grafene, allotropous carbon)
S8 (cyclo-octa-sulfur, with rhombic and monoclinic crystallice forms);
even S
12 and S
18 have been reported, but sulfur is an amazing exception