Which gas is monatomic at STP?
A. Neon.
B. Fluorine.
C. Chlorine.
D. Nitrogen.
E. Oxygen.
The noble gases, group 8A, are the least reactive elements. Look up Neon on the periodic table and notice that it has a completely filled energy shell. When an energy shell is completely filled, the element does not form bonds with other atoms. It is inert or nonreactive. Thus Neon is a monatomic gas. Cl2, F2, and N2 are diatomic gases (bond with another atom. Di means two).