More Atoms

Nitrogen is similar to carbon except it most often has a valence of 3, or in a few cases 5. The usual color for nitrogen is blue. Oxygen is a lot like nitrogen, but with a valence of 2. Oxygen is usually shown as red.

We've seen that carbon combines with hydrogen to make methane. Nitrogen and oxygen also combine with hydrogen. A molecule of one nitrogen atom and 3 hydrogens forms ammonia. A molecule of one oxygen atom and two hydrogens is already familiar as water, or good old H₂O.

Incidentally, the sequence of elements in a condensed formula is pretty much one of convention. We could say OH₂ or H₃N or H₄C instead, but no one ever really does when they're just the simple molecules on their own. You will see some molecular structures with for example H₃C-- in order to show that the carbon is the one with a bond to some other part of the molecule, but that is pretty much the only time the sequence is varied up.

Here are the structures of ammonia and water:

H--N--H
   |
   H

H--O
   |
   H

Nitrogen and oxygen atoms are tetrahedral like carbon atoms are. Some atoms have other geometries. Sulfur has all of its bonds at right angles like the corners of an octahedron. Hydrogen sulfide has almost right angle bonds, though the hydrogens slightly repel each other, making the angle a little more than 90°. Phosphorus has a weird geometry where two of the bonds point in opposite directions like poles of a globe, while the remaining three form a triangular shape around the "equator". However, the phosphate and sulfate groups, where the central atom is surrounded by four oxygen atoms, both have their oxygens in a tetrahedral shape. Nitrates have three oxygens in a flat triangle shape around a central nitrogen atom. It's not tetrahedral because the nitrogen atom actually has 5 bonds in this instance.