We are home schooling my son, and right now we are covering chemistry- ionic bonds, covalent bonds, etc. We've talked about how carbon is "special" in that it can form very complex molecules (organic chemistry), and I have done some hand waving by saying that it is its ability to make four bonds that causes that specialness.
He, quite reasonably, asked why nitrogen, with its ability to make three covalent bonds, couldn't be almost as special as carbon. I told him that it probably can make pretty complicated molecules, but that only being able to make three bonds is a limiting factor. That didn't seem to satisfy him, and to be honest, it doesn't totally satisfy me.
So why is carbon so special? Is it just the four bonds vs. three, or is it something else?