Chapter 4: Elements and Compounds (or Twenty Million Dishes - and Counting - from Only Ninety Ingredients )

Figure 4-1. The variety of spices available in some markets is impressive  but pales in comparison to the number of different recipes can be made from them. So it is with the Periodic Table - pure elements number in the dozens, but the number of known compounds exceeds 20,000,000! (Image: "Spice Market" by Tord Sollie is licensed under CC BY-NC-ND 2.0.)

We saw in the previous chapter that hydrogen and oxygen can react to form water, H₂O, a reaction that unleashes considerable energy. But why is each oxygen atom satiated when it forms bonds to two hydrogen atoms? If each O-H bond formed decreases potential energy, why stop at two? Why not make H₃O or H₄O? Because these compounds do not form we can surmise, based on the arguments we made in the last chapter, that formation of these species would increase, not decrease, the potential energy relative to water. This is true, and there are some surprisingly simple guidelines that allow you to predict it.

Our goal in this chapter is to give you the tools to recognize what compounds “make sense”. In other words, we want you to recognize that H₂O is a reasonable formula for the compound formed between hydrogen and oxygen, but that H₃O and H₄O are not; indeed, they are offensive ideas (at least to those with some rather peculiar sensitivities)! This is really just another way of looking at a concept we introduced in Chapter 1, namely the fact that oxygen usually makes only two bonds. Carbon usually makes four. Hydrogen, only one. But why? It's easy enough to memorize such patterns, but our goal is to help you see Nature's logic behind those patterns because that will allow you to understand exceptions to "the rules" when they crop up. Rules here is in quotes because is no such set list of arbitrary things that Nature can or can't do when it comes to bonding. What we see in chemistry is the manifestation of sometimes complex series of events that result from bonds be made and broken, events driven by the tendency to reduce chemical potential energy. Chemists aim to articulate what the fundamental patterns of such events are so they can be applied more effectively in making new compounds or in understanding the way Nature does so.

To answer these questions we’ll need to take a closer look at the Periodic Table of the Elements; we introduced it in the last chapter, but its value in determining what compounds are reasonable - and which are not - is impossible to overstate. It is the go-to resource for summarizing vast amounts of knowledge about matter, the first step in understanding the vast puzzle Nature has so kindly provided us with.