THE BLUE SPHERES BELOW REPRESENT ATOMS HOW TO

The electrons in the 1s orbital are not used because the amount of energy needed to use those electrons is greater than the energy that would be released upon bond formation (they are held tightly to the nucleus by the electromagnetic force). Since each C-H bond is formed from bonding orbitals we can use the model for bonding where these four bonding orbitals arise from the “hybridization” of the pre-existing 2s and three 2p atomic orbitals. We also know, from experiment, that the shape of the methane molecule is tetrahedral there is a carbon at the center and the four C–H bonds pointing towards the corners of a four-sided figure. We begin with what we know: in methane the carbon atoms make four bonds, one to each of four hydrogen atoms. As previously we use the hybridization model to explain the behaviors we observe. What happens if we combine hydrogen with carbon? Do we get a compound with properties intermediate between the two? Absolutely not, as you might have expected when considering the differences between diamonds and graphite. It therefore makes sense that a carbon atom would form as many bonds as possible, resulting in the most stable possible molecular species. Remember that the formation of a bond always lowers the energy of a system. Carbon has a total of six electrons, two of which are in a filled (1s) quantum shell, and four valence electrons it is these valence electrons that can take part in bonding. To answer these questions we need to return to the ideas (introduced in Chapter 2) about the quantization of electron energy levels. In this discussion, we will be building on the ideas introduced when we talked about diamond, graphite, and graphene.

We will now consider in greater detail why this is so, what forms the bonds can take, and what are the consequences of this fact. As in all its compounds and its elemental forms, carbon is tetravalent, which means that it always forms four bonds. The simplest such compound is methane CH4, a major component of natural gas. There are many such molecules, and collectively they are known as hydrocarbons. In keeping with our ongoing attempt to keep things simple (or better put, as simple as possible), let us start by examining the types of molecules that can be formed by combining carbon with hydrogen. Now we move on to molecules involving atoms of carbon and other elements. Similarly atoms of carbon can be linked together in various ways to form diamond, graphite, and graphene (see Chapter 3). We have already seen an example of how a covalent bond can form between two hydrogen atoms producing molecular (H2) as opposed to the atomic form of hydrogen. Carbon: Always Tetravalent and Often TetrahedralĪtoms combine in many different ways. We are carbon-based life forms! Carbon compounds are also used in a wide range of synthetic materials, such as pharmaceuticals, polymers, and high-tech materials we also consume a lot of carbon compounds by burning them for fuel. Carbon is a key building block of the major molecules of life: proteins, nucleic acids, lipids, and carbohydrates. Under the conditions that persist on the surface of the Earth carbon compounds are stable enough to hang around but not stable enough to persist forever, so they are not dead ends.

Carbon is distinguished from most other elements in its ability to form a vast array of diverse compounds by bonding with itself and other elements with bonds that are not too strong and not too weak. Carbon is the fourth most abundant element in the universe (~3,032 atoms per million), after hydrogen (~705,700 atoms per million), helium (~275,200 atoms per million), and oxygen (~5,920 atoms per million). To extend our discussion to the wider world of what we might call heterogenous molecules, that is, molecules made up of atoms of more than one element, we will begin with carbon. Both involve understanding the concepts underlying how atoms interact. Modern chemistry is concerned (largely) with putting atoms together to form new and useful molecules.

THE BLUE SPHERES BELOW REPRESENT ATOMS HOW TO

Much of the efforts of alchemists, early chemists, and the modern refining industry involve determining how to (economically) separate specific types of atoms (elements) away from others.

Moreover, pure elements are rare in nature. While this makes things simpler (although you might not agree after thinking about the many forms of carbon), it leaves out the vast majority of the compounds that exist in the world and their chemistries. Up until this point we have considered only bonds between atoms of the same element.

The blue spheres below represent atoms