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...a congenial arrangement for atoms

Let’s re-examine that oxygen atom illustrated in the section on atoms.

Notice the arrangement of electrons in the shells. The inner shell has a single pair of electrons. That pair fills the inner shell. The outer shell has two pairs of electrons and two unpaired electrons. Again quantum mechanics is to blame for the configuration. Oxygen has 8 protons so is limited to 8 electrons in order to maintain electrical neutrality. Quantum mechanical rules require electron shells to be filled in such a way that the number of unpaired electrons is minimized. Second electron shells require 8 electrons fill them so the oxygen atom by itself is two shy of having its outer shell filled.

Now suppose a second oxygen atom was in the neighborhood of our first one. The two oxygen atoms will in a sense merge their outer shells so that the two unpaired electrons in each atom allow both atoms to, at least part of the time, have filled outer shells as they share the unpaired electrons. The illustration below pictures that situation. That is why in nature, oxygen gas is normally found as a di-atomic molecule. Molecule is the name we give certain combinations of atoms. In a sense many atoms are more content in these combinations with themselves or other atoms.

The basis for the atom’s “contentment” with these combinations is worth exploring. It involves the notion of energy. Most of us have some idea of the difference between high energy and lower energy. A hot stove exhibits higher energy then a cold stove. A weight hung from a pulley has low energy sitting on the floor and higher energy when raised to the height of the pulley. A convenient way to think about the energy state of an object is that if you have to do work to change to that state, it is a higher energy state. That together with the fact that, left to themselves, things move to the lowest energy state available to them allows us to understand something about the nature of molecules. Turn off the stove and it cools off. Let go of the rope attached to the weight and it falls back to the floor.

Imagine that we might somehow grasp each of the atoms in the molecule above and try to pull them apart. We would find that force applied over some distance would be required to separate them. That means that atoms-separated is a higher energy configuration for this two atom system than the atoms-together configuration. The oxygen molecule then is stable, meaning that it does not spontaneously disassemble itself. Remember though that we are subject to quantum mechanical rules in this realm so a more precise statement would be that the probability of spontaneous separation of the oxygen atoms is so low as to not have any practical effect on a bottle of oxygen gas.

Hydrogen also forms diatomic molecules. The hydrogen atom has only a single proton as its nucleus and thus only a single electron in its first shell. In order to complete its only shell it will glom onto another single hydrogen atom to form an H₂ molecule as oxygen forms an O₂ molecule. Now if we carefully mix hydrogen and oxygen molecules in a single container all the atoms are content with their partners and nothing happens.

Suppose however that we add energy at one point in the container by firing a spark plug. The energy from the spark will break up some of the hydrogen and oxygen pairs in the neighborhood and resulting single hydrogen and oxygen atoms will seek lower energy states. It turns out the four hydrogen atoms each with a single unpaired electron and two oxygen atoms with two unpaired electrons each will combine in two H₂O molecules with a much lower energy configuration than the two separate H₂ and one separate O₂ molecules. So what happens to the energy given up by the atoms in forming these lower energy water (H₂O) molecules? Well, like the initial spark, it goes ultimately into breaking up more H₂ and O₂ molecules - and so on and so forth until the container runs our of either H₂ or O₂ molecules or, more likely, the container is destroyed in the attempt to contain the energy released in forming water from hydrogen and oxygen.

So with only a few rudimentary notions of atoms, molecules and energy levels we have predicted cataclysmic results and should advise out friends not to apply energy of any kind to a mixture of hydrogen and oxygen gasses.

As entertaining as explosions may be, we need to push along with our examination of molecules. So far we have studied only simple molecules with two or three atoms participating. The same principles apply to molecules with dozens, hundreds or more atoms in them, bound together by the rules of quantum mechanics. Below is a segment of a DNA molecule.

In our example of the hydrogen-oxygen explosion, the mixture of gasses before the application of a spark were in what is called a metastable state – a state that is stable but even small upsets may push it over the brink to fall into a much lower state. In that case the byproduct of that fall was violent destruction of the neighborhood. In the case of very complex molecules, many of them are metastable, easily broken up into pieces. Most often that kind of breakup just breaks down the complex molecule, releasing much less energy than the hydrogen explosion. The DNA molecule illustrated above is an extreme example of a complex molecule which is metastable – easily altered or even destroyed. The kind of shared electron bonds in the molecules we have seen are called covalent bonds.

Next we will visit other combinations of atoms not, strictly speaking, molecules.