In the last article, we went over some of the physical characteristics of water and why too much of it is not good. In the process, we built a little on our understanding of potential and kinetic energy. This time, let’s get a little bit into why water is such a big deal that NASA spends so much time and money looking for it on other planets. And moons. And asteroids. And comets.
Water is a small, simple molecule—one oxygen atom bonded to two hydrogen atoms, but it’s easily the most important chemical in the world, from the biological perspective. If you look at a water molecule, it’s sort of triangular-shaped, with the larger oxygen atom is on one side (let’s call it the top) and the two smaller hydrogen atoms are on the other side (let’s call this side the bottom). If you will remember, we’ve mentioned a couple of times about electrons and chemical bonds. Isn’t it interesting how different subjects we talk about in these articles keep connecting back and forth with each other? Anyway, electrons, as you will recall, are the particles of atoms that orbit the nucleus (which is where the protons and neutrons are). Electrons are negatively-charged and protons are positively charged. The chemical bonds which cause the oxygen and the hydrogens to stick together to form a water molecule result from an interaction of their electrons, which results in a covalent bond. A quick review of chemical bonds is probably in order, now.
As we’ve mentioned several times, hydrogen is the smallest atom, with one proton (no neutrons) in the nucleus and one electron. Oxygen has eight protons and eight neutrons in the nucleus with eight electrons in orbit around it. Hydrogen is not “happy” (meaning “stable”) with just one electron. It wants to have two in order to be most stable. Oxygen is not “happy” with eight electrons. It needs to have ten to be in its most stable configuration. So each hydrogen atom needs to gain one electron and the oxygen atom needs to gain two.
Some atoms can just completely lose or gain an electron to get “happy”. When they do, they become what are called “ions”. Sodium and Chlorine are examples of atoms that can become ions. Sodium has one too many electrons to be stable, and it has a very weak attraction to that electron, so it’s easy to lose it. When it does, Sodium becomes an ion with a positive charge. Remember that protons are positively-charged and electrons are negatively-charged? In most atoms, the number of protons equals the number of electrons, so the positive charges balance out the negative charges and the overall atom is neutral (has 0 electric charge). When sodium loses an electron, it now has one more proton than it does electrons, so it has an extra positive charge—therefore it becomes a positive ion. Chlorine is just the opposite. It really, really wants to attract an extra electron to be stable. When it does, it now has one, excess, negatively-charged, electron, so it becomes a negative ion. When a positive ion and a negative ion come close to one another, they stick together because, as everyone knows, opposites attract. This type of bond is called an “ionic bond”. The molecule formed in this particular case is sodium chloride, which is commonly referred to as table salt.
Most atoms, though, cannot change the number of electrons they have, so they can’t become ions or form ionic bonds. They still want to be “happy”, however, and there is another way they can get their electrons right and be happy. Instead of completely losing an electron or gaining one, atoms can also “share” electrons with other atoms. This is the case for oxygen and hydrogen. As we said, oxygen needs to gain two electrons and each of the hydrogens needs to gain one. To achieve this, the oxygen atom “shares” one of its electrons with the hydrogen atom and the hydrogen shares its electron with the oxygen. Now there are two electrons (the one from the hydrogen and the one from the oxygen) orbiting BOTH the hydrogen atom and the oxygen. This is a big oversimplification, but you can think of it like this: before the sharing, the electron around the hydrogen just went in a circular orbit around the hydrogen nucleus and the electron from the oxygen (along with the other seven electrons around the oxygen nucleus) just orbited the oxygen in a circle. Once the electrons were shared, they are now orbiting both the oxygen and the hydrogen in sort of a figure-eight-shaped orbit, alternating between going around the hydrogen and then going around the oxygen. This pair of shared electrons forms a very strong bond between the atoms, called a “covalent bond”. The oxygen in a water molecule forms covalent bonds with two hydrogen atoms, which is why its molecular formula is H2O.
Covalent bonds are, by far, the most common type of chemical bonds. The covalent bond in a water molecule is, however, a special type of bond, called a “polar” covalent bond. The word “polar” literally means “having poles” or “having different ends”. The Earth has a north and a south pole. A magnet has a north and a south pole. A battery has a positive pole and a negative pole. In the case of a water molecule, it is polar because of its shape and the characteristics of the atoms that comprise it. Oxygen has a greater attraction for electrons than hydrogen does. This means that, although there are two electrons shared between the oxygen and each hydrogen, they aren’t EQUALLY shared—the electrons actually spend more time up around the oxygen than they do down around the hydrogens. Since electrons carry a negative charge, this means that the oxygen side of the molecule (the top) is actually a little bit negative and the hydrogen side of the molecule (the bottom) is a little bit positive. In the image below, this “partially negative” and “partially positive” charge on different sides of the water molecule are indicated by the symbol gamma, which looks like a fancy, curly “d” with a minus sign, for partial negative and gamma with a plus sign for partial positive. This is what being “polar” means in chemistry. There are lots of polar molecules to be found, but water is special, because of its size and shape, and it’s polarity leads to some very important characteristics.
Source: University of Hawai’i
When you have a container full of something, the molecules are generally just randomly arranged, like if you dumped a bunch of red, yellow and blue marbles into a jar—there would be a few red ones here, a yellow one there—just random. In a jar of water, however, the molecules are NOT randomly arranged. Because of the slightly negative side (top) and the slightly positive side (bottom), the water molecules arrange themselves in a sort of orderly fashion where the top of one molecule is attracted to the bottom of the adjacent molecule, which is, in turn, oriented so it is close to the top of a third, and so on. They aren’t randomly arranged. This property of water is why water is so important. It is why, among other things, ice floats, how trees can get so tall and why there is life on Earth,. We’ll expand more on those ideas, and how the polar nature of water makes it unique in the next article.