The analogy that electrons orbiting a nucleus resembles planets orbiting a star is surly flawed but serves to conjure up a general picture. The most obvious problem is that while each star orbit generally contains but one planet, each orbital shell about a nucleus contains multiple electrons. The first orbit has a max of 2, the second has a max of 8, the third - 18, and so on in a complex but prescribed pattern. Most of the time, the number of (negative) electrons orbiting a nucleus equals the number of (positive) protons in the nucleus, thereby balancing the atom electrically.
Something Alan hit on is the idea of electron sharing, a whole field of study in itself. If we look at an atom of oxygen, we’ll find 8 protons in the nucleus, and normally 8 electrons. So, that would be 2 electrons in the inner shell, and 6 in the outer. For whatever reason, atoms really really prefer exactly 8 electrons in the outer shell, so having only 6 leaves the atom wanting more even though it is electrically balanced. (Side note: Try tearing an electron away from an oxygen atom sometime, very extremely difficult). This gives cause to 2 common solutions:
Many times, if there is any hydrogen nearby, 2 hydrogen atoms, with one proton and one electron each, will hook-up and share their electrons with the oxygen atom. This is a suitable compromise. By sharing 2 electrons with the 2 hydrogen atoms, the oxygen atom has it’s 8 electrons in the outer shell (if only shared or part time), and the entire unit of 3 atoms is electrically neutral (3 atoms, 10 total protons, 10 total electrons). This chemical bonding, or sharing of electrons, of 1 oxygen atom and 2 hydrogen is what makes one molecule of water (H2O).
Other times 2 oxygen atoms will hook-up chemically to form one molecule of oxygen gas (O2). Think of this – 2 oxygen atoms, each of which would like 2 more electrons in their outer shells, will bond chemically, meaning they will each share 2 of their outer electrons with the other atom, allowing both the illusion of having 8 in the outer shell (10 total), and still maintain an overall electrical balance (2 atoms, 16 total protons, 16 total electrons).
So you see, pretty much the whole of chemistry is based on electrons, and the sharing thereof.
Many other possibilities are possible with oxygen alone: O3, OH-, H3O+, and many more. One last point here. Because it has such a hunger for 2 electrons to complete the outer shell, oxygen is a very reactive substance. There are a few other types of atoms that naturally have 8 electrons in the outer shell and are electrically balanced. These are chemically inert, meaning they simply do not react chemically with other substances (because there is no need, no excess or deficiency of electrons). You could say they don’t share well at all.
Sorry if this got too long.
Radar
[This message has been edited by Radar (edited 01-27-2006).]
