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The charge is a whole number because we say it is. The +1, -2, or whatever refers to the polarity and the number of net electronic charges present. Charge is actually measured in coulombs, and an electron or proton has a charge of -/+ 1.60 x 10-19 coulombs. This is the smallest increment of charge that we can measure (supposedly the smallest increment that exists, but who knows what subatomic particles we may discover in the future). If the charge is +3.20 x 10-19, then we say the ion has a charge of +2.As far as the atom "keeping track", it's actually quite simple. It doesn't. Electrons can adhere to an existing atom, creating an anion. They can leave an existing atom, creating a cation. The tendency towards this happening depends on electrostatics. As an electron approaches an atom, it can feel repulsions caused by the electrons already present, and attractions caused by the protons present. If the attractive forces are stronger, as tends to be the case with the nonmetals (they're smaller, so the electron can get closer to the nucleus), the electron will adhere to the atom. If the repulsive forces are stronger, the electron will tend to veer away.Sometimes, an atom with strong attractive forces approaches an atom with weak attractive forces. When this occurs, an electron will leave the weakly attractive atom and hop over to the strongly attractive atom.Now, to comment on something that stevewillie said, most elements do not have an equal number of protons and neutrons. That's really only partially true, even for elements from helium to calcium (less than a quarter of the stable elements). After that (and even some elements before that), the atoms prefer to have more neutrons than protons. By the time you get to bismuth (the last element with a stable isotope), the ratio of neutrons to protons is about 1.5 : 1.
Thanks for the responses. We got John Law's question answered. I haven't figured out why some questions which I regard as silly and non-scientific get substanial responses while to-the-point scientific questions end up in limbo. (Actually I have figured it out, but I don't like the answer.)
Quote from: stevewillie on 22/09/2008 20:33:35Thanks for the responses. We got John Law's question answered. I haven't figured out why some questions which I regard as silly and non-scientific get substanial responses while to-the-point scientific questions end up in limbo. (Actually I have figured it out, but I don't like the answer.) So what's the answer?
My take is that a lot of people think that to-the-point scientific questions are boring while silly questions allow a lot of latitude for banter. That's OK for some of the forums like "That Can't Be True" but this was a chemistry question that deserved a response from someone.