Was it possible to knock the positive charge loose from the mass of the hydrogen nucleus? Nothing physicists did could manage to do that. In 1914 Rutherford decided the attempt should be given up. He suggested that the hydrogen nucleus, for all its high mass, should be considered the unit of positive electrical charge, just as the electron was the unit of negative electrical charge. He called the hydrogen nucleus a “proton” from the Greek word for “first” because it was the nucleus of the first element. [Illustration: _One proton balances 1837 electrons._] Why the proton should be so much more massive than the electron is still one of the unanswered mysteries of physics. The Proton-Electron Theory What about the nuclei of elements other than hydrogen? All the other elements had nuclei more massive than that of hydrogen and the natural first guess was that these were made up of some appropriate number of protons closely packed together. The helium nucleus, which had a mass four times as great as that of hydrogen, might be made up of 4 protons; the oxygen nucleus with a mass number of 16 might be made up of 16 protons and so on. This guess, however, ran into immediate difficulties. A helium nucleus might have a mass number of 4 but it had an electric charge of +2. If it were made up of 4 protons, it ought to have an electric charge of +4. In the same way, an oxygen nucleus made up of 16 protons ought to have a charge of +16, but in actual fact it had one of +8. Could it be that something was cancelling part of the positive electric charge? The only thing that could do so would be a negative electric charge[1] and these were to be found only on electrons as far as anyone knew in 1914. It seemed reasonable, then, to suppose that a nucleus would contain about half as many electrons in addition to the protons. The electrons were so light, they wouldn’t affect the mass much, and they would succeed in cancelling some of the positive charge. Thus, according to this early theory, _now known to be incorrect_, the helium nucleus contained not only 4 protons, but 2 electrons in addition. The helium nucleus would then have a mass number of 4 and an electric charge (atomic number) of 4 - 2, or 2. This was in accordance with observation. This “proton-electron theory” of nuclear structure accounted for isotopes very nicely. While oxygen-16 had a nucleus made up of 16 protons and 8 electrons, oxygen-17 had one of 17 protons and 9 electrons, and oxygen-18 had one of 18 protons and 10 electrons. The mass numbers were 16, 17, and 18, respectively, but the atomic number was 16 - 8, 17 - 9, and 18 - 10, or 8 in each case.