![]() ![]() Many carbon atoms, however, have 8 neutrons. 6 protons + 6 neutrons = 12 nucleons, and so this variety is called Carbon-12. For example, the most common variety of carbon has six neutrons, in addition to its six protons. The number of neutrons in an atom can vary, but is typically close to the number of protons, for light elements (atoms with small numbers of protons). 26 protons means iron, 92 protons for uranium, and so on. Any atom with one proton is a hydrogen atom. For example, any atom with six protons in its nucleus is a carbon atom. Protons and neutrons ("nucleons," as they're referred to collectively) are much heavier than electrons, and that's why most of the mass of an atom is in the nucleus.Ītoms are classified according to the number of protons in their nuclei. ![]() The nucleus is populated by two kinds of particles: positively charged "protons," and electrically neutral particles, called "neutrons." These particles are nearly identical to each other, except in electric charge. And this gave a much clearer picture of what's inside an atom. They found new, more powerful, more exotic ways to bombard atoms, and built better detectors to measure the result. Protons and neutronsPhysicists didn't stop here. So the atom is found to be in two parts: a tiny nucleus at the center, which contains a positive charge, and the surrounding region, which is sparsely populated by negatively charged electrons. The electrons aren't in the nucleus, but instead surround it. (We'll get to electric charge in the next page.) Rutherford found that the region at the center, which we now call the "nucleus," carries a positive charge. Now the atom was already known to contain tiny particles called "electrons," which carry a negative electric charge. When an alpha particle hits this region, it bounces back. The bulk of an atom is concentrated in a very small region at the center. Rutherford tried to figure out how this could happen, and the best scenario he could come up with was, in fact, the truth. What he found was that a few of the particles, and not nearly as few as he expected, bounced back more or less in the direction from which they came. ![]() The experiment used a very thin gold foil as the target, and Rutherford expected most of the alpha particles to go on through, and be detected on the other side. It's not important here.) He found the results quite remarkable. (Don't worry about what alpha particles are. This is what Ernest Rutherford was doing early in this century, in an experiment in which gold atoms were bombarded with alpha particles. That is, you bombard them with other microscopic particles, and see what happens. Now the way this was discovered is the way that most things are learned about microscopic particles. Electrons and nucleiNot long after the discovery of the atom, it was discovered that the atom is made up of two different regions. What's in an Atom What's in an Atom * Electrons and nucleiAt first glance., well, not really at first glance, since at first glance you can't even see an atom, because they're so small, but if you could see them. ![]()
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