Protons, neutrons and electrons

Rub a balloon on your hair, lift it away, and your hair rises after it from a centimetre away. A minute earlier it did nothing. Weigh the balloon on a kitchen scale before and after, and the reading does not move. Yet something has moved onto it. That something carries a charge - and it weighs next to nothing.

This is video two of seven in Atomic structure. If atoms and elements still feel shaky, go back to Atoms, elements and compounds first.

Now, an atom is not a solid ball. Inside it are three kinds of smaller particle: protons, neutrons and electrons. Every atom has a nucleus, a tiny, dense core. The protons and the neutrons are packed together in the nucleus. The electrons are outside the nucleus, in shells around it. A shell is a set distance from the nucleus where electrons are found. Any drawing of an atom is far from scale. The nucleus is much smaller than a drawing can show, and the video How big an atom is puts a number on that. Each particle has a relative charge. Relative means compared with the others, with no unit. A proton is plus one, an electron is minus one, and a neutron is zero. Each has a relative mass as well. A proton has a relative mass of one, and so does a neutron. An electron's relative mass, though, is very small - far less than a proton's. Where is almost all of an atom's mass: in the nucleus, or out in the shells? In the nucleus. The protons and neutrons sit there, with a relative mass of one each. The electrons outside are tiny by comparison. Very small is not the same as nothing. An electron does have mass. It is so small, though, that gaining or losing a few makes no difference you could ever weigh.

Now, the charge column. A whole atom has no overall charge; it is neutral. Neutral means the charges add up to zero. Neutral sounds like neutron, but they are different things. Neutral describes a total of zero. The neutron is a particle, the one with no charge at all. Take a particle with twelve protons, twelve neutrons and twelve electrons. Each proton brings plus one, and each electron brings minus one. Twelve lots of plus one, twelve of minus one: what is the overall charge? Zero. Twelve lots of plus one make plus twelve. Twelve lots of minus one make minus twelve. Plus twelve and minus twelve cancel. The neutrons never came into that sum. Make it fourteen neutrons instead of twelve, and the total is still zero. So an atom is neutral because its protons and electrons are equal in number. Now, take one electron away from that same particle. The nucleus is untouched: twelve protons, and now eleven electrons. Twelve protons, eleven electrons: what is the overall charge now? Plus one. Plus twelve and minus eleven leave plus one over. It is no longer a neutral atom - and only the electrons changed. The protons stayed exactly where they were. A student explained it like this: an atom has no charge because there is nothing charged inside it. The reason sounds sensible, but it describes the wrong thing. What is wrong with that student's reason? There are charged particles inside. Protons are positive and electrons are negative. So the atom has no overall charge because equal numbers of them cancel. Put the two columns together and you have your handle for this video: heavy in the nucleus, light outside it, and the charges cancel to nothing.

Back to the balloon and your hair, because the table now explains it. Which particle moved onto the balloon: protons, neutrons or electrons? Electrons. They sit on the outside of atoms, in the shells, so rubbing can move them. Protons and neutrons are held in the nucleus. And that is why the scale did not move. Each electron's relative mass is very small, so even a huge number of them is too light to weigh. Your hair, then, gave those electrons up. Its atoms now have more protons than electrons, so the hair is positive, and the negative balloon pulls it towards itself.

Now check it yourself; the table answers every one of these. Eight protons, ten neutrons, eight electrons: what is the overall charge? It is neutral. Eight protons and eight electrons cancel, and the ten neutrons add nothing. Here's a different kind: which two particles are in the nucleus? Protons and neutrons. The electrons are outside the nucleus, in shells. Now one step further: nine protons and ten electrons. What is the charge? Minus one. Plus nine and minus ten leave one minus over, because this particle has gained an electron.

Tap the thumb once you could draw the atom from memory, with each particle in its place and its charge beside it. That one is done. If the table has not settled yet, keep the video, and give it a day before you look again.

Next in the chapter: How big an atom is.

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