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Atoms can chemically combine with other atoms to form molecules. Molecules usually have very different properties from the atoms they comprise. For example, water is composed of molecules that consist of two atoms of hydrogen and one atom of oxygen (hence, H2O). Obviously water is appreciably different from either hydrogen or oxygen. Likewise, the molecules of table salt consist of an atom of sodium and an atom of chlorine, neither of which would be particularly appetizing on French fries.

Hydrogen, oxygen, sodium, and chlorine are all elements. Water and salt are called compounds. Salt water, however, is a mixture rather than a compound because the water and the salt maintain their own properties.

The number of electrons in an atom is usually the same as the number of protons. But in certain circumstances, electrons can be dislodged from atoms. That's how electricity happens.

The words electron and electricity both derive from the ancient Greek word ηλεκτρον (elektron), which you might expect means something like "little tiny invisible thing." But no—ηλεκτρον is actually the Greek word for "amber," which is the glasslike hardened sap of trees. The reason for this unlikely derivation is that the ancient Greeks experimented with rubbing amber with wool, which produces something we now call static electricity. Rubbing wool on amber causes the wool to pick up electrons from the amber. The wool winds up with more electrons than protons, and the amber ends up with fewer electrons than protons. In more modern experiments, carpeting picks up electrons from the soles of our shoes.

Protons and electrons have a characteristic called charge. Protons are said to have a positive (+) charge and electrons are said to have a negative (–) charge. Neutrons are neutral and have no charge. But even though we use plus and minus signs to denote protons and electrons, the symbols don't really mean plus and minus in the arithmetical sense or that protons have something that electrons don't. The use of these symbols just means that protons and electrons are opposite in some way. This opposite characteristic manifests itself in how protons and electrons relate to each other.

Protons and electrons are happiest and most stable when they exist together in equal numbers. An imbalance of protons and electrons will attempt to correct itself. When the carpet picks up electrons from your shoes, eventually everything gets evened out when you touch something and feel a spark. That spark of static electricity is the movement of electrons by a rather circuitous route from the carpet through your body back to your shoes.

Another way to describe the relationship between protons and electrons is to note that opposite charges attract and like charges repel. But this isn't what we might assume by looking at the diagram of the atom. It looks like the protons huddled together in the nucleus are attracting each other. The protons are held together by something stronger than the repulsion of like charges, and that something is called the strong force. Messing around with the strong force involves splitting the nucleus, which produces nuclear energy. In this chapter, we're merely fooling around with the electrons to get electricity.

Static electricity isn't limited to the little sparks produced by fingers touching doorknobs. During storms, the bottoms of clouds accumulate electrons while the tops of clouds lose electrons; eventually, the imbalance is evened out with a stroke of lightning. Lightning is a lot of electrons moving very quickly from one spot to another.

The electricity in the flashlight circuit is obviously much better mannered than a spark or a lightning bolt. The light burns steadily and continuously because the electrons aren't just jumping from one place to another. As one atom in the circuit loses an electron to another atom nearby, it grabs another electron from an adjacent atom, which grabs an electron from another adjacent atom, and so on. The electricity in the circuit is the passage of electrons from atom to atom.

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