Everything is made of atoms. They are the smallest unit that defines what we call chemical properties. Each atom is made of a very tiny nucleus surrounded by a large space with electrons in it. There are different kinds of atoms. For a stable atom, each one will have equal numbers of protons and electrons so that it doesn't have any electric charge. The simplest kind of atom to make is hydrogen. This atom has one proton and one electron. You can make a helium atom with two protons, two neutrons, and two electrons. The periodic table of the elements gives the rules for how each type of atom is built.
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