ATOMS

next paragraph). In a compound the molecule is the smallest part that has all the characteristics of the compound. Consider water, for example. Depending on the temperature, it may exist as a liquid (water), a solid (ice), or a gas (steam). Regardless of the temperature, it will still have the same composition. If we start with a quantity of water, divide this and pour out one half, and continue this process enough times, we will end up with a quantity of water that cannot be further divided without ceasing to be water. This quantity is called a molecule of water. If this molecule of water is divided, instead of two parts of water, we will have one part of oxygen and two parts of hydrogen (H₂O). ATOMS Molecules are made up of smaller particles called ATOMS. An atom is the smallest particle of an element that retains the characteristics of that element. The atom of one element, however, differs from the atoms of all other elements, Since over 100 elements are known, there must be over 100 different atoms, or a different atom for each element. Just as thousands of words are made by a combination of the proper letters of the alphabet, so thousands of different materials are made by the chemical combination of the proper atoms. Any particle that is a chemical combination of two or more atoms is called a molecule. The oxygen molecule has two atoms of oxygen, and the hydrogen molecule has two molecules of hydrogen. Sugar, on the other hand, is a compound composed of atoms of carbon, hydrogen, and oxygen. These atoms are combined into sugar molecules. Since the sugar molecules can be broken down by chemical means into smaller and simpler units, we cannot have sugar atoms. In figure 2-1 you will see that the atoms of each element are made up of electrons, protons, and, in most cases, neutrons, which are collectively called subatomic particles. Further- more, the electrons, protons, and neutrons of one element are identical to those of any other element. The reason there are different elements is that the number and arrangement of electrons and protons within the atom are different for the different elements. The electron is considered to be a small negative charge of electricity. The proton has a positive charge of electricity equal and opposite to the charge of the electron. Scientists have measured the mass and size of the electron and proton. They know how much charge each has. The electron and proton each have the same quantity of charge, although the mass of the proton is about 1837 times that of the electron. In some atoms, a neutral particle exists called a neutron. The neutron is a mass about equal to that of a proton, but it has no electrical charge. According to a popular theory, the electrons, protons, and neutrons of the atoms are thought to be arranged in a manner similar to a miniature solar system. The protons and neutrons form a heavy nucleus with a positive charge, around which the very light electrons revolve. Figure 2-1.—Structure of simple atoms. 2-2