Atoms

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An atom is the smallest subdivision of matter. An atom consists of three types of particles. The nucleus, or center, of an atom contains neutral particles, called neutrons, and positively charged particles, called protons. Around the nucleus is a thinly populated region that contains electrons, particles with a negative electrical charge. You can picture an atom as a miniature solar system. The sun represents the protons and neutrons. Electrons orbit the nucleus much like planets orbit the sun (except that planets remain in the same plane). In an atom, the number of protons and the number of electrons are equal; therefore, the atom is neutral or has no electrical charge. If an atom gains or loses electrons, the charge of the atom becomes negative or positive, respectively. Charged atoms are called ions. If you have ever shuffled your feet on a carpet and then touched a door knob, you are familiar with an electrical charge. An element is a substance which cannot decompose into other substances by ordinary chemical means. One atom of an element will have all the characteristics of that element. Although you may not realize it, you are already familiar with several elements. For example, the element carbon is in diamonds, used for jewelry, and in graphite, used in pencil lead. We also use the elements gold and silver for jewelry. We need the element oxygen in the air we breathe. We worry about the carbon dioxide emitted by our cars. Carbon dioxide is a compound, or a combination of two or more elements. In the case of carbon dioxide, two atoms of oxygen combine with one atom of carbon. Another common compound, table salt, is made of the elements sodium and chlorine.
Table 1. Average Composition of the Crust
Element (Symbol) Weight percent
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Oxygen (O) 46.6
Silicon (Si) 27.7
Aluminum (Al) 8.1
Iron (Fe) 5.0
Calcium (Ca) 3.6
Sodium (Na) 2.8
Potassium (K) 2.6
Magnesium (Mg) 2.1...