Elements And Atoms

Elements are substances that cannot be broken down chemically into simpler kinds of matter. More than 100 elements have been identified, though fewer than 30 are important to living things. In fact, more than 90 percent of the mass of all kinds of living things is composed of combinations of just four elements: oxygen, carbon, hydrogen, and nitrogen.

Information about the elements is summarized on a chart known as the periodic table, which appears in the Appendix. Each element has a different chemical symbol. A chemical symbol consists of one, two, or three letters, as shown in Figure 2-1. In most cases, the symbol derives from the first letter or other letters in the name of the element, such as Cl for chlorine. Most of the other symbols are derived from the Latin names of elements. One example is sodium's symbol, Na, from the Latin word natrium.

figure 2-1

The periodic table lists information about the elements, including the atomic number, the chemical symbol, and the atomic mass for each element. A complete periodic table can be found in the Appendix.

Atomic number -Chemical symbol

Atomic mass-

Helium

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