# Review of Elements and Compounds

Formulas and Subscripts

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## Key Questions

• A chemical element is a pure chemical substance consisting of a single type of atom distinguished by its atomic number, which is the number of protons in its atomic nucleus. Elements are divided into metals, metalloids, and non-metals.

• The physical and chemical properties of a compound differ from its component elements. For example, the element sodium (Na) is a soft, silvery gray active metal that reacts explosively with water to form sodium hydroxide ($N a O H$) and hydrogen gas (${H}_{2}$). The element chlorine, which exits as a diatomic gaseous molecule ($C {l}_{2}$), has a green color and is toxic. These two elements react to form sodium chloride ($N a C l$), also called common table salt, which is a white crystalline solid that is not toxic and dissolves in water, but does not react with it.

Figure 23: (a) Sodium metal (immersed in oil to prevent reaction with oxygen and moisture in the air); (b) chlorine gas; (c) the reaction between sodium and chlorine; (d) sodium chloride (common table salt)

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