ALWAYS IN MOTION
 

Molecules also can form from two or more atoms of a single element. Oxygen tends to occur in pairs most of the time in Earth’s atmosphere. Thus an oxygen molecule is sometimes denoted by the symbol O₂, where the O represents oxygen, and the subscript 2 indicates that there are two atoms per molecule. The water molecule is symbolized H₂O because there are two atoms of hydrogen and one atom of oxygen in each molecule. Sometimes oxygen atoms are by themselves; then we denote the molecule simply as O. Sometimes there are three atoms of oxygen grouped together. This is the gas called ozone that has received attention in environmental news. It is written O₃.

Fig. 1. Simplified rendition of molecules in a solid (a), in a liquid (b), in a gas (c). The gas molecules are shown smaller for illustrative purposes only.

Molecules are always moving. The speed with which they move depends on the temperature. The higher the temperature, the more rapidly the molecules move around. In a solid, the molecules are interlocked in a sort of rigid pattern, although they vibrate continuously (Fig. 1a). In a liquid, they slither and slide around (see Fig. 1b). In a gas, they are literally whizzing all over the place, bumping into each other and into solids and liquids adjacent to the gas (see Fig. 1c).