When atoms of elements join together to form a compound, the resulting particles are molecules.
above figure is an example of a molecule of water, consisting of three atoms put together. The natural form of an element is also known as its molecule. Oxygen tends to occur in pairs most of the time in the earth’s atmosphere. Thus, an oxygen molecule is sometimes denoted by the symbol O2. 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 exist all 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, which has received much attention lately in environmental news. It is written O3.
All matter, whether solid, liquid, or gas, is made of molecules. These particles are always moving. The speed with which they move depends on the temperature. The hotter 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 (following Fig. A). In a liquid, they slither and slide around (following Fig. B). In a gas, they rush all over the place, bumping into each other and into solids and liquids adjacent to the gas (following Fig. C).