A molecule is the smallest particle of a substance that can exist and still keep the properties of the whole. For example, if you broke down an of sugar, the elements would not have the characteristics of sugar—its taste or its color, among other things.
Sometimes molecules are very simple, others have thousands of atoms arranged in a complicated pattern. In gases, such as helium and neo», a consists of only one atom. Some molecules contain two or more atoms of the same kind. A molecule of water, for example, is made up of two atoms of hydrogen and one of oxygen.
In contrast, the molecule of pure natural rubber is thought to contain about 75,000 carbon atoms and about 120,000 hydrogen atoms. So you can see that molecules differ greatly in size.
Simple molecules like that of water are only a few billionths of an inch in length. The rubber molecule is thousands of times larger. Some molecules are shaped like footballs, others are long and threadlike.
It is really impossible for us to imagine how small molecules are. For example, let us take 10 cubic centimeters of air. In this space, there are over 300 million billion molecules (3 with 20 zeroes after it). And that bit of air is not packed tightly because it actually contains a great deal of empty space.
How Small Is A Molecule?
Within the fields of molecular biology and pharmacology, a small molecule is low molecular weight (< 900 daltons) organic compound that may regulate a biological process, with a size on the order of 1 nm.