My Science School

MOLECULES

Atoms make up all the objects and substances in our world. But they are rarely single atoms, alone or unattached. They are usually attached or joined to other atoms. For example, the oxygen gas that makes up one-fifth of the air does not float about as single atoms of oxygen, O. It is in the form of oxygen atoms joined together in pairs, O2. Two or more atoms linked or joined together make a molecule. O2 is a molecule of oxygen.

If atoms of one chemical element join or combine with atoms from other elements, this forms a compound. O2 is a molecule of oxygen but not a compound. Two atoms of hydrogen and one of oxygen form H2O, which is a molecule and a compound. Some compounds, like minerals in rocks, have 50 or 100 atoms in each molecule from 15 or 20 different elements. Other compounds, like certain plastics, have millions of atoms in each molecule but usually from only a few elements, mainly carbon, hydrogen, oxygen and nitrogen. The links between atoms are called bonds. There are different types of bonds depending on the atom’s structure and the conditions such as temperature and pressure.

One of the main features of the chemical element carbon is that it joins or bonds easily with many other types of atoms and also with itself. Carbon atoms can join like links in a chain to form enormously long molecules. Often the chain is made of the same groups of atoms, called sub-units, which are repeated hundreds or thousands of times along its length. This type of molecule is called a polymer and the repeated sub-units are monomers. Many types of plastics and artificial fibres like rayon, acrylic and nylon are polymers. So are molecules in living things like cellulose in plants, chitin in insect body casings and the carrier of genetic information, DNA.

The molecule known as DNA, found in our genes, is based on a group of atoms called ribose sugar, which is repeated millions of times in a long, coiled chain.