Molecules

As an atom is the smallest part of an element, and as a compound must contain at least two elements, it follows that the smallest part of a compound that can exist must contain at least two different atoms. This is called a molecule. Although the atoms of elements take part in chemical reactions, they very often do not normally exist as single atoms when they are free. The atoms of such gases as oxygen, hydrogen, nitrogen and chlorine may be considered as very friendly ones, as they almost always occur in pairs, each pair forming a molecule, the smallest particle able to exist alone. On the other hand, the atoms of such gases as argon, helium and neon seem to prefer to remain alone. As the molecules of these gases contain only one atom, they are said to be monoatomic in contrast to the former which are diatomic. Some compounds are so complex that a single molecule may contain several hundreds of atoms and be quite large in comparison with a molecule of hydrogen, the smallest of all. It is possible to take photographs showing the way in which such large molecules are made up by means of a special type of microscope called an electron microscope, but these molecules arc still too small to be seen through the most powerful of optical microscopes.