Molecules that are elements

We are learning how to:

• explain that not only compounds are made up of molecules

• define ‘polyatomic’

• explain why diamond is not polyatomic.

Diatomic and polyatomic molecules

Molecules are not always made of different atoms. Sometimes they are made of only one type of atom. In other words, an element can exist as a molecule.

Some gases are made up of pairs of identical atoms, bonded together as a molecule. Because two atoms are bonded together to form each molecule of the gas, the gas is referred to as a diatomic gas.

Since the molecule is made of only one type of atom, it is an element, not a compound. Oxygen gas is one such example. Oxygen gas is always written O2, not just O. Its molecular model is shown in Fig 9.7.

There are other elements that exist as polyatomic molecules, such as phosphorus and sulfur (Fig 9.8).

A polyatomic molecule is a single unit formed of at least three atoms.

Diamond is a precious stone that consists of a clear and colourless crystalline form of pure carbon. In diamond, the carbon atoms are all bonded to each other in a crystal structure that can be very large. This is called a macromolecule.