Molecules
A molecule is a group of two or more atoms joined together chemically.
A molecule is the smallest part of an element or compound that can exist independently.
Molecules may be made up of atoms of the same element such as Hydrogen gas (H2 - two atoms of Hydrogen), or atoms of different elements such as Sodium Chloride (NaCl).
Substances made up of molecules consisting of atoms of different elements are called COMPOUNDS. (The word "compound" means made up of several parts.)
Some molecules are very large and may consist of hundreds, or even thousands of atoms.
The FORMULA of a substance is a way of describing the composition of the molecules of the substance. For example H2 means the smallest unit of Hydrogen gas consists of two atoms of Hydrogen combined together.
NaCl means the smallest unit of Sodium Chloride consists of one atom of Sodium combined with one atom of Chlorine. It could be written as: Na1Cl1, but it is a convention that the "1s" are never written.
A molecule of Hydrochloric acid is made up of the elements Hydrogen and Chlorine - one atom of Hydrogen and one atom of Chlorine.
A molecule of water is made of the the elements Hydrogen and Oxygen - two atoms of Hydrogen combined with one atom of oxygen.
Sugar (C12H22O11) is made up of 12 carbon atoms, 22 hydrogen atoms and 11 oxygen atoms.
Many elements in particular the gases occur naturally as molecules e.g. H2, O2, N2, Cl2.
NOTE: Remember that a charged atom is called an "ion". It would be more correct to say that the smallest unit of Sodium Chloride consists of one ion of Sodium and one ion of Chlorine.
To work out the formula of a compound you need to know the valency of each of its ions ...
Valency is the number of electrons an atom tends to gain, lose or share in order to satisfy the Octet Rule.
