We can write a chemical formula for an element or compound which exists as a molecule. Using the chemical symbols, this shows how many of each type of atom the molecule contains.
E.g. the element oxygen exists as a diatomic molecule, so the formula is O2. Th e formula for methane is CH4.
If elements can combine to make groups containing different combinations of atoms joined together, these are different compounds, which have different chemical and physical properties. We write them with different formulae which show how many atoms are joined together.
Ion
An ion is a particle (an atom or group of atoms) carrying positive or negative charge. Ions with a positive charge are called Cations. Ions with a negative charge are called Anions.
Most elements only form one sort of ion. It is essential to learn the characteristic charges for the most common ions.
Combining ions
Positive and negative ions combine together to form ionic compounds. Compounds must contain an equal number of positive and negative charges overall. Thus the charges on the ions will determine how many cations and anions will combine.
Ionic compounds
If a compound is ionic it is not appropriate to use the term molecule, because many thousands of ions are joined together. The formula cannot show the number of atom. A chemical formula for an ionic compound shows the relative proportions of the different ions. The formula unit is the simplest formula for the compound and the smallest ‘particle’ that can exist.
E.g. NaCl (sodium chloride) and CaO (calcium oxide)
Naming Compounds
Ionic compounds are named with the metal (positive ion) first, then the non- metal (negative ion).
Note how the end of the non-metal changes to “ide” to show that it is a negative ion. If the ion contains oxygen as well, then the ending changes to “ate” or “ite”
