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(v) Stoichiometry:
All about Formulae
• Atoms combine together in fixed ratios that give them a full outer shell of electrons.
Chemical formulae of compounds tell you these fixed ratios. For example, the formula Mg
shows us that 1 Magnesium atom reacts with 2 Chlorine atoms to form Magnesium
Chloride, and consequently, a stable electronic configuration.
• Constructing chemical formulae: The ‘Criss-Cross’ Method
• Refer back to Topic (iii) – The Periodic Table. You will find that the number of electrons
in the valence shell of an atom of an element depends on its group number. Let’s solve an
example with this formula constructing method
Q. Construct the formula of Aluminium Oxide
Step 1: Derive the elements in the compound.
We can derive that the elements contained in this compound will be Aluminium and
Oxygen.
Step 2: Note their charges.
Aluminum is in Group III, therefore its charge is 3+. Similarly, Oxygen is in Group VI,
therefore its charge is 2-
Step 3: Swap their charges.
• Empirical and Molecular Formulae
• Empirical Formulae are the simplest ratios of atoms in a compound, while Molecular
Formulae are the actual ratios of atoms in a compound. For example, the molecular formula
of ethanoic acid is, thus the ratio of Carbon atoms : Hydrogen atoms : Oxygen atoms is
2:4:2. However, the empirical formula of ethanoic acid is. The previous ratio of 2:4:2 has
been simplified to 1:2:1.
Q. Compound S has the empirical formula CH2O and a relative molecular mass of 60.