# 4.1.1 Symbols & Formulae

### Symbols & Formulae of Elements & Compounds

Element symbols

• Each element is represented by its own unique symbol as seen on the Periodic Table e.g. H is hydrogen
• Where a symbol contains two letters, the first one is always in capital letters and the other is small, eg. sodium is Na, not NA
• Atoms combine together in fixed ratios that will give them full outer shells of electrons
• The chemical formula tells you the ratio of atoms
• Eg. H2O is a compound containing 2 hydrogen atoms which combine with 1 oxygen atom
• The chemical formula can be deduced from the relative number of atoms present
• Eg. if a molecule contains 3 atoms of hydrogen and 1 atom of nitrogen then the formula would be NH3
• Diagrams or models can also be used to represent the chemical formula The ammonia molecule consists of a central nitrogen atom bonded to 3 hydrogen atom

Chemical formulae

• The structural formula tells you the way in which the atoms in a particular molecule are bonded. This can be done by either a diagram (displayed formula) or written (simplified structural formula)
• The empirical formula tells you the simplest whole-number ratio of atoms in a compound
• The molecular formula tells you the actual number of atoms of each element in one molecule of the compound or element e.g. H2 has 2 hydrogen atoms, HCl has 1 hydrogen atom and 1 chlorine atom

Example: Butane

• Structural formula (displayed) • Structural formula (simplified)
`CH3CH2CH2CH3`
• Molecular formula
`C4H10`
• Empirical formula
`C2H5`

Deducing formulae by combining power

• The concept of valency is used to deduce the formulae of compounds
• Valency or combing power tells you how many bonds an atom can make with another atom
• Eg. carbon is in Group IV so a single carbon atom can make 4 single bonds or 2 double bonds
• The following valencies apply to elements in each group: • We can use the combining power of each atom to work out a formula
• Example: what is the formula of aluminium sulfide?
• Write out the symbols of each element and write their combining powers underneath:
• Al         S
3          2
• The formula is then calculated by cross multiplying each atom with the number opposite, hence the formula for aluminium sulfide is Al2S3
Extended Only

### Deducing Formulae of Ionic Compounds

• The formulae of these compounds can be calculated if you know the charge on the ions.
• Below are some common ions and their charges: • For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0
• Example: what is the formula of aluminium sulfate?
• Write out the formulae of each ion, including their charges
• Al3+ SO42-
• Balance the charges by multiplying them out:
Al3+ x 2 = +6 and SO42- x 3 = -6; so +6 – 6 = 0
• So the formula is Al2(SO4)3 ### Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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