CIE IGCSE Chemistry

Revision Notes

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

 

Ammonia-Displayed-Formula, IGCSE & GCSE Chemistry revision notesThe 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)

Butane-Displayed-Formula, IGCSE & GCSE Chemistry revision notes

 

  • 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:

Stoichiometry table, IGCSE & GCSE Chemistry revision notes

 

  • 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:

Ion, Formula And Charge table, IGCSE & GCSE Chemistry revision notes

 

  • 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.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top