CIE IGCSE Chemistry

Revision Notes

3.1.1 Formulae

Test Yourself

Molecular Formulae

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
    • E.g. 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
    • E.g. 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
    • E.g. 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 notes
  • The ammonia molecule consists of a central nitrogen atom bonded to three hydrogen atoms

 

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 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(valency)

  • The concept of valency is used to deduce the formulae of compounds (either molecular compounds or ionic compounds)
  • Valency or combining power tells you how many bonds an atom can make with another atom or how many electrons its atoms lose, gain or share, to form a compound
    • E.g. 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

 

Worked example

What is the formula of aluminium sulfide? 

Answer:

  • We can use the combining power (valency) of each atom to work out a formula

Al

 

S

2

  Write out the symbols of each element and write their combining powers underneath
         
Al   S    
  arrows     The formula is then calculated by cross multiplying each atom with the number opposite
  2    
         
  Al2S3     The formula for aluminium sulfide is Al2S3

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Caroline

Author: Caroline

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.