Syllabus Edition

First teaching 2014

Last exams 2024

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Predicting Molecular Shapes (DP IB Chemistry: SL)

Revision Note

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Predicting Shapes & Bond Angles

  • Before you predict the shape of any molecule work out the Lewis structure to determine the number of bonding and lone pairs
  • Apply the VSEPR rules and you should be successful in deducing the correct shape and bond angle

Worked example

Predict the domain geometry, shape and bond angle in the following molecules or ions:

  1. H2S
  2. NH2Cl
  3. NO2+
  4. ClF2+

Answers:

Answer 1: The total number of valence electrons in H2S is = 1 + 1 + 6 = 8, so there are four pairs of electrons around S

Hydrogen only forms one bond, so there are two bonding pairs and two lone pairs:

Answer 2: The total number of valence electrons in NH2Cl is = 5 + 1 + 1 + 7 = 14

Answer 3: The total number of valence electrons in NO2+= 5 + 6 + 6 -1 = 16  (subtracting one for the positive charge)

Answer 4: The total number of valence electrons in ClF2+= 7 + 7 + 7 -1 = 20  (subtracting one for the positive charge)

Examiner Tip

For Standard Level Chemistry you are only required to know the shape of molecules up to four electron domains.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.