# Shapes of molecules and polyatomic ions

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1. What do we look at to predict the shapes of molecules?
Electron pairs (bonded and non-bonded)
2. What is the formula used to work out the shape of molecules?
Electron pairs = (no. of outer electrons in centre + no. of bonded atoms) / 2
3. What do you have to do to work out the shapes of negative and positive ions?
• For negative ions add 1 electron for every negative charge
• For positive ions take off 1 electron for every positive charge
4. What will the shape of the molecule be if it has two electron pairs?
Linear
5. What will the shape of the molecule be if it has three electron pairs?
Trigonal
6. What will the shape of the molecule be if it has four electron pairs?
Tetrahedral
7. What shape will the molecule be if it has five electron pairs?
Trigonal bipyramidal
8. What will the shape of the molecule be if it has six electron pairs?
Octohedral
9. Does the reulsion of theses electron pairs go in increasing or decreasing order of strength?
non-bonded/non-bonded pairs>non-bonded/bonded pair>bonded/bonded pairs
Decreasing
10. Why is there different bond angles in ammonia (NH3) than expected?
As the repulsion between the lone (non-bonded) pair of electrons and the bonded pairs of electrons is greater than the repulsion between different bonding pairs. (non-bonded/non-bonded pairs>non-bonded/bonded pair>bonded/bonded pairs)  Therefore, the lone pair 'squeezes' the three N-H bonds slightly closer toghether giving a smaller bond angle than would occur in a true tetrahedral shape
11. Why is there different bond angles in water (H2O) than expected?
As the repulsion between the two lone (non-bonded) pair of electrons is greater than the repulsion between different bonding pairs (non-bonded/non-bonded pairs>non-bonded/bonded pair>bonded/bonded pairs)  Therefore, the two lone pairs of electrons push the two O-H bonds closer together giving a smaller bond angle than would occur in a linear shape - water is referred to as bent or v-shaped
12. Work out the shape of BeCl2
• Electron pairs = (no. of outer electrons in centre + no. of bonded atoms) / 2
• Electron pairs = (2 + 2) / 2 = 2
• It is a linear shaped molecule
13. Work out the shape of CH4
• Electron pairs = (no. of outer electrons in centre + no. of bonded atoms) / 2
• Electron pairs = (4 + 4) / 2 = 4
• The molecule is tetrahedral shaped
14. Work out the shape of BF3
• Electron pairs = (no. of outer electrons in centre + no. of bonded atoms) / 2
• Electron pairs = (3 + 3) / 2 = 3
• The molecule is trigonal
15. Work out the shape of NH4+
• Electron pairs = (no. of outer electrons in centre + no. of bonded atoms) / 2
• Electron pairs = (5 + 4 -1) / 2 = 4
• The molecule is tetrahedral shaped.  Remeber to take of one for the positive charge!
 Author: Anonymous ID: 191142 Card Set: Shapes of molecules and polyatomic ions Updated: 2013-01-02 13:59:53 Tags: Chemistry Folders: Description: Advanced Higher Chemistry - Unit One - Shapes of molecules and polyatomic ions Show Answers: