Shapes Of Lewis Diagrams

Question 1

Linear CO2

Answer 1

There are 2 areas of Electron density (ED) around the central atom (CA), C and these areas occupy positions to minimise repulsions in a linear arrangement with bond angles of 180°. Both areas of ED are bonded and O are non-bonded. ∴ the molecular shape is also linear.

Question 2

Trigonal planar BH3

Answer 2

There are 3 areas of ED around the CA, B and these areas occupy positions to minimise repulsions in a trigonal planar arrangement with bond angles of 120°. All three areas of ED are bonded and O are non-bonded. ∴ the molecular shape is also trigonal planar.

Question 3

Bent/ v shaped SO2

Answer 3

There are 3 areas of ED around the CA, S and these areas occupy positions to minimise repulsions in a trigonal planar arrangement with bond angles of 120°. Only 2 areas of ED are bonded and 1 is non-bonded which means the actual molecular shape is bent or ‘V’ shaped.

Question 4

Tetrahedral CH4

Answer 4

There are 4 areas of ED around the CA, Carbon and these areas occupy positions to minimise repulsions in a tetrahedral (parent) arrangement with bond angles of 109.5°. 4 areas of ED are bonded and O are non-bonded. ∴ the molecular shape is also tetrahedral.

Question 5

Trigonal pyramid NH3

Answer 5

There are 4 areas of ED around the CA, Nitrogen and these areas occupy positions to minimise repulsions in a tetrahedral (parent) arrangement with bond angles of 109.5°. Only 3 areas of ED are bonded and 1 is non-bonded. ∴ the molecular shape is trigonal pyramidal.

Question 6

Bent/ v shaped H2O

Answer 6

There are 4 areas of ED around the CA, Oxygen and these areas occupy positions to minimise repulsions in a tetrahedral (parent) arrangement with a bond angle of 109.5°. Only 2 areas of ED are bonded and 2 non-bonded ∴ the molecular shape is bent / ‘V’ shaped.

Question 7

Comparison between - Trigonal planar and bent/ v shaped ( Trigonal planar parent shape)

Answer 7

BH3 and SO2. Both BH3 and SO2 have 3 areas of ED around their CA in the same trigonal planar arrangement. Both molecules have bond angles of 120° because both have 3 areas of ED. Because SO2 only has 2 areas of ED which are bonded (and 1 non-bonded area) it has a bent shape whereas BH3 has all 3 areas of ED which are bonded and as a result is trigonal planar.

Question 8

Comparison between - Tetrahedral and Trigonal pyramid

Answer 8

• Both CH4 and NH3 have 4 areas of ED around their CA in the same tetrahedral arrangement. Both molecules have a bond angle of 109.5° because both have 4 areas of ED. Because NH3 only has 3 areas of ED bonded (and 1 non-bonded area) it has a trigonal pyramid shape whereas CH4 has all 4 areas of ED which are bonded as a result is tetrahedral.

Question 9

Comparison between - bent / v shaped (parent shape Trigonal planer ) and bent / v shaped ( parent shape tetrahedral)

Answer 9

SO2 has 3 areas of ED around the CA and is trigonal planar whereas H2O has 4 areas of ED around the CA and is tetrahedral. SO2 has bond angles of 120° because it has only 3 areas of ED whereas H2O has bond angles of 109.5° because it has 4 areas of ED. SO2 has 2 areas of ED which are bonded and 1 non-bonded area of ED and H2O has 2 areas of ED which are bonded and 2 non-bonded areas of ED which both have a result of Bent / ‘V’ shaped.