Shapes of Molecules

Question 1

The shapes of molecules/molecular ions depend on…

Answer 1

• the no. pairs of electrons in outer shell of central atom
• the type of electron pairs around (on) central atom
• lone pairs repel more than bonding pairs so
• greatest angles: between lone pairs of electrons - bond angles between bonding pairs often reduced as - pushed together by lone pair repulsion
  *lone pair/lone pair angles > lone pair/bonding pair > bonding pair/bonding pair
• generally for every lone pair - reduce bond angle by 2.5°

Question 2

Predicting the shapes of molecules?

Answer 2

1. Find central atom
2. Work out no. electrons in outer shell (dotandcross/periodic table)
3. Mr tells how many atoms central is bonded to -> now work out how many individual bonding electrons shared w central atom
4. / by 2 tells no. shared electron PAIRS
5. compare ts num w total no. bonds tells no. lone pairs
6. Use no. electron pairs + lone pairs

Question 3

Electron pair repulsion theory?

Answer 3

The shape of a molecule is determined by the electron pairs surrounding the central atom

Question 4

Bond angles & Shapes of: 4 electron pairs around central atom?
(0, 1, 2 lone pairs)

Answer 4

0: 109.5°, tetrahedral (e.g. CH4/NH4+)
1: 107°, trigonal pyramidal (e.g. NH3/PF3) (N is positively charged due to dative)
2: 104.5°, non-linear/bent/v-shaped (e.g. H2O)

Question 5

Bond angles & Shapes of: 2 electron pairs around central atom?
(0 lone pairs)

Answer 5

180°, linear (e.g. BeCl2, CO2)

Question 6

Bond angles & Shapes of: 3 electron pairs around central atom?
(0, 1 lone pairs)

Answer 6

0: 120°, trigonal planar (BCl3)
1: 119°, non-linear/bent (SO2)

Question 7

Bond angles & Shapes of: 5 electron pairs around central atom?
(0, 1, 2 lone pairs)

Answer 7

0: 120° (between bonds lying on central plane) and 90° (bonds pointing up & down to central plane), trigonal bipyramidal (e.g. PCl5)
1: 87° (between broken line & solid) and 102° (between broken & wedge), seesaw (SF4)
2: 87.5°, distorted T (ClF3)

Question 8

Bond angles & Shapes of: 6 electron pairs around central atom?
(0, 1, 2 lone pairs)

Answer 8

0: 90°, octahedral (SF6)
1: 81.9°, 90°, square pyramidal (IF5)
2: 90°, square planar (XeF4)