Shapes of Molecules

Question 1

What are bond angles?

Answer 1

• The angle between any two bonds on a molecule (not between lone pairs.)

Question 2

Why do molecules have specific shapes with specific bond angles?

Answer 2

• Bonds repel each other equally (without any lone pairs) because they all contain negative electrons that repel one another (want to be as far apart as possible.)

Question 3

Why do lone pairs of electrons repel more strongly than bonding pairs of electrons?

Answer 3

• Because they are more compact.

Question 4

Give the order of repulsion between bonding pairs/ lone pairs in a molecule.

Answer 4

lp- lp > bp -lp > bp -bp

Question 5

Bonding pairs and lone pairs are …… …….. that repel each other.

Answer 5

• Charge.
• Clouds.

Question 6

How do pairs of electrons in outer shells of atoms arrange themselves?

Answer 6

• Pairs of electrons in outer shells of atoms arrange themselves as far apart as possible to minimise repulsion.

Question 7

What is the shape of a molecule determined by?

Answer 7

• Determined by the number of electron pairs on the central atom (central atom TENDS to be first atom in the formula of molecule - but, this isn’t always the case.)

() - extra info for clarification.

Question 8

What do lone pairs do to the shape of molecules?

Answer 8

• Lone pairs push bonding pairs closer together (repel the bonding pairs away from itself strongly.)
• GENERALLY (not all the time), for every lone pair you add to molecule, you reduce the remaining bond angles by 2.5 degrees.

() extra info for clarification.

Question 9

If you are asked to determine shape of molecule that is an ion, what would you do?

Answer 9

• Add/ remove electrons FROM THE CENTRAL ATOM and then continue with your drawing.

Question 10

What is the shape of molecule that has 2bp, no lone pairs. What is the bond angle?

Answer 10

• SHAPE: Linear.
• Bond angle: 180⁰

Question 11

What is the shape of molecule that has 3bp, no lone pairs. What is the bond angle?

Answer 11

• Trigonal planar (flat molecule.)
• 120⁰

Question 12

What is the shape of molecule that has 4bp, no lone pairs. What is the bond angle?

Answer 12

• Tetrahedral (3D shape.)
• 109.5⁰

Question 13

What is structure of tetrehedral molecule?

Answer 13

• 3D structure.
• 1 bond coming towards you and one bond going away from you.
• Atom/ bond coming towards you represented by wedges.
• Atom/ bond going away from you represented by dotted line.

Question 14

If we see a wedge/ dotted line on a shapes drawing, what does this mean?

Answer 14

• Molecule is 3D.
• Wedge: part of molecule coming towards you.
• Dotted line: part of molecule going away from you.

Question 15

What is the shape of molecule that has 5bp, no lone pairs. What are the bond angles?

Answer 15

• Trigonal bypyrimidal - one wedged line and one dotted line.
• 90⁰ and 120⁰ (trigonal planar set up is in horizontal plane with vertical axis in vertical plane. Any vertical bond will hit ANY horizontal bond at 90⁰/ angle between bonds in trigonal planar will be 120.)

Question 16

What is the shape of molecule that has 6bp, no lone pairs. What is the bond angle?

Answer 16

• Octahedral (two wedged lines/ two dotted lines.)
• Bond angle = 90⁰ between ALL bonds.

Question 17

What is shape of molecule with 2bps and 1lp of electrons? What is the bond angle? What shape is this shape BASED ON?

Answer 17

• Based on trigonal planar (because, we have 3 electron pairs.)
• BOND ANGLE: 117.5⁰
• SHAPE: V- shaped.

() - extra info for clarification

Question 18

What is shape of molecule with 3bps and 1lp of electrons? What is the bond angle? What shape is this shape BASED ON?

Answer 18

• Based on tetrehedral (because we have 4 electron pairs.)
• BOND ANGLE: 107⁰
• SHAPE: Pyramidal

() - extra info for clarification

Question 19

What is shape of molecule with 2bps and 2lps of electrons? What is the bond angle?

Answer 19

SHAPE: V- shaped.
Bond angle: 104.5⁰ (2.5⁰ less than 3bp and 1bp - ie pyramidal.)

Question 20

What is shape of molecule with 3bp and 2lps of electrons? What is the bond angle? Why does the bond angle remain unchanged?

Answer 20

• SHAPE: Trigonal Planar (with one wedged/ one dashed line.)
• BOND ANGLE: 120⁰
• Bond angles remain same because the two lone pairs repel equally from opposite sides (same repulsion upwards as downwards) so, no bps are brought closer together.

Question 21

What is shape of molecule with 4bps and 2lp of electrons? What is the bond angle? What shape is this shape BASED ON? Why do the bond angles remain the same?

Answer 21

• Based on octahedral (due to having 6 pairs of electrons.)
• SHAPE: Square planar
• ANGLE: 90⁰
• Bond angles remain same because the two lone pairs repel equally from opposite sides (same repulsion upwards as downwards) so, no bps are brought closer together.