C4 Resonance

Question 1

describe the Lewis model of bonding

Answer 1

• doesn’t model delocalised electron behaviour
• doesn’t explain relative bond strengths (sigma vs pi)

Question 2

describe the atomic orbital model of bonding

Answer 2

• good at predicting shapes and bonding
• explains bond strengths
• predicts localised bonding

Question 3

describe resonance theory

Answer 3

• can be applied to Lewis or atomic orbital model of bonding
• predicts delocalised bonding

Question 4

state a limitation of Lewis structures

Answer 4

• unable to accurately describe the structure of the carbonate anion and many others

Question 5

what is resonance theory?

Answer 5

• states that there can be various possible structures for a molecule and that no individual one of them is correct
• they all contribute to the ‘true structure’

eg. carbonate ion
- none of the resonance structures really exist
- the truth is somewhere in between

Question 6

what can resonance structures also be called?

Answer 6

canonical forms

Question 7

state some rules for drawing resonance structures

Answer 7

• written by moving electrons
• only the position of electrons change (atomic nuclei must not change position)
• all structures must be valid Lewis structures
• double-headed arrow required between structures

Question 8

describe the use of curly arrows when drawing resonance structures

Answer 8

• represent the movement of a pair of electrons
• must start from an electron-rich species and be directed towards electron-deficient species or species happy to accept electrons
• SOURCE TO SINK

Question 9

state potential sources of electrons in resonance structures

Answer 9

• negative charge
• lone pair
• bond (sigma or pi)

Question 10

state potential sinks for electrons in resonance structures

Answer 10

• positive charge
• partial positive charge
• atom happy to accept electrons (eg. oxygen, nitrogen, halogen (electronegative), double bond)

Question 11

how is formal charge on an atom calculated?

Answer 11

(no. of valence e- in normal atom) - (no. unbonded e- + 1/2 bonded e-)

Question 12

are nucleophiles a source or sink of electrons?

Answer 12

source

Question 13

are electrophiles a source or sink of electrons?

Answer 13

sink

Question 14

how do resonance structures contribute different amounts?

Answer 14

• structures with more covalent bonds are more stable so contribute more
• structures with charge separation are less stable so contribute less
• structures where negative charge is on an electronegative atom are more stable than when on an electropositive atom so contribute more

Question 15

what does resonance do to molecules?

Answer 15

• stabilises them
• larger number of resonance structures for a molecule means more stable