Unit 8 - Lewis Structures

Question 1

Equivalent Resonance Structures

Answer 1

• lewis structures are equivalent resonance forms as a result of three equivalent ways of pushing e-‘s from some kind of atom
• the have the same number of formal chargers and multiple bonds between the same kinds of atoms

Question 2

Composite Representation

Answer 2

• shows the possible ways the bonds could be with the partial charges on it
• don’t show the dots
• doesn’t show molecular structure or e- distribution
• usually cannot be drawn on non-equivalent resonance forms
• we only use equivalent resonance forms to draw composite picture, calculate bond order, and calculate partial charge
• invoked when a single lewis structure doesn’t exist
• we say the bonding resonates between various resonance forms (aka contributions, structures, or hybrids)

Question 3

Bond Order

Answer 3

(number of bonding pairs participating in resonance)/(number of equivalent bonds)

• BO = 1 single bond
• BO = 2 double bond
• BO = 3 triple bond
• can have non-integer BO bc you can be inbetween these types of bonds
• the higher the BO, the stronger the bond, and the shorter the bond length

Question 4

Partial Charge

Answer 4

sum of charge of each equivalent atom/(number of equivalent atoms)

Partial charges are determined by the relative electronegativities of the atoms in a bond; the more electronegative atom has a partial negative charge, and the less electronegative atom has a partial positive charge

Question 5

How to see Which Structure is most Preferred

Answer 5

• obey octet rule if possible
• minimize formal charges
• place + FC on less electronegative atoms and - on more electronegative atoms

Question 6

Apparent Octet Expansion

Answer 6

• may occur for Z > 12 central atoms with the steric number > 4
• SN = number of bonded outer atoms + number of lone pairs

Question 7

Molecular Structure

Answer 7

• VSEPR = valence shell electron pair repulsion
• assumption: - bonding e- pairs and lone pairs will stay as far apart as possible around a central atom
• look at diagrams for this
• THIS IS NOT ON EXAM 2

Question 8

Bond Dissociation Energy

Answer 8

• Bond dissociation energy is the energy required to break a bond homolytically, producing radicals
• Bond dissociation energy increases as more electrons are added between the atoms. A double bond has more electrons than a single bond, and a triple bond has more electrons than a double bond
• its also harder to break a triple than a double and a double than a single because the bond order increases from single to double to triple therefore strength of the bond increases