Chapter 13 - Bonding: General Concepts

Question 1

Explain electronegativity and how it relates to polarity

Answer 1

Electronegativity: the relative ability of an atom to attract shared electrons
- The polarity of a bond depends on the electronegativity difference of
the bonded atoms
- The spatial arrangement of polar bonds in a molecule determines whether
the molecule has a dipole moment

Question 2

Explain bond energy

Answer 2

The energy necessary to break a covalent bond

• Increases as the number of shared pairs increases
• Can be used to estimate the enthalpy change for a chemical reaction

Question 3

Explain Lewis structures

Answer 3

• Show how the valence electron pairs are arranged among the atoms in a molecule or polyatomic ion
• Stable molecules usually contain atoms that have their valence orbitals
  filled
  – Leads to a duet rule for hydrogen
  – Leads to an octet rule for second-row elements
  – The atoms of elements in the third row and beyond can exceed the octet rule
• Several equivalent Lewis structures can be drawn for some molecules, a concept called resonance
• When several nonequivalent Lewis structures can be drawn for a molecule, formal charge is often used to choose the most appropriate structure(s)

Question 4

Explain VSEPR

Answer 4

Based on the idea that electron pairs will be arranged around a central
atom in a way that minimizes the electron repulsions
Can be used to predict the geometric structure of most molecules

Question 5

Go through the steps of drawing a Lewis structure

Answer 5

1. Choose the central atom
   a. The atom with the lowest subscript in the chemical formula
   b. The atom that can form the most bonds
   c. The least electronegative atom
2. Draw all of the atoms, connected by single bonds
3. Count the total number of valence electrons from all atoms and molecule charge
4. Determine leftover electrons not used in bonding
5. Use the remaining electrons to complete the octet of each atom, beginning with the most electronegative atom
6. Assign formal charges on each atom
7. Use lone pairs to create additional bonds and minimize formal charges

Question 6

Go through all possible steric numbers and their bond angles.

Answer 6

SN = 2

• AX2 = linear (180) sp
• AXE = linear (180) sp

SN = 3
o AX3 – trigonal planar (120) sp2
o AX2E – Bent (slightly less than 120) sp2

• SN = 4
o AX4 – tetrahedral (109.5) sp3
o AX2E2 – Bent (104.5) sp3
o AX3E – trigonal pyramidal (107.5) sp3

Question 7

How do you calculate bond order?

Answer 7

BO = # bonds in resonance / # bonded atoms in resonance excluding central atom

Question 8

Explain the behaviour of dipoles

Answer 8

• Dipoles are oriented randomly in space
• When a magnetic field is applied, dipoles orient themselves with the field
• Higher dipole moment = higher boiling point because of strength of bonds (strong dipole-dipole interactions)
• Higher polarizability = higher boiling point (strong induced-dipole-induced dipole interactions)
• Dipole moment is more significant on boiling point than polarizability (dipole-dipole stronger than induced dipole-induced dipole)
• Lone pair dipoles always point towards the lone pair