Topic 6: Lewis structure, VSEPR, and intermolecular forces

Question 1

In polyatomic molecules, what is the feature of the atom to be placed in the center?

Answer 1

The central atom is the one with the most number of bonds or the one that needs to share the greatest number of electrons.

Question 2

Explain the relationship between resonance and electron delocalisation in molecular orbitals

Answer 2

• Molecular orbital theory states that when atoms come together, the electrons are shared in a cloud space called molecular orbitals which means that the electrons distributed at every point in that orbital rather than being confined in 1 bond.
• The resonance structures suggest different valid possibilities of the position of valence electrons which form bonds that “in-between” single and double bond.

Question 3

What is the difference between formal charges and polarity?

Answer 3

• Formal charges are theoritical charges that are assigned to atoms assuming that electrons are shared evenly.
• Polarity is when the atoms don’t share electrons evenly, thus creating partial charges for the atoms -> electric dipole

Question 4

Should we follow Octet rule or formal charges when there are different possible arrangements?

Answer 4

If not specified, it’s best to follow Octet rule even if it results in formal charges rather than trying to minimize formal charges and break the Octet rule.

Question 5

Describe the electron group arrangements and molecular shape for molecules containing between 2 and 4 electrons groups

Answer 5

• 2 electron groups: linear
• 3 electron groups: trigonal planar
  + 0 lone pair: trigonal planar
  + 1 lone pair: bent
• 4 electron groups: tetrahedral
  + 0 lone pair: tetrahedral
  + 1 lone pair: trigonal pyramidal
  + 2 lone pairs: bent

Question 6

What can VSEPR model tell that Lewis structure cannot?

Answer 6

The 3D arrangement of electron groups and molecular shapes of atoms.

Question 7

How are electron group in VSEPR model positioned and what is it for in terms of energy level?

Answer 7

Electron groups will repel each other, so to minimize this repulsive forces, electron groups are best to placed as far from each other as possible.
This way, the potential energy of the molecule is at lowest.

Question 8

What is the difference between assumption of electrons being localised or delocalised in VSEPR and molecular orbital theory/Lewis structure?

Answer 8

• In molecular orbital theory and Lewis structure, electrons are assumed to be delocalised between many nuclei.
  + why we have resonance structure and molecular orbitals
• In VSEPR, the electrons are assumed to be localised in the bonds or lone pairs indicated by Lewis structure.
  + defined bond

Question 9

Why is the degree of angle between electron groups smaller in molecules with more number of lone pairs?

Answer 9

Lone pairs take up more space because they repel more strongly than electron groups/bonding electrons
-> push the bonding electrons closer
-> smaller bond angle

Question 10

What is the difference between arrangement and shape in VSEPR?

Answer 10

• Arrangement: includes all electron groups
• Shape: determined only by the number of bonds between atoms

Question 11

Identify some intermolecular forces

Answer 11

• Dispersion forces: present in all molecules
• Hydrogen bonding: only count as H bonds when bonded with very electronegative element: N,F,O
• Dipole-dipole interaction: present in polar molecules
• Dipole-induced dipole interactions

Question 12

Explain dispersion forces and factors influencing the interaction

Answer 12

• Dispersion force: instantaneous dipole-induced dipole force
  + electron cloud is distorted creating surplus and deficit electron density at 2 ends
  + the difference in electron density results in dispersion forces or attraction between opposite charges of multiple atoms
• Strength of interaction depends on:
  + polarisability: how easy it is for electron cloud of an atom or molecule to be distorted; the larger, the more electrons and atom or molecule have, the higher polarisability
  + surface contact area: affect the overall shape which impact interaction strength

Question 13

What is one the direct measurement of strength of intermolecular forces?

Answer 13

Boiling point
Stronger intermolecular forces -> higher boiling point

Question 14

Explain hydrogen bond and identify the elements and bonds that may undergo H bonding

Answer 14

• When H bonds with very electronegative element, it results in unusually strong dipole-dipole interaction. -> leaves H positive partial charged
• Also, when H bonds with other element, it leaves a partially exposed nucleus as it only has 1 electron. -> easy to bond

–> H bond arises between the hydrogen atom of this molecule to the lone pairs of F,O,N of another molecule

Question 15

Why does water has a really high boiling point? (relate to H bonds)

Answer 15

Water has a really strong H bond and can form in total 4 H bonds (2 H with 1 O and the 2 lone pairs of O bond with 2 H of other water molecules)
–> dense and extensive 3D networks of H bonds

Question 16

How does H-bond networks affect properties of molecules?

Answer 16

Extensive H-bond networks can impact the solubility and mixing of molecules, especially boiling point.