Delocalized Electron Model (Lewis Structures, Resonance Structures, VSEPR)

Question 1

Equation for formal charge

Answer 1

FC = V-L-B/2

V= number of valence electrons
L= number of lone pair electrons
B= number of bonded electrons

Question 2

Significance of FC

Answer 2

Structures with lower absolute values of FC are the more stable (lower in energy) structures

Question 3

Exceptions to the octet rule

Answer 3

1) Odd number of valence electrons
2) Octet deficient molecules
3) Valence shell expansion

Question 4

Exception 1: Odd number of valence electrons

Answer 4

For molecules with an odd number of valence electrons, it is impossible for every atom to have an octet

• Radical species: molecules with an unpaired electron.
• Radicals are usually very reactive!

Question 5

Exception 2: Octet deficient molecules

Answer 5

• B or Al tend to form compounds in which the atom has fewer than eight electrons around it (incomplete octet)

Question 6

Exception 3: Valence shell expansion

Answer 6

• Elements that can allow more than eight electrons to fit around their central atom exceed the octet rule

Question 7

What are the principles that VSEPR theory is based on?

Answer 7

• Valence electron pairs repel each other
• The geometry around the central atom will be such as to minimize the repulsion

Question 8

How to claculate steric number

Answer 8

SN = number of atoms bonded to central atoms + number of lone pairs

Question 9

VSEPR for molecules without lone pairs

Answer 9

Question 10

Lone pairs vs. electrons in bonds

Answer 10

• Electrons in bonds take up less space than lone pairs
• Lone pairs take up more space, so experience more repulsion

Question 11

VSEPR for molecules with lone pairs

Answer 11