CHEM1040

Octet Rule & Electrons

100

Which of the following molecules violates the octet rule?
a. CO     b. BF₃
c. CH₄     d. NH₃

b. BF₃

200

The total number of valence electrons in SO₃^2-

26

VSEPR & Molecular Geometry

100

Which correctly describes a "group" in VSEPR theory?
a. Only single bonds
b. Any set of localized valence electrons (bond or lone pair)
c. Only nonbonding electrons
d. Only sigma bonds

b. Any set of localized valance electrons (bond or lone pair)

200

Define the difference between electron-group arrangement and molecular shape

Electron-group arrangement counts all groups (bonds and lone pairs) around the central atom
Molecular shape describes the 3D arrangement of nuclei (positions only), so lone pairs influence shape but are not shown as atoms

Molecular Polarity

100

Which molecule is nonpolar?
a. H₂O
b. NH₃
c. CH₂Cl₂
d. CO₂

d. CO₂

200

Predict whether SO₂ is polar or nonpolar and justify.

POLAR: trigonal planar arrangement with one lone pair (S). Shape is bend, so S=O bond dipoles do not cancel.

Lewis & Resonance

100

How does formal charge help identify the best resonance structure?

Structures will the smallest magnitude of formal charges are preferred (avoiding like charges on adjacent atoms)
ex: negative charge preferred on more electronegative atoms

200

Draw the Lewis structure for NO₃^- and determine number of resonance forms

Three resonance forms (N=O bond delocalized over each oxygen)

Bond Angles

100

The bond angle in a trigonal planar molecule is approximately
a. 90
b. 109.5
c. 120
d. 180

c. 120

200

Why do real bond angles deviate from ideal groups? (ex H₂O vs NH₃ vs CH₄)

Lone pairs repel more strongly than bonding pairs and multiple bonds have greater electron density so they compress adjacent bond angles relative to their ideal geometry. Real bond angles deviate (tetrahedral 109deg but NH₃ is 107deg)