Covalent Bonding Quiz - Answer Key

Complete Solutions with Lewis Structures

Section A Simple Molecules

Problem 1: Methane (CH4)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
C 1 4 4 4
H 4 1 4 1

Total valence electrons: 4 + 4 = 8

Central atom: Carbon (C)
C H H H H

4 single bonds, 0 lone pairs

Problem 2: Water (H2O)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
H 2 1 2 1
O 1 6 6 2

Total valence electrons: 2 + 6 = 8

Central atom: Oxygen (O)
Lone pairs on oxygen: 2
O H H

2 single bonds, 2 lone pairs on O

Problem 3: Ammonia (NH3)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
N 1 5 5 3
H 3 1 3 1

Total valence electrons: 5 + 3 = 8

Central atom: Nitrogen (N)
Lone pairs on nitrogen: 1
N H H H

3 single bonds, 1 lone pair on N

Problem 4: Hydrogen Fluoride (HF)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
H 1 1 1 1
F 1 7 7 1

Total valence electrons: 1 + 7 = 8

Lone pairs on fluorine: 3
H F

1 single bond, 3 lone pairs on F

Problem 5: Hydrogen Sulfide (H2S)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
H 2 1 2 1
S 1 6 6 2

Total valence electrons: 2 + 6 = 8

Central atom: Sulfur (S)
Lone pairs on sulfur: 2
S H H

Same structure as water! 2 single bonds, 2 lone pairs on S

Section B Carbon Compounds

Problem 6: Methanol (CH3OH)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
C 1 4 4 4
H 4 1 4 1
O 1 6 6 2

Total valence electrons: 4 + 4 + 6 = 14

Lone pairs on oxygen: 2
C H H H O H

Problem 7: Methylamine (CH3NH2)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
C 1 4 4 4
H 5 1 5 1
N 1 5 5 3

Total valence electrons: 4 + 5 + 5 = 14

Lone pairs on nitrogen: 1
C H H H N H H

Problem 8: Ethane (C2H6)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
C 2 4 8 4
H 6 1 6 1

Total valence electrons: 8 + 6 = 14

Bonds per carbon: 4 (1 C-C bond + 3 C-H bonds)
C C H H H H H H

Problem 9: Chloromethane (CH3Cl)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
C 1 4 4 4
H 3 1 3 1
Cl 1 7 7 1

Total valence electrons: 4 + 3 + 7 = 14

Lone pairs on chlorine: 3
C H H H Cl

Problem 10: Dichloromethane (CH2Cl2)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
C 1 4 4 4
H 2 1 2 1
Cl 2 7 14 1

Total valence electrons: 4 + 2 + 14 = 20

Total lone pairs: 6 (3 on each Cl)
C H H Cl Cl

Section C More Compounds

Problem 11: Phosphine (PH3)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
P 1 5 5 3
H 3 1 3 1

Total valence electrons: 5 + 3 = 8

Lone pairs on phosphorus: 1 (same as ammonia!)
P H H H

Problem 12: Nitrogen Trifluoride (NF3)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
N 1 5 5 3
F 3 7 21 1

Total valence electrons: 5 + 21 = 26

Central atom: Nitrogen (N)
Lone pairs on nitrogen: 1
Lone pairs on each fluorine: 3
N F F F

Each F has 3 lone pairs (not all shown for clarity)

Problem 13: Oxygen Difluoride (OF2)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
O 1 6 6 2
F 2 7 14 1

Total valence electrons: 6 + 14 = 20

Central atom: Oxygen (O)
Lone pairs on oxygen: 2
O F F

Each F has 3 lone pairs (not all shown)

Problem 14: Silicon Tetrafluoride (SiF4)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
Si 1 4 4 4
F 4 7 28 1

Total valence electrons: 4 + 28 = 32

Lone pairs on silicon: 0
Lone pairs on each fluorine: 3
Si F F F F

Same structure as CH4! Each F has 3 lone pairs.

Section D Challenge Problems

Problem 15: Ethanol (C2H5OH)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
C 2 4 8 4
H 6 1 6 1
O 1 6 6 2

Total valence electrons: 8 + 6 + 6 = 20

C C O H H H H H H

Problem 16: Hydrogen Peroxide (H2O2)

Atom How many? Valence e⁻ each Total from this atom Bonds needed (each)
H 2 1 2 1
O 2 6 12 2

Total valence electrons: 2 + 12 = 14

Lone pairs on each oxygen: 2
H O O H

Section E Conceptual Questions

Problem 17

What is the difference between an ionic bond and a covalent bond?

Ionic bond: Electrons are transferred from one atom to another. Typically occurs between metals and nonmetals. Creates charged ions that attract each other.

Covalent bond: Electrons are shared between atoms. Typically occurs between nonmetals. Creates molecules.

Problem 18

Why does carbon typically form 4 bonds while oxygen typically forms 2 bonds?

Carbon has 4 valence electrons and needs 4 more to complete its octet (8 - 4 = 4). It forms 4 bonds to share 4 pairs of electrons.

Oxygen has 6 valence electrons and needs only 2 more to complete its octet (8 - 6 = 2). It forms 2 bonds and keeps 2 lone pairs.

Problem 19

What is electronegativity?

Electronegativity is how much an atom wants to grab or attract electrons.

  • Atoms with high electronegativity (like F, O, N) are "electron greedy"
  • Atoms with low electronegativity (like H, metals) don't hold electrons as tightly
  • When drawing Lewis structures, the least electronegative atom usually goes in the center

Problem 20: Isomers

What are isomers?

Isomers are molecules that have the same molecular formula (same atoms, same number of each) but are arranged differently.

Think of it like building blocks - you have the same pieces but can build different structures. Isomers often have different properties (boiling point, smell, etc.) even though they contain the exact same atoms.

Problem 21: Isomers of C2H6O

Isomer 1: Ethanol (CH3CH2OH)

Oxygen at the END of the chain

C H H H C H H O H

Liquid at room temp, BP 78°C

Isomer 2: Dimethyl Ether (CH3OCH3)

Oxygen in the MIDDLE of the chain

C H H H O C H H H

Gas at room temp, BP -24°C

Key point: Same atoms (2 C, 6 H, 1 O) but VERY different properties because of different arrangements!

Problem 22

Complete the table:

Molecule Total Valence e⁻ # Single Bonds # Lone Pairs (total)
H2O 8 2 2
CH4 8 4 0
NH3 8 3 1
HF 8 1 3
H2S 8 2 2
How to verify: Each single bond uses 2 electrons. Each lone pair uses 2 electrons.
Total e⁻ = (# bonds × 2) + (# lone pairs × 2)

What's Next?

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