CHEM 219 HONORS ORGANIC CHEMISTRY I:
QUESTIONS WITH ANSWERS AND
RATIONALES (STRUCTURE, BONDING, AND
MECHANISTIC PATHWAYS) | RICE UNIVERSITY
1. The atomic number of boron is 5. What is the correct
ground-state electron configuration for a neutral boron
atom?
A) 1s² 2s² 2p¹
B) 1s² 2s² 2p³
C) 1s² 2s¹ 2p²
D) 1s² 2s³
Answer: A | Rationale: Boron (Z=5) has 5 electrons. The
ground-state configuration fills the 1s (2 electrons), 2s (2
electrons), and places the remaining electron in the 2p
subshell, giving 1s² 2s² 2p¹. Option B is nitrogen's
configuration; options C and D violate the Aufbau principle .
2. Which of the following correctly describes the
hybridization state and geometry of a carbon atom bonded
to four single bonds with a tetrahedral arrangement?
A) sp, linear
B) sp², trigonal planar
C) sp³, tetrahedral
D) sp³d, trigonal bipyramidal
,Answer: C | Rationale: A carbon atom with four sigma bonds
and no lone pairs is sp³ hybridized with a tetrahedral
geometry and bond angles of approximately 109.5°. This is
the fundamental hybridization state for saturated carbon
centers .
3. Which carbon(s) in the following molecule is (are) sp
hybridized? CH₃–CH₂–C≡C–H
A) Carbon 1 only
B) Carbon 2 only
C) Carbons 3 and 4
D) Carbon 4 only
Answer: C | Rationale: sp hybridization occurs when a
carbon forms two π bonds (as in a triple bond or two double
bonds). In CH₃–CH₂–C≡C–H, the two carbons involved in the
triple bond (carbons 3 and 4) each have two π bonds,
making them sp hybridized. The CH₃ and CH₂ carbons are
sp³ .
4. What is the formal charge on the oxygen atom in a
hydronium ion (H₃O⁺)?
A) 0
B) +1
C) -1
D) +2
,Answer: B | Rationale: Formal charge = valence electrons -
(non-bonding electrons + ½ bonding electrons). For oxygen
in H₃O⁺: 6 - (2 + ½(6)) = 6 - (2 + 3) = +1. This calculation
correctly assigns the positive charge to the oxygen atom .
5. Which of the following statements about resonance
structures is true?
A) Resonance structures must have the same number of
electrons but may have different atom connectivity.
B) Resonance structures must have the same number of
electrons and the same atom connectivity.
C) It is acceptable for resonance structures to have
different numbers of electrons if formal charges balance.
D) Resonance structures can have atoms in different
positions within the molecule.
Answer: B | Rationale: Resonance structures are alternative
Lewis structures for the same molecule or ion that reflect
electron delocalization. Critically, all resonance structures
must depict the same number of electrons and maintain the
same connectivity of atoms; only electron arrangements (π
electrons, lone pairs) and formal charges may differ.
Changing atom connectivity or electron count invalidates
the resonance relationship .
6. The valence shell electron pair repulsion (VSEPR) theory
is a tool for determining:
A) The hybridization state of an atom
B) The shape or geometry around an atom
, C) The formal charge on an atom
D) The molecular formula of a compound
Answer: B | Rationale: VSEPR theory predicts the three-
dimensional arrangement of electron pairs (bonding and
lone pairs) around a central atom, thus determining the
molecular geometry. It is a foundational tool for
understanding molecular shape in organic chemistry .
7. Which of the following pairs of atoms forms a purely
covalent (nonpolar) bond?
A) O-H
B) N-N
C) K-S
D) Cl-Br
Answer: B | Rationale: A purely covalent bond occurs when
the electronegativity difference (ΔEN) between two atoms is
0 (or very close to 0). The N-N bond is between two identical
atoms, so ΔEN = 0, making it purely covalent. O-H and Cl-Br
have small ΔEN (polar covalent), while K-S has a large ΔEN
(ionic character) .
8. How many constitutional isomers are possible for the
molecular formula C₆H₁₄?
A) 4
B) 5
C) 6
QUESTIONS WITH ANSWERS AND
RATIONALES (STRUCTURE, BONDING, AND
MECHANISTIC PATHWAYS) | RICE UNIVERSITY
1. The atomic number of boron is 5. What is the correct
ground-state electron configuration for a neutral boron
atom?
A) 1s² 2s² 2p¹
B) 1s² 2s² 2p³
C) 1s² 2s¹ 2p²
D) 1s² 2s³
Answer: A | Rationale: Boron (Z=5) has 5 electrons. The
ground-state configuration fills the 1s (2 electrons), 2s (2
electrons), and places the remaining electron in the 2p
subshell, giving 1s² 2s² 2p¹. Option B is nitrogen's
configuration; options C and D violate the Aufbau principle .
2. Which of the following correctly describes the
hybridization state and geometry of a carbon atom bonded
to four single bonds with a tetrahedral arrangement?
A) sp, linear
B) sp², trigonal planar
C) sp³, tetrahedral
D) sp³d, trigonal bipyramidal
,Answer: C | Rationale: A carbon atom with four sigma bonds
and no lone pairs is sp³ hybridized with a tetrahedral
geometry and bond angles of approximately 109.5°. This is
the fundamental hybridization state for saturated carbon
centers .
3. Which carbon(s) in the following molecule is (are) sp
hybridized? CH₃–CH₂–C≡C–H
A) Carbon 1 only
B) Carbon 2 only
C) Carbons 3 and 4
D) Carbon 4 only
Answer: C | Rationale: sp hybridization occurs when a
carbon forms two π bonds (as in a triple bond or two double
bonds). In CH₃–CH₂–C≡C–H, the two carbons involved in the
triple bond (carbons 3 and 4) each have two π bonds,
making them sp hybridized. The CH₃ and CH₂ carbons are
sp³ .
4. What is the formal charge on the oxygen atom in a
hydronium ion (H₃O⁺)?
A) 0
B) +1
C) -1
D) +2
,Answer: B | Rationale: Formal charge = valence electrons -
(non-bonding electrons + ½ bonding electrons). For oxygen
in H₃O⁺: 6 - (2 + ½(6)) = 6 - (2 + 3) = +1. This calculation
correctly assigns the positive charge to the oxygen atom .
5. Which of the following statements about resonance
structures is true?
A) Resonance structures must have the same number of
electrons but may have different atom connectivity.
B) Resonance structures must have the same number of
electrons and the same atom connectivity.
C) It is acceptable for resonance structures to have
different numbers of electrons if formal charges balance.
D) Resonance structures can have atoms in different
positions within the molecule.
Answer: B | Rationale: Resonance structures are alternative
Lewis structures for the same molecule or ion that reflect
electron delocalization. Critically, all resonance structures
must depict the same number of electrons and maintain the
same connectivity of atoms; only electron arrangements (π
electrons, lone pairs) and formal charges may differ.
Changing atom connectivity or electron count invalidates
the resonance relationship .
6. The valence shell electron pair repulsion (VSEPR) theory
is a tool for determining:
A) The hybridization state of an atom
B) The shape or geometry around an atom
, C) The formal charge on an atom
D) The molecular formula of a compound
Answer: B | Rationale: VSEPR theory predicts the three-
dimensional arrangement of electron pairs (bonding and
lone pairs) around a central atom, thus determining the
molecular geometry. It is a foundational tool for
understanding molecular shape in organic chemistry .
7. Which of the following pairs of atoms forms a purely
covalent (nonpolar) bond?
A) O-H
B) N-N
C) K-S
D) Cl-Br
Answer: B | Rationale: A purely covalent bond occurs when
the electronegativity difference (ΔEN) between two atoms is
0 (or very close to 0). The N-N bond is between two identical
atoms, so ΔEN = 0, making it purely covalent. O-H and Cl-Br
have small ΔEN (polar covalent), while K-S has a large ΔEN
(ionic character) .
8. How many constitutional isomers are possible for the
molecular formula C₆H₁₄?
A) 4
B) 5
C) 6
