CHEM 219 Modules 1–8 Problem Set
(2026/2027) – Principles of Organic
Chemistry | Portage Learning
Module 1: Bonding and Molecular Structure
20 Questions and Answers
1. What is the hybridization of carbon in methane (CH4)?
Answer: sp3 hybridization. Carbon forms four equivalent sigma bonds using four
sp3 hybrid orbitals, resulting in a tetrahedral geometry with bond angles of
approximately 109.5 degrees.
2. Draw the Lewis structure for CO2 and determine its molecular geometry.
Answer: The Lewis structure of CO2 shows carbon double-bonded to two oxygen
atoms (O=C=O) with no lone pairs on carbon. The molecular geometry is linear
with a bond angle of 180 degrees. The carbon is sp hybridized.
3. What is the difference between a sigma bond and a pi bond?
Answer: A sigma bond is formed by head-on (end-to-end) overlap of atomic
orbitals, allowing free rotation around the bond axis. A pi bond is formed by
sideways overlap of parallel p-orbitals, which restricts rotation. Sigma bonds are
generally stronger than pi bonds. A double bond consists of one sigma and one pi
bond; a triple bond consists of one sigma and two pi bonds.
4. Define electronegativity and explain how it affects bond polarity.
Answer: Electronegativity is the ability of an atom to attract shared electrons in a
covalent bond. When two atoms with different electronegativities form a bond, the
electron density shifts toward the more electronegative atom, creating a dipole
moment. The greater the electronegativity difference, the more polar the bond. If
the difference exceeds approximately 1.7, the bond is considered ionic rather than
covalent.
5. What is VSEPR theory and how is it used to predict molecular geometry?
1
, Answer: VSEPR (Valence Shell Electron Pair Repulsion) theory states that
electron pairs around a central atom arrange themselves to minimize repulsion.
Both bonding pairs and lone pairs are considered. Lone pairs occupy more space
than bonding pairs. The theory predicts geometries such as linear (2 electron
domains), trigonal planar (3 domains), tetrahedral (4 domains), trigonal
bipyramidal (5 domains), and octahedral (6 domains).
6. Determine the hybridization of the central atom in NH3.
Answer: The nitrogen atom in NH3 has three bonding pairs and one lone pair,
giving it four electron domains. Therefore, nitrogen is sp3 hybridized. The
molecular geometry is trigonal pyramidal (not tetrahedral) because the lone pair
occupies one of the tetrahedral positions, giving bond angles of approximately 107
degrees (slightly less than the ideal 109.5 degrees due to lone pair repulsion).
7. What is the molecular geometry of water (H2O)? Explain using VSEPR
theory.
Answer: Water has two bonding pairs and two lone pairs on oxygen, giving four
electron domains (sp3 hybridization). The molecular geometry is bent or V-shaped
with a bond angle of approximately 104.5 degrees. The two lone pairs compress
the H-O-H bond angle below the ideal tetrahedral angle of 109.5 degrees.
8. Explain the concept of formal charge and how to calculate it.
Answer: Formal charge is the charge assigned to an atom in a molecule assuming
electrons are shared equally. It is calculated as: Formal Charge = (valence
electrons of atom) - (nonbonding electrons) - (1/2 x bonding electrons). The most
stable Lewis structure is typically the one with formal charges closest to zero and
any negative formal charges on the most electronegative atoms.
9. What is resonance and why is it important in organic chemistry?
Answer: Resonance occurs when a molecule or ion can be represented by two or
more valid Lewis structures that differ only in the placement of electrons (not
atoms). The actual structure is a hybrid of all resonance forms, which is more
stable than any single contributing structure. Resonance is critical in organic
chemistry for understanding the stability of carbocations, carbanions, and
molecules with conjugated systems.
2
,10. What is the hybridization of carbon in ethyne (C2H2)?
Answer: In ethyne (acetylene), each carbon is sp hybridized. Each carbon forms
one sigma bond to hydrogen, one sigma bond to the other carbon, and two pi
bonds to the other carbon (forming a triple bond). The geometry around each
carbon is linear with bond angles of 180 degrees.
11. Define dipole moment and explain how molecular geometry affects it.
Answer: A dipole moment is a measure of the separation of positive and negative
charge in a molecule, represented as a vector. Molecular geometry determines
whether individual bond dipoles cancel or add. In symmetrical molecules like
CO2 (linear) and CCl4 (tetrahedral), bond dipoles cancel, resulting in zero net
dipole moment. In asymmetrical molecules like H2O (bent) and NH3 (trigonal
pyramidal), bond dipoles do not cancel, giving a net dipole moment.
12. What is the octet rule and what are its common exceptions?
Answer: The octet rule states that atoms tend to form bonds until they have eight
electrons in their valence shell. Common exceptions include: (1) hydrogen and
helium, which follow the duet rule (2 electrons); (2) elements in period 3 and
beyond, which can have expanded octets (e.g., SF6 with 12 electrons around
sulfur); (3) molecules with odd numbers of electrons (free radicals like NO); and
(4) boron and beryllium compounds, which can be electron-deficient (e.g., BF3
with 6 electrons around boron).
13. Explain the difference between polar covalent, nonpolar covalent, and ionic
bonds.
Answer: Nonpolar covalent bonds form between atoms with similar
electronegativities (difference less than 0.4), where electrons are shared nearly
equally (e.g., H-H). Polar covalent bonds form between atoms with moderate
electronegativity differences (0.4 to 1.7), where electrons are shared unequally,
creating partial charges (e.g., H-Cl). Ionic bonds form when the electronegativity
difference exceeds approximately 1.7, resulting in nearly complete electron
transfer from one atom to another, producing cations and anions (e.g., NaCl).
14. How many sigma and pi bonds are present in a molecule of ethene (C2H4)?
Answer: Ethene has five sigma bonds (four C-H sigma bonds and one C-C sigma
bond) and one pi bond (the second bond of the C=C double bond). Each carbon is
3
, sp2 hybridized with bond angles of approximately 120 degrees in a trigonal planar
arrangement.
15. What is a Lewis acid and a Lewis base?
Answer: A Lewis acid is an electron-pair acceptor (e.g., BF3, AlCl3, H+), while a
Lewis base is an electron-pair donor (e.g., NH3, H2O, OH-). The interaction
between a Lewis acid and base forms a coordinate covalent bond, where both
electrons in the bond come from the Lewis base. This concept is broader than
Bronsted-Lowry acid-base theory and is particularly useful in organic chemistry.
16. Explain the concept of bond energy and how it relates to bond stability.
Answer: Bond energy (bond dissociation energy) is the amount of energy required
to break a bond homolytically, splitting the shared pair equally between the two
atoms. Higher bond energy indicates a stronger, more stable bond. Bond energies
also determine whether a reaction is exothermic or endothermic: if the bonds
formed in the products are stronger than the bonds broken in the reactants, the
reaction is exothermic.
17. What is the molecular orbital theory and how does it differ from valence
bond theory?
Answer: Molecular orbital (MO) theory describes electrons in molecules as
occupying molecular orbitals that extend over the entire molecule, formed by the
combination (linear combination) of atomic orbitals. MO theory can explain
paramagnetism and bond order in ways valence bond theory cannot. Valence bond
theory focuses on localized bonds between specific pairs of atoms using hybrid
orbitals. MO theory predicts bonding and antibonding orbitals, and bond order =
(bonding electrons - antibonding electrons) / 2.
18. Determine the formal charges on all atoms in the nitrate ion (NO3-).
Answer: In the best resonance structure of NO3-, the nitrogen atom has a formal
charge of +1, one oxygen has a formal charge of 0 (double-bonded to N), and two
oxygens each have a formal charge of -1 (single-bonded to N). However, due to
resonance, the actual structure distributes the negative charge equally among all
three oxygens, giving each oxygen an average formal charge of -2/3.
4
(2026/2027) – Principles of Organic
Chemistry | Portage Learning
Module 1: Bonding and Molecular Structure
20 Questions and Answers
1. What is the hybridization of carbon in methane (CH4)?
Answer: sp3 hybridization. Carbon forms four equivalent sigma bonds using four
sp3 hybrid orbitals, resulting in a tetrahedral geometry with bond angles of
approximately 109.5 degrees.
2. Draw the Lewis structure for CO2 and determine its molecular geometry.
Answer: The Lewis structure of CO2 shows carbon double-bonded to two oxygen
atoms (O=C=O) with no lone pairs on carbon. The molecular geometry is linear
with a bond angle of 180 degrees. The carbon is sp hybridized.
3. What is the difference between a sigma bond and a pi bond?
Answer: A sigma bond is formed by head-on (end-to-end) overlap of atomic
orbitals, allowing free rotation around the bond axis. A pi bond is formed by
sideways overlap of parallel p-orbitals, which restricts rotation. Sigma bonds are
generally stronger than pi bonds. A double bond consists of one sigma and one pi
bond; a triple bond consists of one sigma and two pi bonds.
4. Define electronegativity and explain how it affects bond polarity.
Answer: Electronegativity is the ability of an atom to attract shared electrons in a
covalent bond. When two atoms with different electronegativities form a bond, the
electron density shifts toward the more electronegative atom, creating a dipole
moment. The greater the electronegativity difference, the more polar the bond. If
the difference exceeds approximately 1.7, the bond is considered ionic rather than
covalent.
5. What is VSEPR theory and how is it used to predict molecular geometry?
1
, Answer: VSEPR (Valence Shell Electron Pair Repulsion) theory states that
electron pairs around a central atom arrange themselves to minimize repulsion.
Both bonding pairs and lone pairs are considered. Lone pairs occupy more space
than bonding pairs. The theory predicts geometries such as linear (2 electron
domains), trigonal planar (3 domains), tetrahedral (4 domains), trigonal
bipyramidal (5 domains), and octahedral (6 domains).
6. Determine the hybridization of the central atom in NH3.
Answer: The nitrogen atom in NH3 has three bonding pairs and one lone pair,
giving it four electron domains. Therefore, nitrogen is sp3 hybridized. The
molecular geometry is trigonal pyramidal (not tetrahedral) because the lone pair
occupies one of the tetrahedral positions, giving bond angles of approximately 107
degrees (slightly less than the ideal 109.5 degrees due to lone pair repulsion).
7. What is the molecular geometry of water (H2O)? Explain using VSEPR
theory.
Answer: Water has two bonding pairs and two lone pairs on oxygen, giving four
electron domains (sp3 hybridization). The molecular geometry is bent or V-shaped
with a bond angle of approximately 104.5 degrees. The two lone pairs compress
the H-O-H bond angle below the ideal tetrahedral angle of 109.5 degrees.
8. Explain the concept of formal charge and how to calculate it.
Answer: Formal charge is the charge assigned to an atom in a molecule assuming
electrons are shared equally. It is calculated as: Formal Charge = (valence
electrons of atom) - (nonbonding electrons) - (1/2 x bonding electrons). The most
stable Lewis structure is typically the one with formal charges closest to zero and
any negative formal charges on the most electronegative atoms.
9. What is resonance and why is it important in organic chemistry?
Answer: Resonance occurs when a molecule or ion can be represented by two or
more valid Lewis structures that differ only in the placement of electrons (not
atoms). The actual structure is a hybrid of all resonance forms, which is more
stable than any single contributing structure. Resonance is critical in organic
chemistry for understanding the stability of carbocations, carbanions, and
molecules with conjugated systems.
2
,10. What is the hybridization of carbon in ethyne (C2H2)?
Answer: In ethyne (acetylene), each carbon is sp hybridized. Each carbon forms
one sigma bond to hydrogen, one sigma bond to the other carbon, and two pi
bonds to the other carbon (forming a triple bond). The geometry around each
carbon is linear with bond angles of 180 degrees.
11. Define dipole moment and explain how molecular geometry affects it.
Answer: A dipole moment is a measure of the separation of positive and negative
charge in a molecule, represented as a vector. Molecular geometry determines
whether individual bond dipoles cancel or add. In symmetrical molecules like
CO2 (linear) and CCl4 (tetrahedral), bond dipoles cancel, resulting in zero net
dipole moment. In asymmetrical molecules like H2O (bent) and NH3 (trigonal
pyramidal), bond dipoles do not cancel, giving a net dipole moment.
12. What is the octet rule and what are its common exceptions?
Answer: The octet rule states that atoms tend to form bonds until they have eight
electrons in their valence shell. Common exceptions include: (1) hydrogen and
helium, which follow the duet rule (2 electrons); (2) elements in period 3 and
beyond, which can have expanded octets (e.g., SF6 with 12 electrons around
sulfur); (3) molecules with odd numbers of electrons (free radicals like NO); and
(4) boron and beryllium compounds, which can be electron-deficient (e.g., BF3
with 6 electrons around boron).
13. Explain the difference between polar covalent, nonpolar covalent, and ionic
bonds.
Answer: Nonpolar covalent bonds form between atoms with similar
electronegativities (difference less than 0.4), where electrons are shared nearly
equally (e.g., H-H). Polar covalent bonds form between atoms with moderate
electronegativity differences (0.4 to 1.7), where electrons are shared unequally,
creating partial charges (e.g., H-Cl). Ionic bonds form when the electronegativity
difference exceeds approximately 1.7, resulting in nearly complete electron
transfer from one atom to another, producing cations and anions (e.g., NaCl).
14. How many sigma and pi bonds are present in a molecule of ethene (C2H4)?
Answer: Ethene has five sigma bonds (four C-H sigma bonds and one C-C sigma
bond) and one pi bond (the second bond of the C=C double bond). Each carbon is
3
, sp2 hybridized with bond angles of approximately 120 degrees in a trigonal planar
arrangement.
15. What is a Lewis acid and a Lewis base?
Answer: A Lewis acid is an electron-pair acceptor (e.g., BF3, AlCl3, H+), while a
Lewis base is an electron-pair donor (e.g., NH3, H2O, OH-). The interaction
between a Lewis acid and base forms a coordinate covalent bond, where both
electrons in the bond come from the Lewis base. This concept is broader than
Bronsted-Lowry acid-base theory and is particularly useful in organic chemistry.
16. Explain the concept of bond energy and how it relates to bond stability.
Answer: Bond energy (bond dissociation energy) is the amount of energy required
to break a bond homolytically, splitting the shared pair equally between the two
atoms. Higher bond energy indicates a stronger, more stable bond. Bond energies
also determine whether a reaction is exothermic or endothermic: if the bonds
formed in the products are stronger than the bonds broken in the reactants, the
reaction is exothermic.
17. What is the molecular orbital theory and how does it differ from valence
bond theory?
Answer: Molecular orbital (MO) theory describes electrons in molecules as
occupying molecular orbitals that extend over the entire molecule, formed by the
combination (linear combination) of atomic orbitals. MO theory can explain
paramagnetism and bond order in ways valence bond theory cannot. Valence bond
theory focuses on localized bonds between specific pairs of atoms using hybrid
orbitals. MO theory predicts bonding and antibonding orbitals, and bond order =
(bonding electrons - antibonding electrons) / 2.
18. Determine the formal charges on all atoms in the nitrate ion (NO3-).
Answer: In the best resonance structure of NO3-, the nitrogen atom has a formal
charge of +1, one oxygen has a formal charge of 0 (double-bonded to N), and two
oxygens each have a formal charge of -1 (single-bonded to N). However, due to
resonance, the actual structure distributes the negative charge equally among all
three oxygens, giving each oxygen an average formal charge of -2/3.
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