Chemistry 100: General Chemistry
Exam 2 Review | 50 Questions with Correct Answers & Rationales | 2025/2026
Institutional Course Assessment | Fast-Writing Delivery
This Exam 2 Review is designed to reinforce the core concepts covered in the second half
of Chemistry 100: General Chemistry. It includes 50 carefully constructed multiple-choice
questions organized across eight key domains—atomic structure and periodic trends,
chemical bonding, molecular geometry and VSEPR theory, chemical reactions and
equation balancing, stoichiometry, solutions and concentration, acids and bases, and gas
laws with thermochemistry. Each question is accompanied by the correct answer (shown in
teal) and a concise rationale explaining the underlying reasoning. Use this review to
identify areas where you need additional study, practice applying concepts to new
situations, and build confidence for exam day. We recommend working through each
section systematically, verifying your answers before reading the rationale, and reviewing
any topics where you find inconsistencies.
Section 1: Atomic Structure & Periodic Trends (Q1–Q7)
1. An atom of potassium-39 (K) has 19 protons, 20 neutrons, and 19 electrons. What is its
approximate atomic mass in atomic mass units (amu)?
A. 38 amu B. 39 amu
C. 40 amu D. 58 amu
Answer: B. 39 amu
Rationale: Atomic mass equals protons plus neutrons: 19 + 20 = 39 amu. Electrons have negligible
mass (~0.0005 amu each) and do not contribute significantly to the total atomic mass.
2. Carbon-14 and carbon-12 are isotopes of carbon. Which statement correctly describes
their relationship?
A. They have different numbers of protons B. They have the same number of protons
and different atomic masses but different numbers of neutrons
C. They have the same number of neutrons D. They have different numbers of
but different numbers of protons electrons
Answer: B. They have the same number of protons but different numbers of neutrons
Rationale: Isotopes have the same number of protons (same element) but different numbers of
neutrons, giving different mass numbers. Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons;
both have 6 protons.
1
, 3. What is the correct electron configuration for a neutral atom of sulfur (atomic number
16)?
A. 1s² 2s² 2p⁶ 3s² 3p⁴ B. 1s² 2s² 2p⁶ 3s² 3p⁶
C. 1s² 2s² 2p⁵ 3s² 3p⁵ D. 1s² 2s² 2p⁶ 3s¹ 3p⁵
Answer: A. 1s² 2s² 2p⁶ 3s² 3p⁴
Rationale: Sulfur has 16 electrons filling orbitals in order: 1s²(2) + 2s²(4) + 2p⁶(10) + 3s²(12) +
3p⁴(16). The 3p subshell holds 4 of its 6 possible electrons.
4. How many valence electrons does a neutral atom of phosphorus (atomic number 15)
have?
A. 3 B. 5
C. 7 D. 15
Answer: B. 5
Rationale: Phosphorus is in Group 15 (Group 5A) and its configuration ends in 3s² 3p³, giving 5
valence electrons in the outermost shell. Group number (for main-group elements) equals the
number of valence electrons.
5. As you move from left to right across Period 3 (Na to Ar), what happens to atomic
radius?
A. It increases because additional electron B. It decreases because increasing nuclear
shells are added charge pulls electrons closer
C. It remains constant because the number D. It first decreases, then increases at the
of electron shells stays the same noble gases
Answer: B. It decreases because increasing nuclear charge pulls electrons closer
Rationale: Across a period, atomic radius decreases because the nuclear charge (number of
protons) increases while electrons are added to the same shell. The stronger effective nuclear
charge pulls the electron cloud closer.
6. Which element has the highest first ionization energy?
A. Sodium (Na) B. Magnesium (Mg)
C. Aluminum (Al) D. Neon (Ne)
Answer: D. Neon (Ne)
Rationale: Ionization energy increases across a period and peaks at noble gases. Neon has a
complete octet, making it extremely difficult to remove an electron. Among these options, Na
(Group 1) has the lowest IE and Ne the highest.
7. Which statement correctly describes the trend in electronegativity across the periodic
table?
2
Exam 2 Review | 50 Questions with Correct Answers & Rationales | 2025/2026
Institutional Course Assessment | Fast-Writing Delivery
This Exam 2 Review is designed to reinforce the core concepts covered in the second half
of Chemistry 100: General Chemistry. It includes 50 carefully constructed multiple-choice
questions organized across eight key domains—atomic structure and periodic trends,
chemical bonding, molecular geometry and VSEPR theory, chemical reactions and
equation balancing, stoichiometry, solutions and concentration, acids and bases, and gas
laws with thermochemistry. Each question is accompanied by the correct answer (shown in
teal) and a concise rationale explaining the underlying reasoning. Use this review to
identify areas where you need additional study, practice applying concepts to new
situations, and build confidence for exam day. We recommend working through each
section systematically, verifying your answers before reading the rationale, and reviewing
any topics where you find inconsistencies.
Section 1: Atomic Structure & Periodic Trends (Q1–Q7)
1. An atom of potassium-39 (K) has 19 protons, 20 neutrons, and 19 electrons. What is its
approximate atomic mass in atomic mass units (amu)?
A. 38 amu B. 39 amu
C. 40 amu D. 58 amu
Answer: B. 39 amu
Rationale: Atomic mass equals protons plus neutrons: 19 + 20 = 39 amu. Electrons have negligible
mass (~0.0005 amu each) and do not contribute significantly to the total atomic mass.
2. Carbon-14 and carbon-12 are isotopes of carbon. Which statement correctly describes
their relationship?
A. They have different numbers of protons B. They have the same number of protons
and different atomic masses but different numbers of neutrons
C. They have the same number of neutrons D. They have different numbers of
but different numbers of protons electrons
Answer: B. They have the same number of protons but different numbers of neutrons
Rationale: Isotopes have the same number of protons (same element) but different numbers of
neutrons, giving different mass numbers. Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons;
both have 6 protons.
1
, 3. What is the correct electron configuration for a neutral atom of sulfur (atomic number
16)?
A. 1s² 2s² 2p⁶ 3s² 3p⁴ B. 1s² 2s² 2p⁶ 3s² 3p⁶
C. 1s² 2s² 2p⁵ 3s² 3p⁵ D. 1s² 2s² 2p⁶ 3s¹ 3p⁵
Answer: A. 1s² 2s² 2p⁶ 3s² 3p⁴
Rationale: Sulfur has 16 electrons filling orbitals in order: 1s²(2) + 2s²(4) + 2p⁶(10) + 3s²(12) +
3p⁴(16). The 3p subshell holds 4 of its 6 possible electrons.
4. How many valence electrons does a neutral atom of phosphorus (atomic number 15)
have?
A. 3 B. 5
C. 7 D. 15
Answer: B. 5
Rationale: Phosphorus is in Group 15 (Group 5A) and its configuration ends in 3s² 3p³, giving 5
valence electrons in the outermost shell. Group number (for main-group elements) equals the
number of valence electrons.
5. As you move from left to right across Period 3 (Na to Ar), what happens to atomic
radius?
A. It increases because additional electron B. It decreases because increasing nuclear
shells are added charge pulls electrons closer
C. It remains constant because the number D. It first decreases, then increases at the
of electron shells stays the same noble gases
Answer: B. It decreases because increasing nuclear charge pulls electrons closer
Rationale: Across a period, atomic radius decreases because the nuclear charge (number of
protons) increases while electrons are added to the same shell. The stronger effective nuclear
charge pulls the electron cloud closer.
6. Which element has the highest first ionization energy?
A. Sodium (Na) B. Magnesium (Mg)
C. Aluminum (Al) D. Neon (Ne)
Answer: D. Neon (Ne)
Rationale: Ionization energy increases across a period and peaks at noble gases. Neon has a
complete octet, making it extremely difficult to remove an electron. Among these options, Na
(Group 1) has the lowest IE and Ne the highest.
7. Which statement correctly describes the trend in electronegativity across the periodic
table?
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