WGU D425 Introduction to Chemistry Objective Assessment
OA Question Bank (Latest 2026/2027 Edition) – 100% Correct
Questions, Answers & Detailed Rationales
Total Questions: 50
Time Allowed: 90 Minutes
Passing Score: Competency-Based (70%+)
Instructions: Select the BEST answer for each question based on introductory chemistry
principles, periodic trends, nomenclature rules, and quantitative problem-solving skills.
SECTION 1: ATOMIC STRUCTURE & PERIODIC PROPERTIES
Questions 1–10
1. An atom of a neutral element contains 17 protons, 18 neutrons, and 17 electrons.
What is the mass number of this atom?
A. 17
B. 34
C. 35
D. 52
Correct Answer: C
,Rationale: The mass number equals the sum of protons and neutrons (17 + 18 = 35).
The atomic number is 17 (protons only). Electrons do not contribute to mass number.
2. Which electron configuration represents the ground state of a chlorine atom (atomic
number 17)?
A. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
B. 1s² 2s² 2p⁶ 3s² 3p⁵
C. 1s² 2s² 2p⁶ 3s² 3p⁴ 4s¹
D. 1s² 2s² 2p⁶ 3s¹ 3p⁶
Correct Answer: B
Rationale: Chlorine (Z = 17) has 17 electrons. Following the Aufbau principle, the
configuration fills as 1s² 2s² 2p⁶ 3s² 3p⁵. Option A has 18 electrons (argon). Option C
violates the Aufbau principle by placing an electron in 4s before 3p is complete. Option
D has incorrect orbital filling.
3. How many valence electrons does an atom of sulfur (Group 16, Period 3) possess?
A. 2
B. 4
C. 6
D. 16
Correct Answer: C
, Rationale: For main-group elements, valence electrons equal the group number (using
the 1–18 system). Sulfur is in Group 16, so it has 6 valence electrons. Its electron
configuration ends in 3s² 3p⁴, confirming 6 valence electrons.
4. Which of the following elements has the largest atomic radius?
A. Na
B. Mg
C. Si
D. Cl
Correct Answer: A
Rationale: Atomic radius decreases across a period from left to right due to increasing
effective nuclear charge pulling electrons closer to the nucleus. Sodium (Na) is the
leftmost element in Period 3 among the choices, so it has the largest atomic radius.
5. Which ion does a neutral atom of aluminum (atomic number 13) most likely form?
A. Al²⁺
B. Al³⁺
C. Al³⁻
D. Al⁵⁺
Correct Answer: B
OA Question Bank (Latest 2026/2027 Edition) – 100% Correct
Questions, Answers & Detailed Rationales
Total Questions: 50
Time Allowed: 90 Minutes
Passing Score: Competency-Based (70%+)
Instructions: Select the BEST answer for each question based on introductory chemistry
principles, periodic trends, nomenclature rules, and quantitative problem-solving skills.
SECTION 1: ATOMIC STRUCTURE & PERIODIC PROPERTIES
Questions 1–10
1. An atom of a neutral element contains 17 protons, 18 neutrons, and 17 electrons.
What is the mass number of this atom?
A. 17
B. 34
C. 35
D. 52
Correct Answer: C
,Rationale: The mass number equals the sum of protons and neutrons (17 + 18 = 35).
The atomic number is 17 (protons only). Electrons do not contribute to mass number.
2. Which electron configuration represents the ground state of a chlorine atom (atomic
number 17)?
A. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
B. 1s² 2s² 2p⁶ 3s² 3p⁵
C. 1s² 2s² 2p⁶ 3s² 3p⁴ 4s¹
D. 1s² 2s² 2p⁶ 3s¹ 3p⁶
Correct Answer: B
Rationale: Chlorine (Z = 17) has 17 electrons. Following the Aufbau principle, the
configuration fills as 1s² 2s² 2p⁶ 3s² 3p⁵. Option A has 18 electrons (argon). Option C
violates the Aufbau principle by placing an electron in 4s before 3p is complete. Option
D has incorrect orbital filling.
3. How many valence electrons does an atom of sulfur (Group 16, Period 3) possess?
A. 2
B. 4
C. 6
D. 16
Correct Answer: C
, Rationale: For main-group elements, valence electrons equal the group number (using
the 1–18 system). Sulfur is in Group 16, so it has 6 valence electrons. Its electron
configuration ends in 3s² 3p⁴, confirming 6 valence electrons.
4. Which of the following elements has the largest atomic radius?
A. Na
B. Mg
C. Si
D. Cl
Correct Answer: A
Rationale: Atomic radius decreases across a period from left to right due to increasing
effective nuclear charge pulling electrons closer to the nucleus. Sodium (Na) is the
leftmost element in Period 3 among the choices, so it has the largest atomic radius.
5. Which ion does a neutral atom of aluminum (atomic number 13) most likely form?
A. Al²⁺
B. Al³⁺
C. Al³⁻
D. Al⁵⁺
Correct Answer: B
