WGU D425 INTRODUCTION TO CHEMISTRY -PA AND OA ACTUAL
EXAM(2026) 100%VERIFIED |COMPLETE QUESTIONS AND ANSWERS
WITH RATIONALE | GRADED A+
Domain Topics
Subatomic particles, isotopes, electron configuration,
Atomic Structure &
periodic table organization, atomic radius, ionization
Periodic Trends
energy, electronegativity
Chemical Bonding & Ionic vs. covalent bonds, Lewis dot structures, VSEPR
Naming theory, molecular geometry, nomenclature
Balancing equations, mole concept, limiting reactants,
Stoichiometry &
theoretical yield, percent yield, gas laws (STP), redox
Reactions
reactions
Solutions & Acid- Solutes/solvents, mixtures, pH, percent composition,
Base Chemistry molarity calculations
Matter & Classification of matter, significant figures, metric syste
Measurement physical vs. chemical properties
SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS
(Questions 1-20)
Question 1: What is the atomic number of an element that has 15
protons and 16 neutrons?
A. 15
B. 16
C. 31
D. 1
Correct Answer: A
,Rationale: The atomic number is defined as the number of protons in an
atom's nucleus. With 15 protons, the atomic number is 15, regardless of the
neutron count. The mass number would be 31 (protons + neutrons) .
Question 2: Which subatomic particle has a negative charge and is
found in orbitals around the nucleus?
A. Proton
B. Neutron
C. Electron
D. Positron
Correct Answer: C
Rationale: Electrons are negatively charged subatomic particles that
occupy electron clouds or orbitals surrounding the nucleus. Protons carry a
positive charge, neutrons are neutral, and positrons are antiparticles not
found in standard atomic structure .
Question 3: Which of the following best describes an isotope?
A. Atoms with the same number of neutrons but different protons
B. Atoms with the same number of protons but different numbers of neutrons
C. Atoms with the same mass number but different atomic numbers
D. Atoms with different numbers of electrons only
Correct Answer: B
Rationale: Isotopes of an element have identical proton counts (same
element) but vary in neutron number, leading to different mass numbers. For
example, carbon-12 and carbon-14 are isotopes .
Question 4: What is the charge of the ion formed by calcium (Ca)?
,A. +1
B. +2
C. -1
D. -2
Correct Answer: B
Rationale: Calcium is a metal in Group 2 of the periodic table. Metals in
Group 2 lose 2 electrons to achieve an octet (8 electrons) in their outermost
shell, forming a +2 cation (Ca²⁺) .
Question 5: Which element has the electron configuration 1s² 2s²
2p⁶ 3s² 3p⁶ 4s¹?
A. Potassium (K)
B. Calcium (Ca)
C. Scandium (Sc)
D. Argon (Ar)
Correct Answer: A
Rationale: Summing the electrons: 2+2+6+2+6+1 = 19 electrons, which is
the atomic number of potassium. The 4s orbital fills before 3d, making this
the ground state configuration for K .
Question 6: According to the Aufbau principle, which orbital is filled
immediately after the 4s orbital?
A. 3d
B. 4p
C. 5s
D. 4d
Correct Answer: A
Rationale: The Aufbau principle describes the order of orbital filling: 1s, 2s,
2p, 3s, 3p, 4s, then 3d, then 4p. The 3d orbitals fill after 4s because they are
slightly higher in energy .
, Question 7: What is the mass number of an atom?
A. Number of protons
B. Number of neutrons
C. Number of protons + number of neutrons
D. Number of electrons + number of protons
Correct Answer: C
Rationale: The mass number is the sum of protons and neutrons in an
atom's nucleus. Electrons have negligible mass and are not included in the
mass number calculation .
Question 8: Which of the following is NOT a subatomic particle?
A. Proton
B. Neutron
C. Electron
D. Positron
Correct Answer: D
Rationale: Positrons are antiparticles of electrons and are not found in
standard atomic structure. Protons, neutrons, and electrons are the
fundamental subatomic particles of atoms .
Question 9: How does atomic radius change as you move down a
group?
A. Increases
B. Decreases
C. Stays the same
D. Increases then decreases
Correct Answer: A
EXAM(2026) 100%VERIFIED |COMPLETE QUESTIONS AND ANSWERS
WITH RATIONALE | GRADED A+
Domain Topics
Subatomic particles, isotopes, electron configuration,
Atomic Structure &
periodic table organization, atomic radius, ionization
Periodic Trends
energy, electronegativity
Chemical Bonding & Ionic vs. covalent bonds, Lewis dot structures, VSEPR
Naming theory, molecular geometry, nomenclature
Balancing equations, mole concept, limiting reactants,
Stoichiometry &
theoretical yield, percent yield, gas laws (STP), redox
Reactions
reactions
Solutions & Acid- Solutes/solvents, mixtures, pH, percent composition,
Base Chemistry molarity calculations
Matter & Classification of matter, significant figures, metric syste
Measurement physical vs. chemical properties
SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS
(Questions 1-20)
Question 1: What is the atomic number of an element that has 15
protons and 16 neutrons?
A. 15
B. 16
C. 31
D. 1
Correct Answer: A
,Rationale: The atomic number is defined as the number of protons in an
atom's nucleus. With 15 protons, the atomic number is 15, regardless of the
neutron count. The mass number would be 31 (protons + neutrons) .
Question 2: Which subatomic particle has a negative charge and is
found in orbitals around the nucleus?
A. Proton
B. Neutron
C. Electron
D. Positron
Correct Answer: C
Rationale: Electrons are negatively charged subatomic particles that
occupy electron clouds or orbitals surrounding the nucleus. Protons carry a
positive charge, neutrons are neutral, and positrons are antiparticles not
found in standard atomic structure .
Question 3: Which of the following best describes an isotope?
A. Atoms with the same number of neutrons but different protons
B. Atoms with the same number of protons but different numbers of neutrons
C. Atoms with the same mass number but different atomic numbers
D. Atoms with different numbers of electrons only
Correct Answer: B
Rationale: Isotopes of an element have identical proton counts (same
element) but vary in neutron number, leading to different mass numbers. For
example, carbon-12 and carbon-14 are isotopes .
Question 4: What is the charge of the ion formed by calcium (Ca)?
,A. +1
B. +2
C. -1
D. -2
Correct Answer: B
Rationale: Calcium is a metal in Group 2 of the periodic table. Metals in
Group 2 lose 2 electrons to achieve an octet (8 electrons) in their outermost
shell, forming a +2 cation (Ca²⁺) .
Question 5: Which element has the electron configuration 1s² 2s²
2p⁶ 3s² 3p⁶ 4s¹?
A. Potassium (K)
B. Calcium (Ca)
C. Scandium (Sc)
D. Argon (Ar)
Correct Answer: A
Rationale: Summing the electrons: 2+2+6+2+6+1 = 19 electrons, which is
the atomic number of potassium. The 4s orbital fills before 3d, making this
the ground state configuration for K .
Question 6: According to the Aufbau principle, which orbital is filled
immediately after the 4s orbital?
A. 3d
B. 4p
C. 5s
D. 4d
Correct Answer: A
Rationale: The Aufbau principle describes the order of orbital filling: 1s, 2s,
2p, 3s, 3p, 4s, then 3d, then 4p. The 3d orbitals fill after 4s because they are
slightly higher in energy .
, Question 7: What is the mass number of an atom?
A. Number of protons
B. Number of neutrons
C. Number of protons + number of neutrons
D. Number of electrons + number of protons
Correct Answer: C
Rationale: The mass number is the sum of protons and neutrons in an
atom's nucleus. Electrons have negligible mass and are not included in the
mass number calculation .
Question 8: Which of the following is NOT a subatomic particle?
A. Proton
B. Neutron
C. Electron
D. Positron
Correct Answer: D
Rationale: Positrons are antiparticles of electrons and are not found in
standard atomic structure. Protons, neutrons, and electrons are the
fundamental subatomic particles of atoms .
Question 9: How does atomic radius change as you move down a
group?
A. Increases
B. Decreases
C. Stays the same
D. Increases then decreases
Correct Answer: A
