WGU D425 Intro to Chemistry Objective
Assessment 2025/2026 | Complete Questions,
Accurate Answers & Detailed Rationales –
Latest Updated Study Guide
WGU D425 – Intro to Chemistry
Objective Assessment 2025/2026 | 400 Questions with Answers & RATIONALE
Legend: = CORRECT ANSWER | Bold Question = Highlighted Question
SECTION 1: ATOMIC STRUCTURE & SUBATOMIC PARTICLES
Q1. What are the three subatomic particles that make up an atom?
A. Protons, neutrons, and molecules B. Electrons, ions, and neutrons C. Protons, ions,
and electrons D. Photons, protons, and neutrons E. Protons, neutrons, and
electrons
RATIONALE: An atom is composed of protons (positive charge) in the nucleus,
neutrons (no charge) in the nucleus, and electrons (negative charge) orbiting the
nucleus in energy shells.
Q2. Where are protons and neutrons located in an atom?
A. In the electron cloud B. In the outermost shell C. In the nucleus D. In the
valence shell E. Distributed evenly throughout the atom
RATIONALE: Protons and neutrons are both located in the nucleus, which is the
dense, central core of the atom. Electrons orbit the nucleus in energy levels.
Q3. What is the charge of a proton?
A. −1 B. 0 C. −2 D. +2 E. +1
RATIONALE: A proton carries a charge of +1 (positive). Neutrons are neutral (0),
and electrons carry a charge of −1.
Q4. What is the charge of an electron?
,A. +1 B. 0 C. −1 D. +2 E. −2
RATIONALE: Electrons carry a negative charge of −1. Their mass is approximately
1/1836 that of a proton, making them nearly massless compared to nuclear particles.
Q5. What is the atomic number of an element?
A. The total mass of the atom B. The number of neutrons in the nucleus C. The number
of electrons in the outer shell D. The number of protons in the nucleus E. The
sum of protons and neutrons
RATIONALE: The atomic number uniquely identifies an element and equals the
number of protons in the nucleus. In a neutral atom, it also equals the number of
electrons.
Q6. What is the mass number of an atom?
A. The number of protons only B. The number of electrons and protons C. The number
of neutrons only D. The number of electrons only E. The sum of protons and
neutrons
RATIONALE: Mass number = protons + neutrons. Electrons are excluded because
their mass is negligible compared to protons and neutrons.
Q7. How many neutrons does an atom of Carbon-14 contain?
A. 6 B. 12 C. 7 D. 8 E. 14
RATIONALE: Carbon has atomic number 6 (6 protons). Carbon-14 has a mass
number of 14. Neutrons = 14 − 6 = 8.
Q8. What are isotopes?
A. Atoms of different elements with the same mass number B. Atoms of the same
element with different numbers of neutrons C. Ions with the same charge D. Atoms
with the same number of electrons and protons E. Molecules with the same molecular
formula
, RATIONALE: Isotopes are atoms of the same element (same atomic
number/protons) that differ in the number of neutrons, giving them different mass
numbers.
Q9. Which of the following is NOT an isotope of hydrogen?
A. Protium (¹H) B. Deuterium (²H) C. Tritium (³H) D. Helium-3 (³He) E. All are
isotopes of hydrogen
RATIONALE: Helium-3 has 2 protons, making it helium, not hydrogen. Protium,
deuterium, and tritium all have 1 proton and are isotopes of hydrogen.
Q10. What is the atomic mass of an element on the periodic table?
A. The mass of the most common isotope B. The mass of the lightest isotope C.
The weighted average mass of all naturally occurring isotopes D. The sum of all
isotope masses E. The mass number of the element
RATIONALE: Atomic mass is a weighted average based on the natural abundance
of each isotope, which is why it is often a decimal number rather than a whole number.
Q11. An atom has 17 protons and 18 neutrons. What is its mass number?
A. 17 B. 18 C. 1 D. 35 E. 36
RATIONALE: Mass number = protons + neutrons = 17 + 18 = 35. This describes
Chlorine-35, one of the most common isotopes of chlorine.
Q12. Who proposed the nuclear model of the atom?
A. John Dalton B. J.J. Thomson C. Ernest Rutherford D. Niels Bohr E. Erwin
Schrödinger
RATIONALE: Rutherford's gold foil experiment (1911) demonstrated that atoms
have a small, dense, positively charged nucleus surrounded by mostly empty space,
leading to the nuclear model.
Q13. What experiment led Rutherford to propose the nuclear model?
, A. Cathode ray tube experiment B. Gold foil experiment C. Photoelectric effect D.
Hydrogen emission spectrum E. Double-slit experiment
RATIONALE: In the gold foil experiment, Rutherford fired alpha particles at gold foil.
Most passed through, but some deflected at large angles, revealing a dense positive
nucleus.
Q14. What did J.J. Thomson discover?
A. The proton B. The neutron C. The nucleus D. The electron E. The quantum
RATIONALE: J.J. Thomson discovered the electron in 1897 using cathode ray tube
experiments, showing that atoms contain negatively charged particles.
Q15. Who discovered the neutron?
A. Ernest Rutherford B. James Chadwick C. J.J. Thomson D. Niels Bohr E. Max
Planck
RATIONALE: James Chadwick discovered the neutron in 1932 by bombarding
beryllium with alpha particles and detecting neutral particles emitted.
Q16. In the Bohr model, electrons travel in:
A. Random paths around the nucleus B. Elliptical orbits of varying energy C. Fixed
circular orbits at specific energy levels D. Parabolic paths E. Wave-like clouds
RATIONALE: Niels Bohr proposed in 1913 that electrons occupy fixed circular orbits
called energy levels or shells, and can only emit or absorb energy when jumping
between levels.
Q17. What happens when an electron absorbs energy in the Bohr model?
A. It falls to a lower energy level B. It is ejected from the atom C. It moves to a
higher energy level D. It splits into two particles E. It enters the nucleus
RATIONALE: When an electron absorbs energy (as a photon), it is excited and
jumps to a higher energy level (farther from the nucleus).
Assessment 2025/2026 | Complete Questions,
Accurate Answers & Detailed Rationales –
Latest Updated Study Guide
WGU D425 – Intro to Chemistry
Objective Assessment 2025/2026 | 400 Questions with Answers & RATIONALE
Legend: = CORRECT ANSWER | Bold Question = Highlighted Question
SECTION 1: ATOMIC STRUCTURE & SUBATOMIC PARTICLES
Q1. What are the three subatomic particles that make up an atom?
A. Protons, neutrons, and molecules B. Electrons, ions, and neutrons C. Protons, ions,
and electrons D. Photons, protons, and neutrons E. Protons, neutrons, and
electrons
RATIONALE: An atom is composed of protons (positive charge) in the nucleus,
neutrons (no charge) in the nucleus, and electrons (negative charge) orbiting the
nucleus in energy shells.
Q2. Where are protons and neutrons located in an atom?
A. In the electron cloud B. In the outermost shell C. In the nucleus D. In the
valence shell E. Distributed evenly throughout the atom
RATIONALE: Protons and neutrons are both located in the nucleus, which is the
dense, central core of the atom. Electrons orbit the nucleus in energy levels.
Q3. What is the charge of a proton?
A. −1 B. 0 C. −2 D. +2 E. +1
RATIONALE: A proton carries a charge of +1 (positive). Neutrons are neutral (0),
and electrons carry a charge of −1.
Q4. What is the charge of an electron?
,A. +1 B. 0 C. −1 D. +2 E. −2
RATIONALE: Electrons carry a negative charge of −1. Their mass is approximately
1/1836 that of a proton, making them nearly massless compared to nuclear particles.
Q5. What is the atomic number of an element?
A. The total mass of the atom B. The number of neutrons in the nucleus C. The number
of electrons in the outer shell D. The number of protons in the nucleus E. The
sum of protons and neutrons
RATIONALE: The atomic number uniquely identifies an element and equals the
number of protons in the nucleus. In a neutral atom, it also equals the number of
electrons.
Q6. What is the mass number of an atom?
A. The number of protons only B. The number of electrons and protons C. The number
of neutrons only D. The number of electrons only E. The sum of protons and
neutrons
RATIONALE: Mass number = protons + neutrons. Electrons are excluded because
their mass is negligible compared to protons and neutrons.
Q7. How many neutrons does an atom of Carbon-14 contain?
A. 6 B. 12 C. 7 D. 8 E. 14
RATIONALE: Carbon has atomic number 6 (6 protons). Carbon-14 has a mass
number of 14. Neutrons = 14 − 6 = 8.
Q8. What are isotopes?
A. Atoms of different elements with the same mass number B. Atoms of the same
element with different numbers of neutrons C. Ions with the same charge D. Atoms
with the same number of electrons and protons E. Molecules with the same molecular
formula
, RATIONALE: Isotopes are atoms of the same element (same atomic
number/protons) that differ in the number of neutrons, giving them different mass
numbers.
Q9. Which of the following is NOT an isotope of hydrogen?
A. Protium (¹H) B. Deuterium (²H) C. Tritium (³H) D. Helium-3 (³He) E. All are
isotopes of hydrogen
RATIONALE: Helium-3 has 2 protons, making it helium, not hydrogen. Protium,
deuterium, and tritium all have 1 proton and are isotopes of hydrogen.
Q10. What is the atomic mass of an element on the periodic table?
A. The mass of the most common isotope B. The mass of the lightest isotope C.
The weighted average mass of all naturally occurring isotopes D. The sum of all
isotope masses E. The mass number of the element
RATIONALE: Atomic mass is a weighted average based on the natural abundance
of each isotope, which is why it is often a decimal number rather than a whole number.
Q11. An atom has 17 protons and 18 neutrons. What is its mass number?
A. 17 B. 18 C. 1 D. 35 E. 36
RATIONALE: Mass number = protons + neutrons = 17 + 18 = 35. This describes
Chlorine-35, one of the most common isotopes of chlorine.
Q12. Who proposed the nuclear model of the atom?
A. John Dalton B. J.J. Thomson C. Ernest Rutherford D. Niels Bohr E. Erwin
Schrödinger
RATIONALE: Rutherford's gold foil experiment (1911) demonstrated that atoms
have a small, dense, positively charged nucleus surrounded by mostly empty space,
leading to the nuclear model.
Q13. What experiment led Rutherford to propose the nuclear model?
, A. Cathode ray tube experiment B. Gold foil experiment C. Photoelectric effect D.
Hydrogen emission spectrum E. Double-slit experiment
RATIONALE: In the gold foil experiment, Rutherford fired alpha particles at gold foil.
Most passed through, but some deflected at large angles, revealing a dense positive
nucleus.
Q14. What did J.J. Thomson discover?
A. The proton B. The neutron C. The nucleus D. The electron E. The quantum
RATIONALE: J.J. Thomson discovered the electron in 1897 using cathode ray tube
experiments, showing that atoms contain negatively charged particles.
Q15. Who discovered the neutron?
A. Ernest Rutherford B. James Chadwick C. J.J. Thomson D. Niels Bohr E. Max
Planck
RATIONALE: James Chadwick discovered the neutron in 1932 by bombarding
beryllium with alpha particles and detecting neutral particles emitted.
Q16. In the Bohr model, electrons travel in:
A. Random paths around the nucleus B. Elliptical orbits of varying energy C. Fixed
circular orbits at specific energy levels D. Parabolic paths E. Wave-like clouds
RATIONALE: Niels Bohr proposed in 1913 that electrons occupy fixed circular orbits
called energy levels or shells, and can only emit or absorb energy when jumping
between levels.
Q17. What happens when an electron absorbs energy in the Bohr model?
A. It falls to a lower energy level B. It is ejected from the atom C. It moves to a
higher energy level D. It splits into two particles E. It enters the nucleus
RATIONALE: When an electron absorbs energy (as a photon), it is excited and
jumps to a higher energy level (farther from the nucleus).
