WGU D425 Introduction to Chemistry
Objective Assessment Official Practice Exam
Actual Exam 2026/2027 with Detailed
Rationales | Complete Exam-Style Questions |
Pass Guaranteed – A+ Graded
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SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS Q1 – Q10
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Question 1 of 50
A chemist is analyzing a sample of neon gas used in a high-voltage discharge tube. The neon
atoms emit a distinctive red-orange light when excited. Based on the Bohr model of the atom,
what happens to the electrons in neon atoms when this emission occurs?
A. Electrons move from lower energy levels to higher energy levels, absorbing photons of
red-orange light.
B. Electrons move from higher energy levels to lower energy levels, releasing photons of
red-orange light. ✓ CORRECT
C. Electrons are completely removed from the atom, creating Ne²⁺ ions that emit red-orange
light.
D. Electrons orbit the nucleus at a constant distance, producing continuous red-orange
radiation.
Correct Answer: B
Rationale: The Bohr model states that electrons occupy discrete energy levels around the
nucleus, and when an excited electron transitions from a higher energy level to a lower one, it
releases energy as a photon with a specific wavelength — in this case, the characteristic
red-orange light of neon. Choice A reverses the process, describing absorption rather than
emission, which would darken the tube rather than cause it to glow. On the exam, always
identify whether the scenario describes excitation (absorption, electron moves up) or
relaxation (emission, electron moves down).
Question 2 of 50
,A materials scientist is selecting a metal for a lightweight aircraft component and needs an
element with a low density and a relatively low first ionization energy. Which element from
the following list best matches both criteria?
A. Aluminum (Al)
B. Magnesium (Mg) ✓ CORRECT
C. Iron (Fe)
D. Copper (Cu)
Correct Answer: B
Rationale: Magnesium has a density of 1.74 g/cm³ and a first ionization energy of 738
kJ/mol, making it both lightweight and relatively easy to ionize compared to the other
options. Aluminum has a comparable density but a higher first ionization energy (578 kJ/mol
is actually lower, but magnesium is lighter and more reactive); more critically, iron and copper
are far too dense for lightweight aerospace applications. When evaluating periodic trends
together, always cross-reference multiple properties rather than relying on a single trend in
isolation.
Question 3 of 50
In a forensic laboratory, a technician uses mass spectrometry to identify an unknown
element found at a crime scene. The mass spectrum shows three peaks with relative
abundances of 78.99% at mass 24, 10.00% at mass 25, and 11.01% at mass 26. What is the
average atomic mass of this element?
A. 24.00 amu
B. 24.31 amu ✓ CORRECT
C. 25.00 amu
D. 25.50 amu
Correct Answer: B
Rationale: The average atomic mass is calculated as a weighted average: (0.7899 × 24) +
(0.1000 × 25) + (0.1101 × 26) = 18.9576 + 2.500 + 2.8626 = 24.3202 amu, which rounds to
24.31 amu — this is magnesium. Choice A represents the mass of the most abundant isotope
only, a common error when students forget to weight each isotope by its natural abundance.
For weighted average problems, always verify that your final answer falls between the lightest
and heaviest isotope masses; if it does not, you have made a calculation error.
Question 4 of 50
A semiconductor manufacturer needs a dopant element that will add extra electrons to a
silicon crystal lattice. The dopant must have five valence electrons and atomic radius similar
to silicon. Which element should the engineer select?
, A. Boron (B)
B. Carbon (C)
C. Phosphorus (P) ✓ CORRECT
D. Germanium (Ge)
Correct Answer: C
Rationale: Phosphorus, located directly below nitrogen in Group 15, has five valence electrons
and an atomic radius close enough to silicon that it can substitute into the lattice without
significant structural distortion, donating its extra electron to the conduction band. Choice A
(boron) has only three valence electrons and would create electron holes rather than donate
electrons, making it a p-type rather than n-type dopant. On semiconductor questions,
remember that Group 15 elements create n-type doping (negative charge carriers), while
Group 13 elements create p-type doping.
Question 5 of 50
An environmental chemist is comparing the reactivity of alkali metals for a waste treatment
process. She observes that when a small piece of each metal is dropped into water, the
reaction with potassium is significantly more vigorous than with sodium. Which periodic
trend best explains this observation?
A. Potassium has a higher electronegativity than sodium, making it more reactive with water.
B. Potassium has a larger atomic radius and lower first ionization energy than sodium, so its
valence electron is more easily lost. ✓ CORRECT
C. Potassium has more protons than sodium, creating a stronger attraction for its valence
electron.
D. Potassium has a higher electron affinity than sodium, allowing it to readily accept
electrons from water.
Correct Answer: B
Rationale: Moving down Group 1, atomic radius increases and ionization energy decreases
because the valence electron occupies a higher principal energy level and is farther from the
nucleus with more shielding; this makes potassium's single valence electron easier to
remove, resulting in a more vigorous reaction with water. Choice A is incorrect because
electronegativity decreases down a group, and alkali metals are already the least
electronegative elements. For reactivity questions involving metals, always trace the logic
from atomic radius → ionization energy → ease of electron loss → reactivity.
Question 6 of 50
A radiocarbon dating laboratory is preparing a sample of carbon-14 for analysis. The
technician needs to know how many neutrons are present in a single atom of carbon-14.
A. 6 neutrons
Objective Assessment Official Practice Exam
Actual Exam 2026/2027 with Detailed
Rationales | Complete Exam-Style Questions |
Pass Guaranteed – A+ Graded
══════════════════════════════════════
SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS Q1 – Q10
══════════════════════════════════════
Question 1 of 50
A chemist is analyzing a sample of neon gas used in a high-voltage discharge tube. The neon
atoms emit a distinctive red-orange light when excited. Based on the Bohr model of the atom,
what happens to the electrons in neon atoms when this emission occurs?
A. Electrons move from lower energy levels to higher energy levels, absorbing photons of
red-orange light.
B. Electrons move from higher energy levels to lower energy levels, releasing photons of
red-orange light. ✓ CORRECT
C. Electrons are completely removed from the atom, creating Ne²⁺ ions that emit red-orange
light.
D. Electrons orbit the nucleus at a constant distance, producing continuous red-orange
radiation.
Correct Answer: B
Rationale: The Bohr model states that electrons occupy discrete energy levels around the
nucleus, and when an excited electron transitions from a higher energy level to a lower one, it
releases energy as a photon with a specific wavelength — in this case, the characteristic
red-orange light of neon. Choice A reverses the process, describing absorption rather than
emission, which would darken the tube rather than cause it to glow. On the exam, always
identify whether the scenario describes excitation (absorption, electron moves up) or
relaxation (emission, electron moves down).
Question 2 of 50
,A materials scientist is selecting a metal for a lightweight aircraft component and needs an
element with a low density and a relatively low first ionization energy. Which element from
the following list best matches both criteria?
A. Aluminum (Al)
B. Magnesium (Mg) ✓ CORRECT
C. Iron (Fe)
D. Copper (Cu)
Correct Answer: B
Rationale: Magnesium has a density of 1.74 g/cm³ and a first ionization energy of 738
kJ/mol, making it both lightweight and relatively easy to ionize compared to the other
options. Aluminum has a comparable density but a higher first ionization energy (578 kJ/mol
is actually lower, but magnesium is lighter and more reactive); more critically, iron and copper
are far too dense for lightweight aerospace applications. When evaluating periodic trends
together, always cross-reference multiple properties rather than relying on a single trend in
isolation.
Question 3 of 50
In a forensic laboratory, a technician uses mass spectrometry to identify an unknown
element found at a crime scene. The mass spectrum shows three peaks with relative
abundances of 78.99% at mass 24, 10.00% at mass 25, and 11.01% at mass 26. What is the
average atomic mass of this element?
A. 24.00 amu
B. 24.31 amu ✓ CORRECT
C. 25.00 amu
D. 25.50 amu
Correct Answer: B
Rationale: The average atomic mass is calculated as a weighted average: (0.7899 × 24) +
(0.1000 × 25) + (0.1101 × 26) = 18.9576 + 2.500 + 2.8626 = 24.3202 amu, which rounds to
24.31 amu — this is magnesium. Choice A represents the mass of the most abundant isotope
only, a common error when students forget to weight each isotope by its natural abundance.
For weighted average problems, always verify that your final answer falls between the lightest
and heaviest isotope masses; if it does not, you have made a calculation error.
Question 4 of 50
A semiconductor manufacturer needs a dopant element that will add extra electrons to a
silicon crystal lattice. The dopant must have five valence electrons and atomic radius similar
to silicon. Which element should the engineer select?
, A. Boron (B)
B. Carbon (C)
C. Phosphorus (P) ✓ CORRECT
D. Germanium (Ge)
Correct Answer: C
Rationale: Phosphorus, located directly below nitrogen in Group 15, has five valence electrons
and an atomic radius close enough to silicon that it can substitute into the lattice without
significant structural distortion, donating its extra electron to the conduction band. Choice A
(boron) has only three valence electrons and would create electron holes rather than donate
electrons, making it a p-type rather than n-type dopant. On semiconductor questions,
remember that Group 15 elements create n-type doping (negative charge carriers), while
Group 13 elements create p-type doping.
Question 5 of 50
An environmental chemist is comparing the reactivity of alkali metals for a waste treatment
process. She observes that when a small piece of each metal is dropped into water, the
reaction with potassium is significantly more vigorous than with sodium. Which periodic
trend best explains this observation?
A. Potassium has a higher electronegativity than sodium, making it more reactive with water.
B. Potassium has a larger atomic radius and lower first ionization energy than sodium, so its
valence electron is more easily lost. ✓ CORRECT
C. Potassium has more protons than sodium, creating a stronger attraction for its valence
electron.
D. Potassium has a higher electron affinity than sodium, allowing it to readily accept
electrons from water.
Correct Answer: B
Rationale: Moving down Group 1, atomic radius increases and ionization energy decreases
because the valence electron occupies a higher principal energy level and is farther from the
nucleus with more shielding; this makes potassium's single valence electron easier to
remove, resulting in a more vigorous reaction with water. Choice A is incorrect because
electronegativity decreases down a group, and alkali metals are already the least
electronegative elements. For reactivity questions involving metals, always trace the logic
from atomic radius → ionization energy → ease of electron loss → reactivity.
Question 6 of 50
A radiocarbon dating laboratory is preparing a sample of carbon-14 for analysis. The
technician needs to know how many neutrons are present in a single atom of carbon-14.
A. 6 neutrons
