WGU D425 OBJECTIVE ASSESSMENT 2 EXAM 2026 |
INTRODUCTION TO CHEMISTRY PRACTICE
QUESTIONS
WITH VERIFIED ANSWERS & RATIONALES GRADED A+ |
COMPLETE STUDY GUIDE |
LATEST UPDATE
SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS (Questions 1-50)
Q1. What is the atomic number of an atom containing 58 protons, 58 electrons,
and 82 neutrons?
A) 58
B) 82
C) 116
D) 140
Correct Answer: A
Rationale: The atomic number is defined as the number of protons in an atom's
nucleus. This atom has 58 protons, so its atomic number is 58. The number of
neutrons (82) determines the mass number (140), but does not affect the atomic
number. The number of electrons (58) indicates the atom is neutral.
Why the others are wrong:
B) 82 is the number of neutrons, not the atomic number.
C) 116 is the sum of protons and electrons (58+58), not the atomic number.
D) 140 is the mass number (protons + neutrons = 58 + 82), not the atomic
number.
Q2. Which subatomic particle has a negative charge?
A) Proton
B) Neutron
C) Electron
D) Positron
1
,Correct Answer: C
Rationale: Electrons are negatively charged subatomic particles that orbit
the nucleus of an atom. They have a relative charge of -1 and negligible mass
compared to protons and neutrons.
Why the others are wrong:
A) Protons have a positive charge (+1).
B) Neutrons have no charge (neutral).
D) Positrons are antimatter particles with a positive charge, not typically
found in stable atoms.
Q3. An atom has 6 protons, 6 neutrons, and 6 electrons. What is its mass
number?
A) 6
B) 12
C) 18
D) 24
Correct Answer: B
Rationale: The mass number is the total number of protons and neutrons in the
nucleus. 6 protons + 6 neutrons = 12. The number of electrons does not affect
the mass number.
Why the others are wrong:
A) 6 is the number of protons (atomic number), not the mass number.
C) 18 is the sum of protons + neutrons + electrons (6+6+6), which is not the
mass number.
D) 24 is double the mass number and incorrect.
Q4. What is the charge of a neutron?
2
,A) +1
B) -1
C) 0 (neutral)
D) +2
Correct Answer: C
Rationale: Neutrons are subatomic particles with no electrical charge (neutral).
They are located in the nucleus along with protons and contribute to the mass
of the atom.
Why the others are wrong:
A) +1 is the charge of a proton.
B) -1 is the charge of an electron.
D) +2 is not a charge carried by any subatomic particle.
Q5. Neon (Ne) has three stable isotopes: 90.5% neon-20 (19.992 amu), 0.270%
neon-21 (20.994 amu), and 9.25% neon-22 (21.991 amu). What is the average
atomic mass of neon?
A) 20.18 amu
B) 20.2 amu
C) 20.99 amu
D) 21.0 amu
Correct Answer: A
Rationale: Average atomic mass = (fraction1 × mass1) + (fraction2 × mass2) +
(fraction3 × mass3). (0.905 × 19.992) + (0.00270 × 20.994) + (0.0925 × 21.991)
= 18.09 + 0.0567 + 2.034 = 20.18 amu.
Why the others are wrong:
B) 20.2 amu is a rough estimate but not the precise calculated value.
C) 20.99 amu is the mass of neon-21, not the average.
D) 21.0 amu is the mass of neon-22, not the average.
3
, Q6. Naturally occurring sulfur (S) is composed of 95.02% sulfur-32 (31.972 amu),
0.7500% sulfur-33 (32.971 amu), 4.210% sulfur-34 (33.969 amu), and 0.02000%
sulfur-36 (35.967 amu). What is the average atomic mass of sulfur?
A) 32.06 amu
B) 32.064 amu
C) 33.72 amu
D) 33.720 amu
Correct Answer: B
Rationale: Average atomic mass = (0.9502 × 31.972) + (0.0075 × 32.971) +
(0.04210 × 33.969) + (0.0002 × 35.967) = 30.38 + 0.2473 + 1.430 + 0.00719
= 32.064 amu.
Why the others are wrong:
A) 32.06 amu is a rounded value but less precise than 32.064.
C) 33.72 amu is significantly higher than the actual average.
D) 33.720 amu is also significantly higher than the actual average.
Q7. Naturally occurring gallium (Ga) is composed of 60.11% gallium-69
(68.926 amu) and 39.89% gallium-71 (70.925 amu). What is the average atomic
mass of gallium?
A) 69.64 amu
B) 69.7 amu
C) 69.72 amu
D) 69.9 amu
Correct Answer: C
Rationale: Average atomic mass = (0.6011 × 68.926) + (0.3989 × 70.925) =
41.43 + 28.29 = 69.72 amu.
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INTRODUCTION TO CHEMISTRY PRACTICE
QUESTIONS
WITH VERIFIED ANSWERS & RATIONALES GRADED A+ |
COMPLETE STUDY GUIDE |
LATEST UPDATE
SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS (Questions 1-50)
Q1. What is the atomic number of an atom containing 58 protons, 58 electrons,
and 82 neutrons?
A) 58
B) 82
C) 116
D) 140
Correct Answer: A
Rationale: The atomic number is defined as the number of protons in an atom's
nucleus. This atom has 58 protons, so its atomic number is 58. The number of
neutrons (82) determines the mass number (140), but does not affect the atomic
number. The number of electrons (58) indicates the atom is neutral.
Why the others are wrong:
B) 82 is the number of neutrons, not the atomic number.
C) 116 is the sum of protons and electrons (58+58), not the atomic number.
D) 140 is the mass number (protons + neutrons = 58 + 82), not the atomic
number.
Q2. Which subatomic particle has a negative charge?
A) Proton
B) Neutron
C) Electron
D) Positron
1
,Correct Answer: C
Rationale: Electrons are negatively charged subatomic particles that orbit
the nucleus of an atom. They have a relative charge of -1 and negligible mass
compared to protons and neutrons.
Why the others are wrong:
A) Protons have a positive charge (+1).
B) Neutrons have no charge (neutral).
D) Positrons are antimatter particles with a positive charge, not typically
found in stable atoms.
Q3. An atom has 6 protons, 6 neutrons, and 6 electrons. What is its mass
number?
A) 6
B) 12
C) 18
D) 24
Correct Answer: B
Rationale: The mass number is the total number of protons and neutrons in the
nucleus. 6 protons + 6 neutrons = 12. The number of electrons does not affect
the mass number.
Why the others are wrong:
A) 6 is the number of protons (atomic number), not the mass number.
C) 18 is the sum of protons + neutrons + electrons (6+6+6), which is not the
mass number.
D) 24 is double the mass number and incorrect.
Q4. What is the charge of a neutron?
2
,A) +1
B) -1
C) 0 (neutral)
D) +2
Correct Answer: C
Rationale: Neutrons are subatomic particles with no electrical charge (neutral).
They are located in the nucleus along with protons and contribute to the mass
of the atom.
Why the others are wrong:
A) +1 is the charge of a proton.
B) -1 is the charge of an electron.
D) +2 is not a charge carried by any subatomic particle.
Q5. Neon (Ne) has three stable isotopes: 90.5% neon-20 (19.992 amu), 0.270%
neon-21 (20.994 amu), and 9.25% neon-22 (21.991 amu). What is the average
atomic mass of neon?
A) 20.18 amu
B) 20.2 amu
C) 20.99 amu
D) 21.0 amu
Correct Answer: A
Rationale: Average atomic mass = (fraction1 × mass1) + (fraction2 × mass2) +
(fraction3 × mass3). (0.905 × 19.992) + (0.00270 × 20.994) + (0.0925 × 21.991)
= 18.09 + 0.0567 + 2.034 = 20.18 amu.
Why the others are wrong:
B) 20.2 amu is a rough estimate but not the precise calculated value.
C) 20.99 amu is the mass of neon-21, not the average.
D) 21.0 amu is the mass of neon-22, not the average.
3
, Q6. Naturally occurring sulfur (S) is composed of 95.02% sulfur-32 (31.972 amu),
0.7500% sulfur-33 (32.971 amu), 4.210% sulfur-34 (33.969 amu), and 0.02000%
sulfur-36 (35.967 amu). What is the average atomic mass of sulfur?
A) 32.06 amu
B) 32.064 amu
C) 33.72 amu
D) 33.720 amu
Correct Answer: B
Rationale: Average atomic mass = (0.9502 × 31.972) + (0.0075 × 32.971) +
(0.04210 × 33.969) + (0.0002 × 35.967) = 30.38 + 0.2473 + 1.430 + 0.00719
= 32.064 amu.
Why the others are wrong:
A) 32.06 amu is a rounded value but less precise than 32.064.
C) 33.72 amu is significantly higher than the actual average.
D) 33.720 amu is also significantly higher than the actual average.
Q7. Naturally occurring gallium (Ga) is composed of 60.11% gallium-69
(68.926 amu) and 39.89% gallium-71 (70.925 amu). What is the average atomic
mass of gallium?
A) 69.64 amu
B) 69.7 amu
C) 69.72 amu
D) 69.9 amu
Correct Answer: C
Rationale: Average atomic mass = (0.6011 × 68.926) + (0.3989 × 70.925) =
41.43 + 28.29 = 69.72 amu.
4
