200+ QUESTIONS & ANSWERS WITH
RATIONALES
,WGU D425 Introduction to Chemistry Final Exam 2026/2027 | Practice Questions, Answers &
Rationales | OA Prep
SECTION 1: ATOMIC STRUCTURE & THE PERIODIC TABLE
Question 1
What is the atomic number of an element?
A) The number of neutrons in the nucleus
B) The total mass of the atom
C) The number of protons in the nucleus
D) The number of electrons in the outer shell
E) The sum of protons and neutrons
Answer: C) The number of protons in the nucleus
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. The mass
number (not atomic number) is the sum of protons and neutrons .
Question 2
Which subatomic particle has a negative charge and orbits the nucleus?
A) Proton
B) Neutron
C) Electron
D) Positron
E) Alpha particle
Answer: C) Electron
,Rationale: Electrons carry a charge of -1 and orbit the nucleus in regions called electron shells
or energy levels. Protons are positively charged (+1) and neutrons are neutral (0), both located
in the nucleus .
Question 3
The mass number of an atom is defined as:
A) Number of protons only
B) Number of electrons only
C) Number of neutrons only
D) Number of protons plus neutrons
E) Number of protons minus electrons
Answer: D) Number of protons plus neutrons
Rationale: Mass number = protons + neutrons. Electrons have negligible mass and are not
counted in mass number calculations. The mass number is always a whole number and is
distinct from atomic mass, which is a weighted average of all naturally occurring isotopes .
Question 4
Isotopes of the same element differ in:
A) Number of protons
B) Number of electrons
C) Number of neutrons
D) Atomic number
E) Chemical properties
Answer: C) Number of neutrons
, Rationale: Isotopes have the same atomic number (same number of protons) but differ in the
number of neutrons, giving them different mass numbers. For example, carbon-12 and carbon-
14 both have 6 protons but 6 and 8 neutrons respectively .
Question 5
An atom of carbon-14 contains how many neutrons?
A) 6
B) 7
C) 8
D) 12
E) 14
Answer: C) 8
Rationale: Carbon has an atomic number of 6 (6 protons). Carbon-14 has a mass number of 14.
Neutrons = Mass Number - Atomic Number = 14 - 6 = 8 neutrons .
Question 6
Which scientist discovered the nucleus of the atom through the gold foil experiment?
A) Dalton
B) J.J. Thomson
C) Rutherford
D) Bohr
E) Schrödinger
Answer: C) Rutherford
RATIONALES
,WGU D425 Introduction to Chemistry Final Exam 2026/2027 | Practice Questions, Answers &
Rationales | OA Prep
SECTION 1: ATOMIC STRUCTURE & THE PERIODIC TABLE
Question 1
What is the atomic number of an element?
A) The number of neutrons in the nucleus
B) The total mass of the atom
C) The number of protons in the nucleus
D) The number of electrons in the outer shell
E) The sum of protons and neutrons
Answer: C) The number of protons in the nucleus
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. The mass
number (not atomic number) is the sum of protons and neutrons .
Question 2
Which subatomic particle has a negative charge and orbits the nucleus?
A) Proton
B) Neutron
C) Electron
D) Positron
E) Alpha particle
Answer: C) Electron
,Rationale: Electrons carry a charge of -1 and orbit the nucleus in regions called electron shells
or energy levels. Protons are positively charged (+1) and neutrons are neutral (0), both located
in the nucleus .
Question 3
The mass number of an atom is defined as:
A) Number of protons only
B) Number of electrons only
C) Number of neutrons only
D) Number of protons plus neutrons
E) Number of protons minus electrons
Answer: D) Number of protons plus neutrons
Rationale: Mass number = protons + neutrons. Electrons have negligible mass and are not
counted in mass number calculations. The mass number is always a whole number and is
distinct from atomic mass, which is a weighted average of all naturally occurring isotopes .
Question 4
Isotopes of the same element differ in:
A) Number of protons
B) Number of electrons
C) Number of neutrons
D) Atomic number
E) Chemical properties
Answer: C) Number of neutrons
, Rationale: Isotopes have the same atomic number (same number of protons) but differ in the
number of neutrons, giving them different mass numbers. For example, carbon-12 and carbon-
14 both have 6 protons but 6 and 8 neutrons respectively .
Question 5
An atom of carbon-14 contains how many neutrons?
A) 6
B) 7
C) 8
D) 12
E) 14
Answer: C) 8
Rationale: Carbon has an atomic number of 6 (6 protons). Carbon-14 has a mass number of 14.
Neutrons = Mass Number - Atomic Number = 14 - 6 = 8 neutrons .
Question 6
Which scientist discovered the nucleus of the atom through the gold foil experiment?
A) Dalton
B) J.J. Thomson
C) Rutherford
D) Bohr
E) Schrödinger
Answer: C) Rutherford
