Aleks Placement Test: Chemistry Questions and Answers & Detailed Explanation 2025/2026
Complete Study Guide |Guaranteed Value Pack Graded A+
✓ SECTION 1: ATOMIC STRUCTURE
1. Which subatomic particle has a positive charge?
A) Electron
B) Neutron
C) Proton
D) Positron
Answer: C
Rationale: Protons have a positive charge (+1). Electrons are negatively charged; neutrons are neutral.
2. The atomic number of an element is defined as the number of:
A) Neutrons
B) Protons
C) Electrons in the outermost shell
D) Protons plus neutrons
Answer: B
Rationale: Atomic number = number of protons. It determines the element's identity.
3. How many protons, neutrons, and electrons are in an atom of ⁴⁰₂₀Ca?
A) 20 protons, 20 neutrons, 20 electrons
B) 20 protons, 20 neutrons, 18 electrons
C) 20 protons, 40 neutrons, 20 electrons
D) 40 protons, 20 neutrons, 40 electrons
Answer: A
Rationale: The subscript 20 is the atomic number (protons). The mass number 40 minus 20 = 20 neutrons. A
neutral atom has 20 electrons.
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, Aleks Placement Test: Chemistry Questions and Answers & Detailed Explanation 2025/2026
Complete Study Guide |Guaranteed Value Pack Graded A+
4. Which of the following represents a pair of isotopes?
A) ¹⁴C and ¹⁴N
B) ³²S and ³²S²⁻
C) ³⁵Cl and ³⁷Cl
D) H₂O and D₂O
Answer: C
Rationale: Isotopes are atoms of the same element (same number of protons) with different numbers of
neutrons.
5. An atom with the electron configuration 1s² 2s² 2p³ has how many valence electrons?
A) 3
B) 5
C) 7
D) 2
Answer: B
Rationale: Valence electrons are the electrons in the outermost shell. The configuration shows 2 electrons in
the 2s subshell and 3 in the 2p subshell for a total of 5 valence electrons.
6. The ______ sublevel of an atom has a total of five orbitals.
A) s
B) p
C) d
D) f
Answer: C
Rationale: The d sublevel has five orbitals. The s sublevel has one, p has three, and f has seven.
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, Aleks Placement Test: Chemistry Questions and Answers & Detailed Explanation 2025/2026
Complete Study Guide |Guaranteed Value Pack Graded A+
7. What is the mass number of an atom with 17 protons and 18 neutrons?
A) 17
B) 18
C) 35
D) 36
Answer: C
Rationale: Mass number = protons + neutrons = 17 + 18 = 35.
8. Which subatomic particle determines the identity of an element?
A) Electron
B) Neutron
C) Proton
D) All of the above
Answer: C
Rationale: The number of protons (atomic number) uniquely identifies an element.
9. Who conducted the gold foil experiment that discovered the nucleus?
A) J.J. Thomson
B) Ernest Rutherford
C) John Dalton
D) Robert Boyle
Answer: B
Rationale: Ernest Rutherford shot alpha particles at gold foil and discovered a small, dense, positively
charged nucleus.
10. The "plum pudding model" of the atom was proposed by:
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, Aleks Placement Test: Chemistry Questions and Answers & Detailed Explanation 2025/2026
Complete Study Guide |Guaranteed Value Pack Graded A+
A) Ernest Rutherford
B) John Dalton
C) J.J. Thomson
D) Niels Bohr
Answer: C
Rationale: J.J. Thomson discovered electrons and proposed the plum pudding model, depicting a sphere of
positive charge with electrons scattered throughout.
11. Which particle has no charge and is located in the nucleus?
A) Proton
B) Electron
C) Neutron
D) Positron
Answer: C
Rationale: Neutrons are neutral particles found in the nucleus.
12. The electron cloud contains:
A) Protons and neutrons
B) Electrons only
C) Protons only
D) Neutrons only
Answer: B
Rationale: Electrons circle the nucleus in the electron cloud. Protons and neutrons are in the nucleus.
13. How many electrons does a neutral atom of aluminum (atomic number 13) have?
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Complete Study Guide |Guaranteed Value Pack Graded A+
✓ SECTION 1: ATOMIC STRUCTURE
1. Which subatomic particle has a positive charge?
A) Electron
B) Neutron
C) Proton
D) Positron
Answer: C
Rationale: Protons have a positive charge (+1). Electrons are negatively charged; neutrons are neutral.
2. The atomic number of an element is defined as the number of:
A) Neutrons
B) Protons
C) Electrons in the outermost shell
D) Protons plus neutrons
Answer: B
Rationale: Atomic number = number of protons. It determines the element's identity.
3. How many protons, neutrons, and electrons are in an atom of ⁴⁰₂₀Ca?
A) 20 protons, 20 neutrons, 20 electrons
B) 20 protons, 20 neutrons, 18 electrons
C) 20 protons, 40 neutrons, 20 electrons
D) 40 protons, 20 neutrons, 40 electrons
Answer: A
Rationale: The subscript 20 is the atomic number (protons). The mass number 40 minus 20 = 20 neutrons. A
neutral atom has 20 electrons.
1|Page
, Aleks Placement Test: Chemistry Questions and Answers & Detailed Explanation 2025/2026
Complete Study Guide |Guaranteed Value Pack Graded A+
4. Which of the following represents a pair of isotopes?
A) ¹⁴C and ¹⁴N
B) ³²S and ³²S²⁻
C) ³⁵Cl and ³⁷Cl
D) H₂O and D₂O
Answer: C
Rationale: Isotopes are atoms of the same element (same number of protons) with different numbers of
neutrons.
5. An atom with the electron configuration 1s² 2s² 2p³ has how many valence electrons?
A) 3
B) 5
C) 7
D) 2
Answer: B
Rationale: Valence electrons are the electrons in the outermost shell. The configuration shows 2 electrons in
the 2s subshell and 3 in the 2p subshell for a total of 5 valence electrons.
6. The ______ sublevel of an atom has a total of five orbitals.
A) s
B) p
C) d
D) f
Answer: C
Rationale: The d sublevel has five orbitals. The s sublevel has one, p has three, and f has seven.
2|Page
, Aleks Placement Test: Chemistry Questions and Answers & Detailed Explanation 2025/2026
Complete Study Guide |Guaranteed Value Pack Graded A+
7. What is the mass number of an atom with 17 protons and 18 neutrons?
A) 17
B) 18
C) 35
D) 36
Answer: C
Rationale: Mass number = protons + neutrons = 17 + 18 = 35.
8. Which subatomic particle determines the identity of an element?
A) Electron
B) Neutron
C) Proton
D) All of the above
Answer: C
Rationale: The number of protons (atomic number) uniquely identifies an element.
9. Who conducted the gold foil experiment that discovered the nucleus?
A) J.J. Thomson
B) Ernest Rutherford
C) John Dalton
D) Robert Boyle
Answer: B
Rationale: Ernest Rutherford shot alpha particles at gold foil and discovered a small, dense, positively
charged nucleus.
10. The "plum pudding model" of the atom was proposed by:
3|Page
, Aleks Placement Test: Chemistry Questions and Answers & Detailed Explanation 2025/2026
Complete Study Guide |Guaranteed Value Pack Graded A+
A) Ernest Rutherford
B) John Dalton
C) J.J. Thomson
D) Niels Bohr
Answer: C
Rationale: J.J. Thomson discovered electrons and proposed the plum pudding model, depicting a sphere of
positive charge with electrons scattered throughout.
11. Which particle has no charge and is located in the nucleus?
A) Proton
B) Electron
C) Neutron
D) Positron
Answer: C
Rationale: Neutrons are neutral particles found in the nucleus.
12. The electron cloud contains:
A) Protons and neutrons
B) Electrons only
C) Protons only
D) Neutrons only
Answer: B
Rationale: Electrons circle the nucleus in the electron cloud. Protons and neutrons are in the nucleus.
13. How many electrons does a neutral atom of aluminum (atomic number 13) have?
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