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ACS Chemistry General Exam 2026 Questions
with Correct Answers and Explanations
Q1. Which particle determines the identity of an element?
A) Electron
B) Neutron
C) Proton
D) Nucleus
Answer: C) Proton
Rationale: The number of protons in an atom's nucleus defines the
atomic number (Z), which uniquely identifies an element.
Changing the number of protons changes the element itself, while
electrons and neutrons can vary without altering elemental
identity .
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Q2. What is the correct electron configuration for oxygen (Z =
8)?
A) 1s² 2s² 2p⁴
B) 1s² 2s² 2p⁶
C) 1s² 2p⁶
D) 1s² 2s² 2p³
Answer: A) 1s² 2s² 2p⁴
Rationale: Oxygen has 8 electrons. Filling orbitals in order of
increasing energy (Aufbau principle) gives 1s² (2 electrons), 2s²
(2 electrons), and 2p⁴ (4 electrons). This configuration follows
Hund's rule and the Pauli exclusion principle .
Q3. Which quantum number determines the shape of an orbital?
A) n (principal)
B) l (angular momentum)
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C) mₗ (magnetic)
D) mₛ (spin)
Answer: B) l (angular momentum quantum number)
Rationale: The angular momentum quantum number (l) determines
the shape of an orbital. l = 0 corresponds to s orbitals
(spherical), l = 1 to p orbitals (dumbbell-shaped), l = 2 to d
orbitals, and l = 3 to f orbitals .
Q4. What are the four quantum numbers and their significance?
Answer:
n (principal) = energy level (1, 2, 3...)
l (angular momentum) = shape of orbital (0 to n-1)
mₗ (magnetic) = orientation in space (-l to +l)
mₛ (spin) = electron spin direction (+½ or -½)
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Rationale: These four quantum numbers uniquely describe each
electron in an atom, following the Pauli exclusion principle which
states no two electrons can have the same set of four quantum
numbers .
Q5. An atom has a valence shell configuration of 1s¹. To which
group on the periodic table does it belong?
A) Noble gases
B) Alkali metals
C) Halogens
D) Alkaline earth metals
Answer: B) Alkali metals
Rationale: The configuration 1s¹ represents hydrogen, but more
generally, a valence shell configuration of s¹ indicates Group IA
(alkali metals). These elements have one valence electron and
are highly reactive .
ACS Chemistry General Exam 2026 Questions
with Correct Answers and Explanations
Q1. Which particle determines the identity of an element?
A) Electron
B) Neutron
C) Proton
D) Nucleus
Answer: C) Proton
Rationale: The number of protons in an atom's nucleus defines the
atomic number (Z), which uniquely identifies an element.
Changing the number of protons changes the element itself, while
electrons and neutrons can vary without altering elemental
identity .
,Page 2 of 165
Q2. What is the correct electron configuration for oxygen (Z =
8)?
A) 1s² 2s² 2p⁴
B) 1s² 2s² 2p⁶
C) 1s² 2p⁶
D) 1s² 2s² 2p³
Answer: A) 1s² 2s² 2p⁴
Rationale: Oxygen has 8 electrons. Filling orbitals in order of
increasing energy (Aufbau principle) gives 1s² (2 electrons), 2s²
(2 electrons), and 2p⁴ (4 electrons). This configuration follows
Hund's rule and the Pauli exclusion principle .
Q3. Which quantum number determines the shape of an orbital?
A) n (principal)
B) l (angular momentum)
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C) mₗ (magnetic)
D) mₛ (spin)
Answer: B) l (angular momentum quantum number)
Rationale: The angular momentum quantum number (l) determines
the shape of an orbital. l = 0 corresponds to s orbitals
(spherical), l = 1 to p orbitals (dumbbell-shaped), l = 2 to d
orbitals, and l = 3 to f orbitals .
Q4. What are the four quantum numbers and their significance?
Answer:
n (principal) = energy level (1, 2, 3...)
l (angular momentum) = shape of orbital (0 to n-1)
mₗ (magnetic) = orientation in space (-l to +l)
mₛ (spin) = electron spin direction (+½ or -½)
, Page 4 of 165
Rationale: These four quantum numbers uniquely describe each
electron in an atom, following the Pauli exclusion principle which
states no two electrons can have the same set of four quantum
numbers .
Q5. An atom has a valence shell configuration of 1s¹. To which
group on the periodic table does it belong?
A) Noble gases
B) Alkali metals
C) Halogens
D) Alkaline earth metals
Answer: B) Alkali metals
Rationale: The configuration 1s¹ represents hydrogen, but more
generally, a valence shell configuration of s¹ indicates Group IA
(alkali metals). These elements have one valence electron and
are highly reactive .
