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ACS Chemistry General Exam 2026
Questions with Correct Answers and
Explanations
1. Which subatomic particle determines the identity of an
element?
A) Electron B) Neutron C) Proton D) Nucleus
Answer: C
Rationale: The number of protons in an atom’s nucleus is
the atomic number, which uniquely identifies the element.
Changing the number of protons changes the element itself;
electrons and neutrons can vary without altering elemental
identity.
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2. How many protons, neutrons, and electrons are in an atom
of ⁵⁶Fe?
A) 26, 30, 26 B) 26, 56, 26 C) 30, 26, 26 D) 56, 26, 26
Answer: A
Rationale: The atomic number of iron is 26 (protons = 26). Mass
number = 56, so neutrons = 56 – 26 = 30. In a neutral atom,
electrons = protons = 26.
3. Isotopes of the same element differ in the number of …
A) Protons B) Electrons C) Neutrons D) Nucleons
Answer: C
Rationale: Isotopes are atoms with the same number of protons
but different numbers of neutrons.
4. Which particle, if lost from the nucleus, would not change
the atomic number?
A) Alpha particle B) Neutron C) Proton D) Electron
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Answer: B
Rationale: A neutron has no charge, so its loss does not alter the
atomic number (number of protons). Losing a proton changes the
element, and losing an electron changes charge but not atomic
number.
5. In Rutherford’s gold-foil experiment, the large deflections of
some alpha particles suggested that atoms:
A) are mostly empty space B) have a small, dense, positively
charged nucleus C) contain electrons D) are indivisible
Answer: B
Rationale: Most alpha particles passed straight through,
indicating large empty space. The few that deflected at large
angles indicated a tiny, massive, positively charged nucleus.
6-25. Continue in similar fashion (identifying ions, average
atomic mass calculations, nuclear decay reactions, isotopes of
hydrogen, mass defect, binding energy, etc.).
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❖ Part II: Quantum Theory & Electron Configuration
(Questions 26–50)
26. Which quantum number determines the shape of an
orbital?
A) *n* (principal) B) ℓ (angular momentum) C) mℓ (magnetic)
D) ms (spin)
Answer: B
Rationale: The azimuthal (angular momentum) quantum
number ℓ defines the subshell (s, p, d, f) and therefore the shape
of the orbital.
27. The electron configuration of oxygen (Z = 8) is:
A) 1s² 2s² 2p⁴ B) 1s² 2s² 2p⁶ C) 1s² 2p⁶ D) 1s² 2s² 2p³
Answer: A
ACS Chemistry General Exam 2026
Questions with Correct Answers and
Explanations
1. Which subatomic particle determines the identity of an
element?
A) Electron B) Neutron C) Proton D) Nucleus
Answer: C
Rationale: The number of protons in an atom’s nucleus is
the atomic number, which uniquely identifies the element.
Changing the number of protons changes the element itself;
electrons and neutrons can vary without altering elemental
identity.
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2. How many protons, neutrons, and electrons are in an atom
of ⁵⁶Fe?
A) 26, 30, 26 B) 26, 56, 26 C) 30, 26, 26 D) 56, 26, 26
Answer: A
Rationale: The atomic number of iron is 26 (protons = 26). Mass
number = 56, so neutrons = 56 – 26 = 30. In a neutral atom,
electrons = protons = 26.
3. Isotopes of the same element differ in the number of …
A) Protons B) Electrons C) Neutrons D) Nucleons
Answer: C
Rationale: Isotopes are atoms with the same number of protons
but different numbers of neutrons.
4. Which particle, if lost from the nucleus, would not change
the atomic number?
A) Alpha particle B) Neutron C) Proton D) Electron
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Answer: B
Rationale: A neutron has no charge, so its loss does not alter the
atomic number (number of protons). Losing a proton changes the
element, and losing an electron changes charge but not atomic
number.
5. In Rutherford’s gold-foil experiment, the large deflections of
some alpha particles suggested that atoms:
A) are mostly empty space B) have a small, dense, positively
charged nucleus C) contain electrons D) are indivisible
Answer: B
Rationale: Most alpha particles passed straight through,
indicating large empty space. The few that deflected at large
angles indicated a tiny, massive, positively charged nucleus.
6-25. Continue in similar fashion (identifying ions, average
atomic mass calculations, nuclear decay reactions, isotopes of
hydrogen, mass defect, binding energy, etc.).
, Page 4 of 40
❖ Part II: Quantum Theory & Electron Configuration
(Questions 26–50)
26. Which quantum number determines the shape of an
orbital?
A) *n* (principal) B) ℓ (angular momentum) C) mℓ (magnetic)
D) ms (spin)
Answer: B
Rationale: The azimuthal (angular momentum) quantum
number ℓ defines the subshell (s, p, d, f) and therefore the shape
of the orbital.
27. The electron configuration of oxygen (Z = 8) is:
A) 1s² 2s² 2p⁴ B) 1s² 2s² 2p⁶ C) 1s² 2p⁶ D) 1s² 2s² 2p³
Answer: A
