ACS General Chemistry Exam Practice Test 2026 150
Questions with Correct Answers and Explanations
SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS
(Questions 1–20)
Question 1 of 150
Which of the following elements has the largest atomic radius?
• A. Lithium (Li)
• B. Sodium (Na)
• C. Potassium (K)
• D. Rubidium (Rb)
Correct Answer: D – Rubidium (Rb)
Explanation: Atomic radius increases down a group because additional electron shells are
added. Among the alkali metals (Group 1), Rb is the largest in this list. Francium (Fr) is
larger but not listed.
Question 2 of 150
Which of the following elements has the highest electronegativity?
• A. Fluorine (F)
• B. Oxygen (O)
, • C. Nitrogen (N)
• D. Chlorine (Cl)
Correct Answer: A – Fluorine (F)
Explanation: Fluorine has the highest electronegativity (4.0 on the Pauling scale).
Electronegativity increases from left to right across a period and decreases down a group.
Oxygen is second (3.5), nitrogen third (3.0), and chlorine (3.2) is high but still lower than
fluorine.
Question 3 of 150
Which of the following represents the ground-state electron configuration for a sulfur
atom (atomic number 16)?
• A. 1s² 2s² 2p⁶ 3s¹ 3p⁵
• B. 1s² 2s² 2p⁶ 3s² 3p⁴
• C. 1s² 2s² 2p⁶ 3s² 3p⁶
• D. 1s² 2s² 2p⁶ 3s² 3p²
Correct Answer: B – 1s² 2s² 2p⁶ 3s² 3p⁴
Explanation: Sulfur has 16 electrons. The correct configuration: 1s² (2), 2s² (4), 2p⁶ (10), 3s²
(12), 3p⁴ (16). Option C is argon (18 electrons); option A is chlorine (17 electrons); option D
is silicon (14 electrons).
Question 4 of 150
How many valence electrons does a neutral atom of phosphorus (atomic number 15)
have?
, • A. 2
• B. 3
• C. 5
• D. 7
Correct Answer: C – 5
Explanation: Phosphorus is in Group 15 (VA). Elements in this group have 5 valence
electrons. The electron configuration is [Ne] 3s² 3p³.
Question 5 of 150
Which of the following is an isotope of carbon-12 (¹²C)?
• A. ¹³C
• B. ¹⁴C
• C. ¹¹C
• D. All of the above
Correct Answer: D – All of the above
Explanation: Isotopes are atoms of the same element with the same number of protons (6
for carbon) but different numbers of neutrons. ¹¹C, ¹²C, ¹³C, and ¹⁴C are all isotopes of
carbon.
Question 6 of 150
Which of the following has the largest ionization energy?
• A. Sodium (Na)
Questions with Correct Answers and Explanations
SECTION 1: ATOMIC STRUCTURE & PERIODIC TRENDS
(Questions 1–20)
Question 1 of 150
Which of the following elements has the largest atomic radius?
• A. Lithium (Li)
• B. Sodium (Na)
• C. Potassium (K)
• D. Rubidium (Rb)
Correct Answer: D – Rubidium (Rb)
Explanation: Atomic radius increases down a group because additional electron shells are
added. Among the alkali metals (Group 1), Rb is the largest in this list. Francium (Fr) is
larger but not listed.
Question 2 of 150
Which of the following elements has the highest electronegativity?
• A. Fluorine (F)
• B. Oxygen (O)
, • C. Nitrogen (N)
• D. Chlorine (Cl)
Correct Answer: A – Fluorine (F)
Explanation: Fluorine has the highest electronegativity (4.0 on the Pauling scale).
Electronegativity increases from left to right across a period and decreases down a group.
Oxygen is second (3.5), nitrogen third (3.0), and chlorine (3.2) is high but still lower than
fluorine.
Question 3 of 150
Which of the following represents the ground-state electron configuration for a sulfur
atom (atomic number 16)?
• A. 1s² 2s² 2p⁶ 3s¹ 3p⁵
• B. 1s² 2s² 2p⁶ 3s² 3p⁴
• C. 1s² 2s² 2p⁶ 3s² 3p⁶
• D. 1s² 2s² 2p⁶ 3s² 3p²
Correct Answer: B – 1s² 2s² 2p⁶ 3s² 3p⁴
Explanation: Sulfur has 16 electrons. The correct configuration: 1s² (2), 2s² (4), 2p⁶ (10), 3s²
(12), 3p⁴ (16). Option C is argon (18 electrons); option A is chlorine (17 electrons); option D
is silicon (14 electrons).
Question 4 of 150
How many valence electrons does a neutral atom of phosphorus (atomic number 15)
have?
, • A. 2
• B. 3
• C. 5
• D. 7
Correct Answer: C – 5
Explanation: Phosphorus is in Group 15 (VA). Elements in this group have 5 valence
electrons. The electron configuration is [Ne] 3s² 3p³.
Question 5 of 150
Which of the following is an isotope of carbon-12 (¹²C)?
• A. ¹³C
• B. ¹⁴C
• C. ¹¹C
• D. All of the above
Correct Answer: D – All of the above
Explanation: Isotopes are atoms of the same element with the same number of protons (6
for carbon) but different numbers of neutrons. ¹¹C, ¹²C, ¹³C, and ¹⁴C are all isotopes of
carbon.
Question 6 of 150
Which of the following has the largest ionization energy?
• A. Sodium (Na)
