AP Chemistry Practice Quiz: Atomic Structure and Electron Configuration
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Test your understanding with short quizzes. This quiz has 11 questions to check your progress.
Question 1 of 11
All Questions (11)
A) 1s²2s²2p⁶3s²3p³
B) 1s²2s²2p⁶3s³3p²
C) 1s²2s²2p⁶3s²3p⁵
D) 1s²2s²2p⁶3s¹3p⁴
Correct Answer: A
According to the Aufbau principle, electrons fill the lowest energy subshells first. For 15 electrons, the configuration is 1s², then 2s², then 2p⁶, then 3s², and the remaining 3 electrons go into the 3p subshell, resulting in 1s²2s²2p⁶3s²3p³.
A) Valence electrons
B) Neutrons
C) Core electrons
D) Protons
Correct Answer: D
The atom's nucleus is made of protons, which are positively charged, and neutrons, which have no charge. The electrons are negatively charged and are located outside the nucleus. Therefore, the positive charge of the nucleus is due to its protons.
A) 1s²2s²2p⁶3s²3p⁶4s²
B) 1s²2s²2p⁶3s²3p⁶
C) 1s²2s²2p⁶3s²3p⁴4s²
D) 1s²2s²2p⁶3s²3p⁶4s²3d²
Correct Answer: B
A neutral calcium atom has 20 electrons with the configuration 1s²2s²2p⁶3s²3p⁶4s². To form the Ca²⁺ ion, the atom loses its two outermost (valence) electrons, which are the two electrons in the 4s subshell. The resulting configuration is 1s²2s²2p⁶3s²3p⁶.
A) 18 core, 1 valence
B) 10 core, 9 valence
C) 8 core, 11 valence
D) 2 core, 17 valence
Correct Answer: A
Valence electrons are the electrons in the outermost shell (highest principal energy level, n). In this case, the outermost shell is n=4, which contains 1 electron. All other electrons (1s²2s²2p⁶3s²3p⁶) are core electrons. There are 2+2+6+2+6 = 18 core electrons.
A) The valence electron in Na is in an s-subshell, while the valence electron in Cl is in a p-subshell, which is harder to remove.
B) A Cl atom has more neutrons than an Na atom, increasing the nuclear force.
C) The valence electron in Na is farther from the nucleus and experiences a lower effective nuclear charge than a valence electron in Cl, resulting in a weaker attraction.
D) The valence electrons in Cl are paired in an orbital, and the resulting repulsion makes them easier to remove than the single valence electron in Na.
Correct Answer: C
Both Na and Cl have valence electrons in the n=3 shell. However, Cl has a much higher effective nuclear charge (more protons) than Na. According to Coulomb's law, this greater charge results in a stronger force of attraction on the valence electrons. Additionally, the atomic radius of Na is larger than Cl, meaning its valence electron is farther from the nucleus, weakening the attraction. Both factors make the Na electron easier to remove.
A) It is reduced to one-fourth of the original force.
B) It is reduced to one-half of the original force.
C) It is doubled.
D) It is quadrupled.
Correct Answer: A
Coulomb's law states that the force between two charged particles is inversely proportional to the square of the distance between them (F ∝ 1/r²). If the distance (r) is doubled, the new force will be proportional to 1/(2r)², which is 1/(4r²). Therefore, the force is reduced to one-fourth of its original value.
A) After removing one electron, the remaining electrons rearrange into a less stable configuration.
B) The second electron is removed from a core shell, which is much closer to the nucleus.
C) Removing the first electron increases the number of neutrons, which holds the remaining electrons more tightly.
D) The second electron is being removed from a positive ion (Mg⁺), which has a greater effective nuclear charge and exerts a stronger Coulombic attraction than a neutral Mg atom.
Correct Answer: D
The first ionization removes an electron from a neutral Mg atom. The second ionization removes an electron from a Mg⁺ ion. In the Mg⁺ ion, the same number of protons (12) are now attracting fewer electrons (11). This increases the effective nuclear charge experienced by each remaining electron, leading to a stronger Coulombic attraction that requires significantly more energy to overcome.
A) Vanadium (V)
B) Chromium (Cr)
C) Manganese (Mn)
D) Iron (Fe)
Correct Answer: C
The noble gas Argon (Ar) has an atomic number of 18. The given configuration adds 2 electrons in the 4s subshell and 5 electrons in the 3d subshell. The total number of electrons is 18 + 2 + 5 = 25. The element with atomic number 25 is Manganese (Mn).
A) Core electrons require less energy to remove because they are shielded from the nucleus by the valence electrons.
B) Valence electrons require less energy to remove because they are in higher energy shells, farther from the nucleus.
C) Both core and valence electrons require the same amount of energy to remove because they experience the same nuclear charge.
D) The energy required to remove core or valence electrons depends only on the number of neutrons in the nucleus.
Correct Answer: B
Valence electrons occupy the outermost shells, which are at a greater average distance from the nucleus. According to Coulomb's law, a greater distance results in a weaker force of attraction. Therefore, valence electrons are held less tightly and require less energy (lower ionization energy) to remove compared to core electrons, which are closer to the nucleus.
A) 1s²2s²2p⁶3s²
B) 1s²2s²2p⁶3s²3p⁶4s¹
C) 1s²2s²2p⁶3s¹3p¹
D) 1s²2s²2p⁵
Correct Answer: C
The Aufbau principle states that electrons must fill the lowest available energy levels before filling higher levels. In the configuration 1s²2s²2p⁶3s¹3p¹, the 3s subshell, which can hold two electrons, is not completely filled before an electron is placed in the higher-energy 3p subshell. This represents an excited state, not a ground state.
A) ...has a smaller mass than the 2s electron.
B) ...is, on average, closer to the positively charged nucleus.
C) ...is shielded from the nucleus by the 2s electron.
D) ...experiences less electron-electron repulsion.
Correct Answer: B
The 1s subshell represents a lower principal energy level (n=1) than the 2s subshell (n=2). Electrons in lower energy levels are, on average, much closer to the nucleus. Based on Coulomb's law, the attractive force is much stronger at a shorter distance, thus requiring significantly more energy (higher ionization energy) to remove the electron.