Rucete ✏ AP Chemistry In a Nutshell
1. Structure of the Atom — Practice Questions 3
This problem set explores atomic theory from new perspectives, emphasizing historical experiments, quantum principles, energy quantization, and electron configurations. It is intended to further solidify understanding of atomic structure concepts through unique and challenging questions.
(Multiple Choice — Click to Reveal Answer)
1. What term describes atoms of the same element with different mass numbers?
(A) Ions
(B) Isomers
(C) Isotopes
(D) Allotropes
Answer
(C) — Isotopes have the same atomic number but different numbers of neutrons.
2. Who proposed the indivisible nature of atoms in early atomic theory?
(A) Democritus
(B) Dalton
(C) Rutherford
(D) Bohr
Answer
(A) — Democritus first suggested atoms were indivisible particles.
3. Which particle determines the atomic number of an element?
(A) Neutron
(B) Electron
(C) Proton
(D) Nucleus
Answer
(C) — Atomic number equals the number of protons.
4. Which principle explains why electrons occupy the lowest energy orbitals first?
(A) Hund’s Rule
(B) Aufbau Principle
(C) Pauli Exclusion Principle
(D) Bohr’s Postulate
Answer
(B) — Aufbau principle states electrons fill orbitals from lowest to highest energy.
5. Which of the following is NOT a quantum number?
(A) Principal (n)
(B) Angular momentum (ℓ)
(C) Directional (d)
(D) Spin (ms)
Answer
(C) — “Directional” is not one of the four quantum numbers.
6. Which of the following is NOT found in the nucleus of an atom?
(A) Proton
(B) Neutron
(C) Electron
(D) Positive charge
Answer
(C) — Electrons are found outside the nucleus in orbitals.
7. The discovery of the electron was made using:
(A) Gold foil experiment
(B) Oil drop experiment
(C) Cathode ray tube
(D) X-ray diffraction
Answer
(C) — J.J. Thomson used the cathode ray tube to discover the electron.
8. Which subatomic particle has the least mass?
(A) Proton
(B) Neutron
(C) Alpha particle
(D) Electron
Answer
(D) — Electrons are much lighter than protons or neutrons.
9. The number of protons and neutrons in an atom is called its:
(A) Atomic number
(B) Isotope
(C) Mass number
(D) Charge
Answer
(C) — Mass number = protons + neutrons.
10. Which of the following is a correct symbol for an isotope of carbon with 6 protons and 7 neutrons?
(A) C-6
(B) C-7
(C) ¹³C
(D) ¹¹C
Answer
(C) — 6 protons + 7 neutrons = 13 → ¹³C.
11. The charge of a neutron is:
(A) +1
(B) –1
(C) 0
(D) +2
Answer
(C) — Neutrons are neutral particles.
12. A line spectrum is produced when:
(A) Electrons absorb energy
(B) Electrons are removed completely
(C) Electrons drop to lower energy levels
(D) Electrons gain kinetic energy
Answer
(C) — Line spectra occur when electrons fall to lower energy levels and emit specific photons.
13. Which of these is a valid set of quantum numbers for a 3p electron?
(A) n = 3, ℓ = 1, mℓ = 2
(B) n = 3, ℓ = 1, mℓ = 0
(C) n = 3, ℓ = 2, mℓ = 2
(D) n = 2, ℓ = 1, mℓ = –2
Answer
(B) — p orbitals (ℓ = 1) have mℓ values from –1 to +1; mℓ = 0 is valid.
14. The principal quantum number describes:
(A) Shape of orbital
(B) Orientation in space
(C) Average distance from nucleus
(D) Direction of spin
Answer
(C) — Principal quantum number (n) determines the energy level and average distance of the orbital from the nucleus.
15. What is the maximum number of electrons a single orbital can hold?
(A) 1
(B) 2
(C) 4
(D) 6
Answer
(B) — Each orbital holds up to 2 electrons with opposite spins.
16. In which orbital do electrons have the lowest energy?
(A) 1s
(B) 2s
(C) 3p
(D) 3d
Answer
(A) — The 1s orbital is the lowest energy orbital in any atom.
17. The shape of a p orbital is:
(A) Spherical
(B) Dumbbell
(C) Cloverleaf
(D) Circular
Answer
(B) — p orbitals have a dumbbell shape.
18. Which principle explains why no orbital can have more than two electrons?
(A) Aufbau principle
(B) Hund’s rule
(C) Pauli exclusion principle
(D) Schrödinger’s model
Answer
(C) — The Pauli exclusion principle says no two electrons can have the same four quantum numbers.
19. Which experiment led to the discovery of the nucleus?
(A) Cathode ray experiment
(B) Oil drop experiment
(C) Gold foil experiment
(D) Spectroscopy
Answer
(C) — Rutherford’s gold foil experiment showed that atoms have a dense central nucleus.
20. What is the purpose of an orbital diagram?
(A) Show only the number of electrons
(B) Show electron energy
(C) Show spatial arrangement and spin
(D) Show only the atomic radius
Answer
(C) — Orbital diagrams indicate how electrons are distributed in orbitals and their spins.
21. Which of the following is the correct electron configuration for oxygen?
(A) 1s² 2s² 2p⁴
(B) 1s² 2p⁴ 2s²
(C) 2s² 1s² 2p⁶
(D) 1s² 2s² 2p⁶
Answer
(A) — Oxygen (Z=8) has 8 electrons: 1s² 2s² 2p⁴.
22. How many valence electrons does fluorine have?
(A) 1
(B) 5
(C) 7
(D) 8
Answer
(C) — Fluorine has 7 valence electrons (2s² 2p⁵).
23. Which of these would be classified as an excited state configuration for carbon (Z = 6)?
(A) 1s² 2s² 2p²
(B) 1s² 2s¹ 2p³
(C) 1s² 2s² 2p¹
(D) 1s² 2s¹ 2p²
Answer
(B) — 2s¹ 2p³ indicates an electron has jumped from 2s to 2p: an excited state.
24. What is the quantum number ℓ for a 4f orbital?
(A) 0
(B) 1
(C) 2
(D) 3
Answer
(D) — f orbitals correspond to ℓ = 3.
25. Which orbital holds electrons closest to the nucleus?
(A) 2s
(B) 3s
(C) 1s
(D) 3d
Answer
(C) — The 1s orbital is closest to the nucleus and has the lowest energy.
26. What is the total number of electrons that can occupy all the orbitals in the n = 4 energy level?
(A) 16
(B) 18
(C) 32
(D) 36
Answer
(C) — 2n² = 2(4²) = 32 electrons max at n = 4.
27. Which element has the ground state electron configuration ending in 4s² 3d¹⁰?
(A) Calcium
(B) Zinc
(C) Gallium
(D) Chromium
Answer
(B) — Zinc has the configuration [Ar] 4s² 3d¹⁰.
28. What is the maximum number of electrons that can have ℓ = 2 in the n = 4 energy level?
(A) 6
(B) 10
(C) 14
(D) 18
Answer
(B) — ℓ = 2 means d orbitals → 5 orbitals × 2 electrons = 10.
29. What distinguishes an f-block element in terms of electron configuration?
(A) The last electron enters a p orbital
(B) The last electron enters a d orbital
(C) The last electron enters an f orbital
(D) The last electron enters a 3s orbital
Answer
(C) — f-block elements have their last electrons added to f orbitals.
30. Which of the following transitions would result in the emission of a photon with the most energy?
(A) n = 3 → n = 2
(B) n = 5 → n = 4
(C) n = 4 → n = 3
(D) n = 5 → n = 1
Answer
(D) — Greater difference in energy levels = more energy released → n = 5 → n = 1 transition emits most energy.
31. Which value of ℓ corresponds to the sublevel that holds a maximum of 14 electrons?
(A) 1
(B) 2
(C) 3
(D) 4
Answer
(C) — ℓ = 3 refers to f orbitals, which hold 14 electrons max.
32. Which ion has the same electron configuration as neon?
(A) Mg⁺
(B) Al³⁺
(C) N³⁻
(D) S²⁻
Answer
(C) — N³⁻ has 7 + 3 = 10 electrons = same as Ne (1s² 2s² 2p⁶).
33. In the context of PES (photoelectron spectroscopy), a peak further from the origin represents:
(A) Higher binding energy
(B) Lower binding energy
(C) More valence electrons
(D) Smaller atomic radius
Answer
(A) — In PES, peaks farther from the origin correspond to higher binding energy (core electrons).
34. What does the number of peaks in a PES spectrum correspond to?
(A) Number of protons
(B) Number of orbitals with distinct energy levels
(C) Number of neutrons
(D) Number of isotopes
Answer
(B) — Each peak in PES corresponds to electrons from a specific orbital energy level.
35. Which quantum number determines the number of orientations an orbital can have?
(A) n
(B) ℓ
(C) mℓ
(D) ms
Answer
(C) — mℓ determines the number of orientations (values: –ℓ to +ℓ).
36. Explain why the 2s orbital penetrates closer to the nucleus than the 2p orbital.
Answer
2s orbitals have a greater probability of being found near the nucleus due to their spherical shape and lack of angular nodes compared to 2p orbitals.
37. How many total orbitals are there in the third principal energy level?
Answer
There are 9 orbitals: 1 (3s) + 3 (3p) + 5 (3d) = 9.
38. Why do atoms with filled sublevels tend to be chemically unreactive?
Answer
Filled sublevels, such as in noble gases, result in electron stability and minimal tendency to gain or lose electrons, leading to low reactivity.
39. What is the difference between binding energy and ionization energy?
Answer
Binding energy refers to how tightly an electron is held within an atom (as shown in PES), while ionization energy is the energy required to remove a valence electron.
40. Describe one major limitation of Rutherford’s model of the atom.
Answer
It couldn’t explain why electrons didn’t spiral into the nucleus or the stability of atoms.
41. Why does atomic radius decrease across a period?
Answer
More protons are added, increasing nuclear attraction and pulling the electron cloud closer, reducing size.
42. Define electron shielding and describe its effect on outer electrons.
Answer
Electron shielding occurs when inner electrons reduce the attraction between the nucleus and outer electrons, making them easier to remove.
43. How many electrons can be described with n = 3, ℓ = 2?
Answer
10 electrons — 5 d orbitals (ℓ = 2) × 2 electrons each.
44. Explain how the quantum mechanical model differs from the Bohr model.
Answer
The Bohr model uses fixed orbits; the quantum model uses orbitals based on probabilities and does not specify exact electron paths.
45. What is the electron configuration of Fe³⁺?
Answer
[Ar] 3d⁵ — Iron loses two 4s and one 3d electron.
46. What is the significance of the Heisenberg uncertainty principle in atomic theory?
Answer
It shows that we cannot know both the exact position and momentum of an electron simultaneously, reinforcing the probabilistic nature of orbitals.
47. Why do atoms emit photons at specific wavelengths?
Answer
Electron transitions between discrete energy levels produce photons of specific energy (E = hν), corresponding to specific wavelengths.
48. Describe the relationship between energy level and wavelength in electron transitions.
Answer
Larger energy level changes produce photons with shorter wavelengths and higher frequencies (E = hc/λ).
49. What type of orbital has three orientations in space?
Answer
p orbitals — mℓ = –1, 0, +1 (3 orientations).
50. Give the quantum numbers for the last electron in the sulfur atom (Z = 16).
Answer
n = 3, ℓ = 1, mℓ = 0, ms = –½ — the 3p⁴ electron fills with opposite spin in the middle p orbital.