AP Chemistry: Atomic Structure & Periodicity 2 — Questions and Answers
Question 1: A sample of an element contains isotopes of mass 10.0 amu (20%) and 11.0 amu (80%). What is the average atomic mass?
- 10.2 amu
- 10.5 amu
- 10.8 amu (Correct answer)
- 11.0 amu
Correct answer: 10.8 amu
Weighted average = (0.20)(10.0) + (0.80)(11.0) = 10.8 amu.
Question 2: Which set of quantum numbers (n, l, ml, ms) is NOT allowed?
- 2, 1, 0, +1/2
- 3, 2, -2, -1/2
- 2, 2, 1, +1/2 (Correct answer)
- 1, 0, 0, -1/2
Correct answer: 2, 2, 1, +1/2
For n=2, l can only be 0 or 1, so l=2 is forbidden.
Question 3: How many electrons in a ground-state phosphorus atom (Z=15) are unpaired?
- 1
- 2
- 3 (Correct answer)
- 5
Correct answer: 3
The 3p^3 configuration places one electron in each of three p orbitals by Hund's rule, giving 3 unpaired electrons.
Question 4: Which species is isoelectronic with the neon atom?
- Na+ (Correct answer)
- Cl-
- K+
- Li+
Correct answer: Na+
Na+ has 10 electrons, the same as neon.
Question 5: What is the maximum number of electrons that can occupy the n=3 principal energy level?
- 8
- 9
- 18 (Correct answer)
- 32
Correct answer: 18
The capacity is 2n^2 = 2(3)^2 = 18 electrons.
Question 6: An electron transition releasing the highest-energy photon in a hydrogen atom would be from n=4 to which level?
- n=3
- n=2
- n=1 (Correct answer)
- n=5
Correct answer: n=1
A drop to n=1 spans the largest energy gap, so it emits the highest-energy photon.
Question 7: Which subshell is filled immediately after the 4s subshell according to the aufbau principle?
- 4p
- 3d (Correct answer)
- 5s
- 4d
Correct answer: 3d
After 4s, the 3d subshell fills next due to its lower energy.
AP Chemistry: Atomic Structure & Periodicity 2
A sample of an element contains isotopes of mass 10.0 amu (20%) and 11.0 amu (80%).
What is the average atomic mass?