Chemistry Practice Test 2: Atomic Structure
- Atomic Structure
- 10 questions
- 13 minutes
This second atomic structure test covers protons, neutrons and electrons in atoms and ions, isotopes, early atomic models and electron energy levels. Tip: when an atom becomes an ion, only the number of electrons changes.
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Question 1
An atom has 26 protons and 30 neutrons. What is its mass number?
Correct answer: B
The mass number is protons plus neutrons: 26 + 30 = 56. The value 26 is the atomic number, which counts only the protons.
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Question 2
Which subatomic particle has a negative charge and a mass far smaller than that of a proton?
Correct answer: D
An electron carries a 1− charge and has only about 1/1836 of the mass of a proton. A neutron has almost the same mass as a proton and no charge.
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Question 3
An ion has 16 protons, 16 neutrons and 18 electrons. Which description fits this ion?
Correct answer: A
16 protons make it sulfur, the mass number is 16 + 16 = 32, and two extra electrons give a 2− charge. Adding the electrons to the mass number (16 + 18 = 34) is a common mistake, since electrons are not counted in the mass number.
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Question 4
J. J. Thomson studied cathode rays and found that they were bent by electric and magnetic fields. What did his experiments show?
Correct answer: C
Thomson showed that cathode rays are streams of negative particles found in every atom, which we now call electrons. The dense nucleus came later from Rutherford's gold foil experiment.
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Question 5
Which pair of atoms are isotopes of the same element?
Correct answer: C
Isotopes have the same number of protons but different numbers of neutrons, like chlorine-35 and chlorine-37. Carbon-14 and nitrogen-14 share a mass number but have different numbers of protons, so they are different elements.
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Question 6
Boron has two natural isotopes: 19.9% boron-10 (mass 10.01 amu) and 80.1% boron-11 (mass 11.01 amu). What is the average atomic mass of boron?
Correct answer: A
Weight each mass by its abundance: 0.199 × 10.01 + 0.801 × 11.01 = 10.81 amu. The value 10.51 amu is a simple average that ignores how common each isotope is.
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Question 7
Potassium has atomic number 19. What is the electron configuration of the K+ ion?
Correct answer: B
K+ has 18 electrons because the atom loses its single 4s electron, so its configuration matches argon. The configuration ending in 4s1 is the neutral potassium atom.
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Question 8
What is the maximum number of electrons that the third energy level (n = 3) can hold?
Correct answer: D
An energy level can hold 2n2 electrons, so n = 3 holds 2 × 9 = 18 (in the 3s, 3p and 3d sublevels). The answer 8 confuses this with the octet of valence electrons.
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Question 9
Which electron configuration belongs to an atom in an excited state?
Correct answer: C
In 1s2 2s2 2p5 3s1 an electron sits in 3s while the lower 2p sublevel is not yet full, so the atom (neon, 10 electrons) is excited. The other three follow the normal filling order and are ground states.
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Question 10
In a hydrogen atom, an electron drops from the n = 3 energy level to the n = 2 energy level. What happens?
Correct answer: A
Moving to a lower energy level releases the energy difference as one photon of a fixed energy, which appears as a single line in the emission spectrum. Absorbing a photon would move the electron up, not down.
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