19 exam-style questions on Physics · Atomic spectra, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Ten Physics · Atomic spectra questions with answers
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. In the hydrogen atom, as the quantum number n increases, the allowed energy levels
A
stay equally spaced
B
get farther apart
C
become more negative
D
come closer together
Show answer
Answer: D. En = −13.6/n2 eV: the gaps are 10.2 eV (n = 1 to 2), 1.89 eV (2 to 3), 0.66 eV (3 to 4) and so on, crowding together as E approaches 0. The levels rise towards zero, so they become less negative.
2. The line of longest wavelength in the Balmer series of hydrogen is produced by the transition
A
n = ∞ → n = 2
B
n = 3 → n = 1
C
n = 3 → n = 2
D
n = 4 → n = 2
Show answer
Answer: C. The longest wavelength means the smallest energy drop to n = 2, which is from the next level up, n = 3. The drop from n = ∞ gives the shortest Balmer wavelength (the series limit), and 3 → 1 belongs to the Lyman series.
3. The continuous X-ray spectrum (bremsstrahlung) from an X-ray tube is produced by
A
electrons dropping into vacancies in the K-shell of target atoms
B
the deceleration of fast electrons by the target nuclei
C
the radioactive decay of nuclei in the target material
D
the heating of the filament to a very high temperature
Show answer
Answer: B. Bremsstrahlung means braking radiation: a fast electron slowed in the field of a nucleus loses energy as an X-ray photon, and since it can lose any amount up to eV, the spectrum is continuous. Electrons dropping into K-shell vacancies give the characteristic lines.
4. In an X-ray tube, the filament current is increased while the accelerating voltage is kept the same. This increases
A
the maximum photon energy of the X-rays, but not their intensity
B
both the intensity and the maximum photon energy of the X-rays
C
the wavelengths of the characteristic lines of the X-rays
D
the intensity of the X-rays, but not their maximum photon energy
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Answer: D. A hotter filament emits more electrons, so more X-ray photons are produced each second (higher intensity). The energy of each electron, and so the largest photon energy eV, depends only on the accelerating voltage.
5. Hydrogen atoms in the ground state absorb photons of energy 12.09 eV (En = −13.6/n2 eV). As they return to the ground state, the number of different spectral lines they can emit is
A
3
B
2
C
1
D
6
Show answer
Answer: A. E3 − E1 = −1.51 − (−13.6) = 12.09 eV, so the atoms are raised to n = 3. They can return by 3 → 1, 3 → 2 and 2 → 1, giving three different lines. 6 is the number for atoms raised to n = 4.
6. Characteristic X-ray lines, such as the Kα line, are produced when
A
fast electrons are slowed down suddenly near the nuclei of the target atoms
B
an outer electron drops into a vacancy in an inner shell of a target atom
C
the nuclei of the target atoms give out γ-rays after being struck
D
an inner electron of a target atom is raised to an empty outer shell
Show answer
Answer: B. A fast electron knocks an inner (for example K-shell) electron out of a target atom; an electron from a higher shell drops into the vacancy and emits a photon equal to the energy difference, so the lines are fixed by the target element. The slowing down of electrons near nuclei gives the continuous spectrum.
7. The accelerating voltage of an X-ray tube is doubled. As a result,
A
the cut-off wavelength doubles, but the characteristic lines do not shift
B
the cut-off wavelength halves, and the characteristic lines shift too
C
the cut-off wavelength is unchanged, but the characteristic lines shift
D
the cut-off wavelength halves, but the characteristic lines do not shift
Show answer
Answer: D. The fastest electron gives all its energy eV to a single photon, so λmin = hc/eV halves when V doubles. The characteristic lines come from transitions between inner energy levels of the target atoms, so their wavelengths depend only on the target material.
8. The series of hydrogen lines produced by transitions that end on the n = 3 level is the
A
Balmer series
B
Lyman series
C
Paschen series
D
Brackett series
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Answer: C. Lyman lines end on n = 1 (ultraviolet), Balmer lines on n = 2 (mostly visible), Paschen lines on n = 3 and Brackett lines on n = 4 (both infrared).
9. In the de Broglie picture of the Bohr atom, the circumference 2πr of the n-th orbit, in terms of the de Broglie wavelength λ of the electron in that orbit, is
A
nλ
B
n2λ
C
nλ/2
D
2πnλ
Show answer
Answer: A. Putting λ = h/mv into Bohr's condition mvr = nh/2π gives 2πr = nλ: a whole number of electron waves fits round the orbit, forming a stationary wave.
10. According to Bohr's model, the angular momentum of the electron in the n-th allowed orbit of hydrogen is
A
nh
B
h/2πn
C
nh/2π
D
2πnh
Show answer
Answer: C. Bohr's quantum condition: an electron can revolve only in orbits for which mvr = nh/2π, where n = 1, 2, 3, ...; in these orbits it does not radiate energy.
19, each with the answer and a short explanation, across 2 topics: Bohr model and spectra, X-rays and lasers. Every chapter test draws ten of them, so you can take it many times.
Is the MDCAT Physics chapter 15 test free?
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How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
