Energy Levels and Line Spectra · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Why atoms glow in single colours

Science · Waves, Light & Sound · ages 17-18
Name ______________________   Date ____________

  1. An electron in hydrogen drops from -0.85 eV to -3.40 eV. Take 1 eV = 1.6e-19 J, h = 6.6e-34 J s and c = 3.0e8 m/s. What wavelength is emitted?

    • 4850 nm
    • 48.5 nm
    • 485 nm
    • 4.85 nm

  2. An electron drops from the -1.5 eV level to the -13.6 eV level. What is the emitted photon's energy?

    • 12.1 eV
    • 15.1 eV
    • 1.5 eV

  3. Light of every wavelength, ultraviolet included, shines through cool hydrogen gas. What appears in the spectrum?

    • Bright lines on a dark background
    • Dark lines where the gas swallowed photons
    • A smooth rainbow with no lines

  4. The levels are negative because the electron is trapped, and 0 eV means it just escapes.

    Circle one:   True   False

  5. A made-up atom has levels at -10.0, -5.0, -3.0 and -2.0 eV. Which downward jump emits the photon with the longest wavelength?

    • -5.0 eV to -10.0 eV
    • -2.0 eV to -3.0 eV
    • -3.0 eV to -5.0 eV
    • -2.0 eV to -10.0 eV

  6. How do the dark absorption lines of a gas compare with its bright emission lines?

    • They sit at the same frequencies
    • They sit halfway between the bright lines
    • They use entirely different colours

  7. An electron falls from the -0.9 eV level to the -3.4 eV level. What is the photon energy in eV?

    Answer: ______________

  8. A hydrogen electron sits in the level at -3.40 eV. How much energy, in eV, must it absorb to just escape the atom?

    Answer: ______________

  9. Tom works out the -0.9 eV to -1.5 eV jump as -1.5 minus -0.9 = -0.6 eV and claims the photon carries negative energy. What is his mistake?

    • He subtracted the wrong way round
    • He picked energy levels from two elements
    • He used the wrong units for light

  10. Starlight shows dark lines at exactly the frequencies hydrogen emits when hot. What does that tell you?

    • Hot hydrogen inside the star makes the lines
    • Cooler outer-layer hydrogen absorbed them
    • Dust blocks every colour by the same amount
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Why atoms glow in single colours W1-mt_RlMCSDxE5q-s1

  1. 485 nm · The gap is 2.55 eV = 4.08e-19 J, so wavelength = 6.6e-34 x 3.0e8 / 4.08e-19 = 4.85e-7 m, which is 485 nm.
  2. 12.1 eV · The photon carries the gap: -1.5 minus -13.6 is 12.1 eV.
  3. Dark lines where the gas swallowed photons · Cool hydrogen absorbs photons with exactly a gap energy from its lowest level, cutting dark lines in the ultraviolet part of the spectrum.
  4. True · Bound electrons sit below zero, and zero marks the borderline of freedom.
  5. -2.0 eV to -3.0 eV · Wavelength is h c / E, so the longest wavelength comes from the smallest gap: -2.0 to -3.0 eV is only 1.0 eV.
  6. They sit at the same frequencies · Climbing a gap needs the same photon that falling down it releases.
  7. 2.5 · Take upper minus lower: -0.9 minus -3.4 gives 2.5 eV.
  8. 3.4 · Zero is the escape point, so the electron must climb from -3.40 eV to 0 eV, a gap of 3.4 eV.
  9. He subtracted the wrong way round · The gap is upper minus lower, a positive 0.6 eV that the photon carries away.
  10. Cooler outer-layer hydrogen absorbed them · Matching dark lines mean cooler hydrogen in the star's outer layers absorbed those colours before the light left the star.
Worksheet · LightMySky