Atomic line spectra and energy levelsEdexcel International A Level Physics: Subtopic test

1. 1
   A gas discharge tube containing hydrogen at low pressure emits light when a high voltage is applied across it. When the light is viewed through a diffraction grating, a small number of separate coloured lines is seen against a dark background.

   (a)

   What causes an atom to emit a photon?

   [1 mark]

   • AAn electron moving to a higher energy level
   • BAn electron leaving the atom completely at rest
   • CAn electron moving to a lower energy level
   • DThe nucleus absorbing energy

   (b)

   Why does the light from the tube form separate lines rather than a continuous range of colours?

   [1 mark]

   • AThe atoms have discrete energy levels, so only certain photon energies are possible
   • BAll the atoms in the gas vibrate at the same single frequency
   • CThe electrons in the gas can have any energy but only a few collide
   • DThe diffraction grating only transmits certain wavelengths

   (c)

   Explain why dark lines appear in an absorption spectrum at the same frequencies as the bright lines in the emission spectrum of the same gas.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The lowest three energy levels of a hydrogen atom are n = 1 at −13.6 eV, n = 2 at −3.40 eV and n = 3 at −1.51 eV. Use the Planck constant h = 6.63 × 10⁻³⁴ J s and 1 eV = 1.60 × 10⁻¹⁹ J.

   (a)

   What is the energy of the photon emitted when an electron moves from n = 3 to n = 2?

   [1 mark]

   • A1.51 eV
   • B3.40 eV
   • C4.91 eV
   • D1.89 eV

   (b)

   Which transition emits the photon of highest frequency?

   [1 mark]

   • An = 2 to n = 1
   • Bn = 3 to n = 1
   • Cn = 3 to n = 2
   • Dn = 1 to n = 2

   (c)

   Calculate the frequency of the photon emitted when an electron moves from n = 2 to n = 1.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An atom of an element has energy levels of −5.4 eV (the ground state), −3.1 eV and −1.5 eV. A sample of the element is excited so that some of its atoms are in the −1.5 eV level. Use the Planck constant h = 6.63 × 10⁻³⁴ J s and 1 eV = 1.60 × 10⁻¹⁹ J.

   (a)

   Calculate the frequency of the photon emitted when an atom moves from the −1.5 eV level to the ground state.

   [3 marks]

   (b)

   Atoms in the −1.5 eV level can return to the ground state in more than one way. Deduce how many different photon frequencies may be emitted and calculate the lowest of them.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hydrogen atoms have energy levels n = 1 at −13.6 eV, n = 2 at −3.40 eV and n = 3 at −1.51 eV. Light from a hydrogen discharge tube is analysed with a spectrometer. Use the Planck constant h = 6.63 × 10⁻³⁴ J s, the speed of light c = 3.00 × 10⁸ m s⁻¹ and 1 eV = 1.60 × 10⁻¹⁹ J.

   (a)

   Explain how an emission line spectrum and an absorption line spectrum of an element arise.

   [6 marks]

   (b)

   Hydrogen atoms in the n = 3 level can emit photons as they return to the ground state. Calculate the wavelengths of all the photons that can be emitted and deduce which of them lie in the visible spectrum, 400 nm to 700 nm.

   [6 marks]

   Total for question 4: 12 marks

End of questions