Light and Atomic Spectra: Quiz

Light and Atomic Spectra: Quiz

7 questions

1. Question 1EasyAs the wavelength of light decreases, what happens to its frequency and photon energy?

   ABoth decreaseBBoth increaseCFrequency increases, energy decreasesDBoth stay the same
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   Answer: Both increase

   c = λν, so a shorter wavelength means a higher frequency; E = hν, so the energy rises too.

2. Question 2EasyWhich type of radiation has the highest energy per photon?

   ARadio wavesBInfraredCVisible lightDUltraviolet
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   Answer: Ultraviolet

   Of these, ultraviolet has the shortest wavelength and therefore the highest photon energy, which is why it can damage skin.

3. Question 3MediumWhat is the wavelength of an FM radio signal of frequency 98.1 MHz?

   A3.06 mB0.327 mC2.94 × 10¹⁶ mD3.06 × 10⁻⁶ m
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   Answer: 3.06 m

   λ = c/ν = (2.998 × 10⁸ m/s) ÷ (98.1 × 10⁶ s⁻¹) = 3.06 m. Remember that 1 MHz = 10⁶ Hz.

4. Question 4MediumWhat is the energy of one photon of blue light with a wavelength of 450 nm?

   A4.41 × 10⁻¹⁹ JB4.41 × 10⁻²⁸ JC2.98 × 10⁻²⁵ JD266 J
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   Answer: 4.41 × 10⁻¹⁹ J

   E = hc/λ = (6.626 × 10⁻³⁴ J s × 2.998 × 10⁸ m/s) ÷ (450 × 10⁻⁹ m) = 4.41 × 10⁻¹⁹ J. 4.41 × 10⁻²⁸ J results from leaving λ in nm.

5. Question 5MediumWhy does each element have its own line spectrum?

   AIts atoms have different massesBIts electrons can only have certain energies, which differ from element to elementCIt reflects light of one colourDIts nucleus emits light
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   Answer: Its electrons can only have certain energies, which differ from element to element

   Each line is a photon emitted when an electron drops between two allowed energy levels. The levels are different in every element, so the pattern of lines is unique.

6. Question 6HardIn the Bohr model, which electron drop in hydrogen produces the line at 486 nm?

   An = 2 to n = 1Bn = 3 to n = 2Cn = 4 to n = 2Dn = 4 to n = 3
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   Answer: n = 4 to n = 2

   ΔE = 2.179 × 10⁻¹⁸ J × (1/2² − 1/4²) = 4.086 × 10⁻¹⁹ J, and λ = hc/ΔE = 486 nm. The drop from 3 to 2 gives 656 nm.

7. Question 7HardHow much energy is needed to ionize one mole of hydrogen atoms from the ground state?

   A2.179 × 10⁻¹⁸ kJ/molB328 kJ/molC1312 kJ/molD1.312 × 10⁹ kJ/mol
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   Answer: 1312 kJ/mol

   2.179 × 10⁻¹⁸ J per atom × 6.022 × 10²³ mol⁻¹ = 1.312 × 10⁶ J/mol = 1312 kJ/mol. 328 kJ/mol would be from n = 2.