(c) emission spectral lines from hot gases in terms of emission of photons…

The idea

A hot gas contains atoms whose electrons occupy discrete energy levels. When an electron drops from a higher level to a lower one, the atom loses energy and emits a single photon carrying that exact energy difference. Because only certain level gaps exist, only certain photon energies, and therefore certain frequencies and wavelengths, are emitted. A spectrometer spreads this light into a series of bright lines on a dark background, called an emission line spectrum. For example, excited hydrogen emits visible lines such as the red line near 656 nm and the blue-green line near 486 nm. Each line corresponds to one specific electron transition, so the pattern acts as a fingerprint of the element.

What earns the marks

• States that electrons occupy discrete, quantised energy levels in an atom.
• Explains that an electron transitions from a higher to a lower energy level, losing energy.
• Links the energy loss to the emission of a single photon of energy equal to the level difference.
• Uses ΔE = hf to relate the photon energy to its frequency, and hence to a line wavelength.
• Describes the result as bright lines on a dark background in an emission spectrum.
• Recognises that the set of possible transitions gives a unique line pattern for each element.

Common mistakes and how to put them right

• Saying electrons emit photons while moving between levels continuously.

  Put it right: Emission occurs at the instant of transition, and the photon energy equals the fixed level difference.

• Confusing emission with absorption.

  Put it right: An emission line appears bright on a dark background, while an absorption line is dark on a continuous spectrum.

• Claiming any photon energy can be emitted.

  Put it right: Only energies matching gaps between discrete levels are allowed, so the spectrum is a set of lines, not a continuous band.

• Forgetting that one transition produces one photon, so line brightness depends on how many atoms make that transition, not on the photon energy alone.

Exam tips

• Always name the direction of the transition: higher to lower level for emission, lower to higher for absorption.
• Quote the relationship ΔE = hf when converting a level difference into a frequency or wavelength.
• Describe the spectrum appearance explicitly, for example bright lines on a dark background, to secure the observation mark.