9702 · 22.4
Energy levels in atoms and line spectra flashcards
Revision flashcards for Cambridge 9702 Energy levels in atoms and line spectra (syllabus 22.4). Flip, recall, then mark a real past-paper question.
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What does 'quantisation of energy levels' mean?
Electrons in an atom can only occupy specific, discrete energy values, not any value in between.
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Define the 'ground state' of an electron.
The lowest and most stable energy level an electron can occupy within an atom.
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How does an electron become 'excited'?
It absorbs a photon (or gains energy from a collision) whose energy is sufficient to move it to a higher, unoccupied energy level.
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What happens when an excited electron 'de-excites'?
It transitions to a lower, more stable energy level, emitting a photon with energy precisely equal to the energy difference between the two levels.
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State the formula for photon energy during an electron transition.
$E_{photon} = hf = E_{initial} - E_{final}$ (where $E_{initial} > E_{final}$ for emission).
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What is Planck's constant ($h$)?
A fundamental constant linking the energy of a photon to its frequency: $h = 6.63 \times 10^{-34} \text{ Js}$.
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What is an electronvolt (eV) and its conversion to Joules?
An electronvolt is the kinetic energy gained by an electron accelerated through a potential difference of 1 volt. $1 \text{ eV} = 1.60 \times 10^{-19} \text{ J}$.
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Describe an emission spectrum.
A spectrum showing discrete bright lines against a dark background, formed by photons emitted during electron de-excitation in excited atoms.
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Describe an absorption spectrum.
A spectrum showing dark lines at specific wavelengths against a continuous background, formed when atoms absorb photons, causing electrons to excite.
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Why is a line spectrum considered an atomic 'fingerprint'?
Each element has a unique set of discrete energy levels, leading to a distinct pattern of photon energies that can be absorbed or emitted, identifying the element.
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Why are energy levels in an atom represented by negative values?
They represent the energy required to remove an electron from the atom (ionisation). The zero energy level corresponds to a free electron (at infinity), so bound electrons have negative potential energy relative to this state.
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What is a de-excitation cascade?
When an excited electron returns to a lower state via a series of intermediate energy levels, emitting multiple photons of different energies, rather than in a single jump.
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What are the two main mechanisms for atomic excitation?
1. Photon absorption, which requires a photon with energy exactly matching an energy level difference. 2. Collisional excitation, where a particle transfers at least the required excitation energy to an atomic electron.
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What is the relationship between the speed of light (c), frequency (f), and wavelength (λ)?
The wave equation: $c = fλ$. This is used to convert between a photon's frequency and wavelength.
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How is a continuous spectrum produced?
By a hot, dense object (solid, liquid, or high-pressure gas) that emits radiation at all wavelengths due to strong interactions between atoms.
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What is the key difference in the conditions required for photon absorption vs. collisional excitation?
Photon absorption requires the photon's energy to be *exactly* equal to the energy gap. Collisional excitation only requires the colliding particle's kinetic energy to be *at least* the energy gap.