A hydrogen atom undergoes two different de-excitations. Transition X is a large drop from energy level 3 down to level 1. Transition Y is a smaller drop from energy level 2 down to level 1. Which transition emits a photon with the shortest wavelength?
Step 1: Compare the energy changes.
Step 2: Relate energy to frequency.
Step 3: Relate frequency to wavelength.
Students often think X-rays and gamma rays are the exact same thing if they share the same energy. Remember they are classified by their origin — X-rays come from electron transitions, while gamma rays come from the nucleus.
When explaining why an atom emits a specific frequency of light, you must state both that the electron 'moves to a lower energy level' AND that the 'energy difference corresponds to a specific frequency'.
Use the 'ladder' analogy to remember that electrons cannot exist between energy levels; they must absorb or emit the exact amount of energy required to reach a specific 'rung'.
Remember that an electron can return to the ground state in multiple steps. If it takes three small steps instead of one big jump, it will emit three lower-energy photons instead of one high-energy photon.
If asked about the effect of gamma emission on a nucleus, explicitly state that the energy of the nucleus decreases, but there is NO change to its mass or atomic number.
Photons
Discrete packets or "quanta" of electromagnetic energy.
Energy levels
Fixed distances from the nucleus representing specific amounts of energy where an electron can be found.
Excitation
The process where an electron moves to a higher energy level by absorbing electromagnetic radiation.
Ionisation
The process of an atom gaining or losing an electron to become a charged particle.
Ion
An atom or molecule that has gained or lost one or more electrons, giving it a net electrical charge.
Ground state
The lowest possible energy level closest to the nucleus that an electron can occupy.
De-excitation
The process where an electron moves to a lower energy level, releasing its excess energy as a photon of electromagnetic radiation.
Gamma rays
High-frequency, short-wavelength electromagnetic waves emitted specifically from the nucleus of an unstable atom.
Excited state
A state where a nucleus or atom has more energy than its stable ground state.
Nuclear transition
A change in the internal energy state of a nucleus, which results in the emission or absorption of gamma radiation.
Put your knowledge into practice — try past paper questions for Physics A
Photons
Discrete packets or "quanta" of electromagnetic energy.
Energy levels
Fixed distances from the nucleus representing specific amounts of energy where an electron can be found.
Excitation
The process where an electron moves to a higher energy level by absorbing electromagnetic radiation.
Ionisation
The process of an atom gaining or losing an electron to become a charged particle.
Ion
An atom or molecule that has gained or lost one or more electrons, giving it a net electrical charge.
Ground state
The lowest possible energy level closest to the nucleus that an electron can occupy.
De-excitation
The process where an electron moves to a lower energy level, releasing its excess energy as a photon of electromagnetic radiation.
Gamma rays
High-frequency, short-wavelength electromagnetic waves emitted specifically from the nucleus of an unstable atom.
Excited state
A state where a nucleus or atom has more energy than its stable ground state.
Nuclear transition
A change in the internal energy state of a nucleus, which results in the emission or absorption of gamma radiation.