Photons & Atomic Transitions
The Photon Model
- Photons are fundamental particles that make up all forms of electromagnetic radiation
- A photon is defined as
A massless “packet” or a “quantum” of electromagnetic energy
- This means that the energy transferred by a photon is not continuous but as discrete packets of energy
- In other words, each photon carries a specific amount of energy and transfers this energy all in one go
- This is in contrast to waves which transfer energy continuously
Atomic Energy Levels
- Electrons in an atom can have only certain specific energies
- These energies are called electron energy levels
- They can be represented as a series of stacked horizontal lines increasing in energy
- Normally, electrons occupy the lowest energy level available, this is known as the ground state
- Electrons can gain energy and move up the energy levels if they absorb energy via:
- Collisions with other atoms or electrons
- Absorbing a photon
- A physical source, such as heat
- This is known as excitation, and when electrons move up an energy level, they are said to be in an excited state
- If the electron gains enough energy to be removed from the atom entirely, this is known as ionisation
- When an electron returns to a lower energy state from a higher excited state, it releases energy in the form of a photon
Electron energy levels in atomic hydrogen. Photons are emitted when an electron moves from a higher energy state to a lower energy state
Worked example
Explain how atomic spectra provide evidence for the quantisation of energy in atoms.
Answer:
Step 1: Outline the meaning of atomic spectra
- Atomic spectra show the spectrum of discrete wavelengths emitted or absorbed by a specific atom
Step 2: Describe the relationship between energy and wavelength
- Photon energy is related to frequency and wavelength
- Therefore, photons with discrete wavelengths have discrete energies equal to the difference between two energy levels
Step 3: Explain how atomic spectra give evidence for the quantisation of energy
- Photons arise from electron transitions between energy levels
- This happens when an electron is excited, or de-excited, from one energy level to another, by either emitting or absorbing light of a specific wavelength
- Since atomic spectra are made up of discrete wavelengths, this shows that atoms must contain discrete, or quantised, energy levels
Worked example
The diagram shows the electron energy levels in an atom of hydrogen.
Determine the number of possible wavelengths that can be produced from transitions between the n = 4 excited state and the n = 1 ground state.
Answer:
- There are six possible wavelengths that could be produced from the different energy level transitions
- The possible transitions are:
- n = 4 to n = 3
- n = 4 to n = 2
- n = 4 to n = 1
- n = 3 to n = 2
- n = 3 to n = 1
- n = 2 to n = 1
Exam Tip
Make sure you learn the definition for a photon: discrete quantity / packet / quantum of electromagnetic energy are all acceptable definitions