5.11.1 Electron Energy Levels

• A continuous spectrum is a spectrum that appears to contain all frequencies over a comparatively wide range
  • The Sun’s spectrum is not continuous as it shows dark lines where some frequencies are missing
• Bohr explained the existence of spectral lines using the new photon model of electromagnetic radiation
• In addition, Bohr proposed a model of the atom which includes:
  • Electrons orbiting the nucleus
  • Electrons only occupying specific orbits associated with a discrete set of energies
  • The orbits are also called shells or energy levels
  • Electrons can gain or lose energy by moving from one allowed energy level to another

Electrons in an atom can have only certain specific (discrete) energies within an atom

• Energy is required for an electron to move from a lower to a higher energy level
  • This transition is called an excitation
• Energy is released if the electron moves from a higher to a lower energy level
  • This transition is called a de–excitation

• Electrons can move to a higher or lower energy level by absorbing or emitting electromagnetic radiation with a frequency, f

• The energy required for excitation of a gas can be provided by:
  • A photon of a specific frequency
  • Energy absorbed from the surroundings (through heating)
  • Energy supplied by an electric field

• The energy released during de–excitation is emitted as electromagnetic radiation of a specific frequency
  • This frequency depends on the difference of energy between the specific energy levels involved in the transition

• The energy of an electron is taken to be zero when it is infinitely far from the nucleus
  • The forces of interaction between the electron and the nucleus are practically zero
  • Energy is regarded to be 0 eV and the electron is said to be free from the atom
• Energy levels have negative values
• Therefore, any energy value for an electron inside the atom will be negative
  • This is because external energy is required to remove an electron from its energy level
• The negative values also mean that electrons are confined within the atom

• The value of the energy level is equal to

The amount of energy required to remove an electron from that energy level

• The energy level with the most negative value is the lowest energy level n = 1
  • This is called the ground state
• The value for the energy level of the ground state is equal to

The energy required to remove an electron from the atom

• For hydrogen, the energy value related to the ground state is equal to –13.6 eV
  • Hence, 13.6 eV is the energy required to remove an electron from the ground state of an atom of hydrogen

• The complete removal of an electron from an atom is known as ionisation
  • If a photon of energy more than 13.6 eV is absorbed, then the excess energy after the electron has been ionised will be transferred to it as kinetic energy