3.2.2.3 Energy levels and photon emission

• What must be done to move an electron away from the nuleus

  Work
• Why are electrons attracted to the nucleus

  Electrons are negative and get attracted to the positive nucleus
• What type of energy does an electron have

  Kinetic and electrostatic
• What can electrons do in an atom

  Gain and lose energy
• Why can an electrons only absorb a specific amount of energy

  Energies for electrons in an atom are not continuous
• Define ground state

  Electrons are in the lowest energy state
• When does electrons in an atom become excited

  When they have absorbed energy and moved to higher states
• When does excitation occur

  When electrons absorb exactly the right amount of energy to move to higher energy level
• How can excitation occur? (1)

  Absorbing a photon with exactly the right amount of energy to move between the 2 levels
• How can excitation occur? (2)

  Absorbing exactly the right amount of energy to move between 2 levels AFTER colliding with a free electron that has energy equal to or greater than energy required
• Energy gained by the electron in the atom equals

  The energy lost be the colliding electron
• The free electrons kinetic energy after collision is equal

  To its kinetic energy before the collision minus the energy transferred to the excited electrons on the atom
• How does an atom become ionised

  If an electrons in an atom absorbs enough energy to escape the atom completely
• When does ionisation occur

  When an atom gains or loses an electron and becomes a charged particle called an ion
• What is the ionisation energy

  Minimum energy needed to remove the electron completely from the atom
• How do electrons gain energy

  Absorption of a photon, excitation, collision
• How do electrons lose energy

  Emission of photons
• Why aren't electrons quanta

  They are discrete and has kinetic energy
• Name the 4 things involved in the process

  Cathode, anode, mercury vapour, phosphor coating
• What is the tube filled with?

  Mercury vapour
• What happens once the light is turned on

  Cathode is heated causing thermionic emission
• How does thermionic emission work?

  Free electrons are emitted from a heated filament
• How does excitation and ionisation occur

  If electrons collide with the mercury atoms, energy is transferred, with kinetic energy
• How are photons of UV released

  When electrons in the excited mercury atoms return to their ground state
• What do these photons then do ?

  Strike the phosphors in the coating, energy is re-emitted as visible light
• Purpose of electrode

  Provides potential difference, which accelerates electrons emitted by the cathode
• Purpose of phosphor

  Absorbs UV photons and re-emits as visible photons
• Purpose of mercury vapour

  Releases ultra violet photons
• Why a person viewing a fluorescent bulb through a diffraction grating observes specific peaks in the spectrum

  It only emits certain frequencies of light atoms in the phosphor coating, discrete energy levels, photons emitted are produced so electrons move between energy levels so they can only have certain energy values
• Difference in fluorescent tube and an incandescent

  Continuous and discrete
• How is line spectra produced?

  When a photon or electron de-excites and release energy in the form of visible light
• What do line spectra lead to?

  Discrete energy levels as particular energies give out a particular wavelength
• How does a fluorescent tube work?

  Each element produces a range of photons with fixed values of wavelength and frequency
• Purpose of the white coating?


  It absorbs high-energy photons emitted by the vapour and gives out visible light
• Purpose of mercury atoms?

  If collides with electrons, energy may be transferred causing ionisation or excitation
• What happens when de-excitation happens?

  Photons are released similar to UV light