Waves 1.5

Question 1

Bohr’s Model of the Atom (Definition)

Answer 1

Suggested that electrons associated with an atom were not found in all possible orbits but are only found in certain fixed discrete orbitals of the atom

Model based on observed photons associated with spectral analysis of hydrogen

Matches experimental evidence perfectly for hydrogen and other single ions but fails when applied to more complex atoms

Question 2

Bohr’s 3 Assumptions

Answer 2

1. Electrons within atom orbit nucleus in fixed value energy levels
   - Energy levels different from each atom
   - Orbiting electrons do not radiate
2. Electrons in one energy level can “jump” to another by absorption or emission of a photon
   - Photon energy must exactly equal the difference between the two energy levels

E = h. f = (E2 - E1)

3. The angular momentum (L) of an electron in any energy level is a fixed amount

L = m. v. r = nh/2∏

• Where n is the energy level number (principal quantum number)

Question 3

Bohr’s Model tof the Atom

Answer 3

• Orbital in which electron is found determined by its energy
• Electron normally found in lowest possible orbit (ground state electron)
• Ground state electron receive energy by absorbing photon or colliding with high speed electron
• When atom absorbs energy, electron elevated to higher level and atom is in excited state
• Electron in excited atom returns to ground state by moving between energy levels and emits single photon of radiation equal in energy to difference between levels
• Each electron transition from higher to lower energy level results in emission of photon and specific spectral line
• Above zero level is continuous region which corresponds to free electrons with kinetic energy

Question 4

Ionisation Energy

Answer 4

The amount of energy required to completely remove an electron from an atom

Question 5

Electrons transition to higher energy levels by…

Answer 5

Absorption of energy from…

• An incident photon
• A collision with a fast moving electron
• Thermal excitation

Question 6

Photon/Atom Interactions (i.e. Incident Photon)

Answer 6

Energy of absorbed photon must be equal to energy difference between two energy levels in the atom

All photon energy must be absorbed in single electron transition and only from ground state

Question 7

Collision with fast moving electron

Answer 7

Atom may absorb parts of the energy necessary to raise an electron between energy levels if struck by a fast moving particle
- Colliding particle loses that amount of its energy

If energy of photon or fast moving electron is greater than ionisation energy then atom will be ionised and extra energy will be in the form of kinetic energy associated with the ejected electron

Question 8

Thermal excitation

Answer 8

Significant number of atoms may be in excited state high temperature due to thermal excitation

Question 9

Spectral Analysis

Answer 9

Techniques develop to cause gas to emit or absorb certain frequencies of light
- Emitted light passed through either prism or diffraction grating to produce series of lines in unique patterns that could be used to identify the gas’ chemical composition

Question 10

Emission Spectrum

Answer 10

Produced if gas is energised in some way to produce the light

Resulting spectrum consists of series of bright coloured lines called spectral lines

Question 11

Absorption Spectrum

Answer 11

Created when a beam of white light is passed through a cool sample of gas

Resulting spectrum usually consists of a continuous spectrum of colours with black lines indicating the absorbed frequencies

Question 12

Classifications of Spectra

Answer 12

Spectra can be classified as…

• Continuous
• Line
• Band

Question 13

Line Emission Spectra

Answer 13

A series of coloured lines separated by black spaces are observed

Question 14

Line Absorption Spectra

Answer 14

Absorption spectrum results when light passes through a vapour of a substance located between the source and the observer

Light of specific frequencies are absorbed resulting in appearance of dark lines in spectrum

Dark lines correspond to frequencies of light absorbed by gaseous atoms when electrons make transitions from ground state to excited energy levels

Question 15

Fluorescence (Definition)

Answer 15

Occurs when an electron is raised to excited states by absorbing a photon or by electron collision

Question 16

Fluorescence (Ultraviolet and X-ray)

Answer 16

Excited electron returns to ground state by series of downward transitions which result in emission of light (photons) in the visible region of EM spectrum