Bohr’s Model of the Atom1913

Scientists noticed that the laws of Classical Physics that applied to large objects did not seem to be able to explain the behaviour of electrons giving rise to:

Planck’s Quantum Theory The new experimental field of “Spectroscopy”

demonstrated that atoms absorb and release bundles of energy.

Bohr used both quantum theory and spectroscopy in formulating his model of the atom.

Bohr’s model

Bohr’s model of the Atom

Atoms and their electrons can only exist in certain specific Energy States.

When moving within these allowed energy states (stationary states) the electron does NOT emit (release) energy.

Bohr’s model of the Atom

For the Hydrogen atom:

En = -R /n2

R = 2.18 x 10-18 J/atomE1 = -R /12 = - 21.8 x 10-19 J/atom

E2 = -R /22 = - 5.44 x 10-19 J/atom

E3 = -R /32 = - 2.42 x 10-19 J/atom

Fixed Energy Levels

n = Principal Quantum number; indicates the orbit.

Energy Levels in a Hydrogen atom

Why is there a negative sign in En?

Bohr defined the point of zero potential energy as the point when the electron was infinitely removed from the atom.

Why is there a negative sign in En?

When an electron moves closer to the nucleus, it feels an increasing attraction and the potential energy of the electron (and atom) decreases

Why is there a negative sign in En?

When a free electron (at infinity) “falls” into E1 (n=1) ,

2.18 x 10-18 J ( or 13.6 ev) is released.

Why is there a negative sign in En?

For the Hydrogen atom:

E1 = -R /1E2 = -R /4 E3 = -R /9 Note that because of the

negative sign, E1 is the lowest in energy

(instead of the highest)

Electron Jumps Between Energy Levels

2. Each Energy level corresponds to an orbit.

3. The electron can travel in an orbit without radiating energy

4. An electron may only change its energy by jumping from one allowed level to another.

Electron Transitions-Excitation

Energy Conversion:

Radiant energy (EM radiation) Potential energy (of electron)

Electron jumps up a level

Electron Transitions

Excitation Relaxation

Electron moves To a higher E level

Energy is Absorbed

Form of Energy Heat, light, electrical

Electromagnetic radiation

Electron Transitions-Relaxation

Energy Conversion:

Potential energy (of electron) Radiant energy (EM radiation)

Electron jumps down a level

Electron Transitions

Excitation Relaxation

Electron moves To a higher E level

To a lower E level

Energy is Absorbed Released

Form of Energy Heat, light, electrical

Electromagnetic radiation

Energy of an atom or electron is “quantized”

Atoms can only absorb or release the amount of energy necessary to move from one allowed energy level to another.

Excitation - Energy Absorbed

n=1

n=2

n=4

n=3

n=5n=6

Excitation - Energy Absorbed

n=1

n=2

n=4

n=3

n=5n=6

Absorbance of Energy(Excitation)

Relaxation - Energy Released

n=1

n=2

n=4

n=3

n=5n=6

Absorbance of Energy(Excitation)

Emission of Energy(Relaxation)

Relaxation - Energy Released

n=1

n=2

n=4

n=3

n=5n=6

Absorbance of Energy(Excitation)

Emission of Energy(Relaxation)

E2 to E6 E6 to E2

Relaxation - Energy Released

n=1

n=2

n=4

n=3

n=5n=6

Absorbance of Energy(Excitation)

Emission of Energy(Relaxation)

E2 to E6 E6 to E2

BOTH TRANSITIONS INVOLVE THE SAME AMOUNT OF ENERGY. THUS INVOLVE THE SAME FREQUENCY OF EM RADIATION

What is observed during these transitions?

Background information:

•Electromagnetic energy (aka light energy) is often released and absorbed by chemical systems.

•The part of this radiation that we see is called the visible spectrum

Background information:

•Planck’s equation: E = hfStates that the greater the frequency of the radiation, the greater the energy of the photons.

What happens to white light as it passes through a prism or diffraction grating (basis of spectroscopic

instruments)?

It is dispersed/separated according to the different colours (wavelengths, frequencies)

Continuous spectrum

What happens to a sample of hydrogen gas (in a discharge tube) when an electric current is run

through it?

Electrical energy is absorbed and excites the electrons

Relaxation follows

H2 emits a purplish light is emitted.

Separating the light by a prism or diffraction grating produces…

Hydrogen Emission Spectrum

Each coloured line is produced by a specific relaxation transition.

Why do we only see 4 lines if many more transitions are possible?

Hydrogen Emission Spectrum

Only the 4 transitions in the BALMER SERIES involve EM radiation in the VISIBLE range

Spectroscopy

Analysis of the way matter absorbs or releases radiant energy

Examples: IR-spectra, atomic emission spectra, NMR (nuclear magnetic resonance), absorption spectra

Spectroscopy is used to:

a) identify elements or compoundsAtomic Spectra: Fingerprints of Elements

Spectroscopy is used to:

b) obtain information about bonding and structures of compounds

IR spectrum

Spectroscopy is used to:

c) Quantitatively determine the concentrations of substances present.

Two main types of atomic spectra:Emission and Absorption Spectra

“Bright-line Spectra” is atomic emission spectra in the visible range.

Absorption vs. Emission Spectra of H2

Note that the spectral lines are on the same wavelength.

Advantages/Uses of Spectroscopy

a) can definitively distinguish between substances with very similar physical and chemical properties.

eg. Members of the alkali metals family

K Na

Advantages/Uses of Spectroscopy

b) Sample is distant Eg. Applications in Astronomy

Astronomers have made the first direct detection and chemical analysis of an atmosphere of a planet that exists outside our solar system.

Spectrum of the hot gases in a nearby star-forming region, the Omega Nebula

(M17)

Advantages/Uses of Spectroscopy

b) Sample quantity is limited eg. in Forensics “Crystal meth”

The End