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Light as a Particle• In 1888, Heinrich Hertz discovered that electrons could be ejected from a sample by
shining light on it. This is known as the photoelectric effect. Note the effects of changing:– The intensity of the light.
- no matter how low the intensity there is still a current??– The frequency of light –Threshold Frequency
- Must have sufficient energy to eject an electron
Black Body Radiation
Matter will emitted radiation when heated. How?
The amount of radiation, and its frequency, depends on the temperature
The higher the T, the lower the l, i.e. higher n
White Light
White light : Continuous spectrum Contains all frequencies in equal amounts
Prism: disperses light into its components
Light Emission From Atomic Gas
Atomic emissions - Not continuous - Why?
-Result from changes in the electron
motion around the nucleus. -Types of changes in motion are restricted corresponding to specific frequencies – i.e. quantized
Emission from the atoms
Otherwise electron will collapse into the nucleus, losing energy as radiation
The electron can change to a lower orbit
A photon is emitted when the electron changes from higher orbit to lower orbit
The electron remains in a stable trajectoryaround the nucleus
i.e its kinetic energy is in balances with the electron nuclear potential energy.
Energy Levels
12345
12
3
4
5
Frequency
Ene
rgy
-ve
0 Energy of free electron
-ve energy change=> more stable than free electron
Atomic SpectraHydrogen Spectrum : Anders Ångström (1817-1874)
In 1885, Johann Balmer (1825-1898) showed that the wavelengths of H could be described by:
1/l = (1.0974*107 m-1)*(1/4 -1/n2)
This equation was later generalized by, Johannes Rydberg (1854-1919) to described all the spectral lines of H as:
1/l = R*|1/n12 -1/n2
2|
R = 1.0974*107 m-1 = Rydberg Constant
Rydberg Equation n1 final
n2 initial
Calculate the wavelength of a photon emitted when a hydrogen atom changes to the n = 4 state from the n = 5 state. What type of electromagnetic radiation is this?
Exercise
1/l = R*|1/n12 -1/n2
2|
1/l = (1.0974*107 m-1 )*|1/42 -1/52|
1/l = (1.0974*107 m-1 )*|1/16 -1/25|
1/l = (1.0974*107 m-1 )*|0.0225|
1/l = 246920 m-1
l = 0.0000040500 m = 4.0500*10-6 m = 4.0500 mm
Visible light
Final n1 = 4; initial n2 = 5
The Bohr Model of the Hydrogen Atom
In 1913, Neils Bohr (1885-1962) proposed an explanation H hydrogen based on three postulates:
1. The orbital angular momentum of electrons in an atom is quantized. Only those electrons whose orbitals correspond to integer multiples of h/2π are “allowed”.
2. Electrons within an allowed orbital can move without radiating (so that there is no loss of energy).
3. The emission or absorption of light occurs when electrons ‘jump’ from one orbital to another
The energy of an electron in the n th orbital of a hydrogen atom
E= - RhC/n2
n = principle quantum number
For any atomic system:
Energy Level of Electrons
En=- Ry Z/n2 Z = atomic number
Ry =RhC= 2.179*10-18 J = Rydberg unit
Energy is negative, i.e. means its stabilized
E/RhC = -1/n2 = -1, -1/4, -1/9, -1/16 ….
Bohr calculated the radius of each orbital:
r = ao (n2/Z) ao= Bohr radius
Absorption, Emission and Energy Levels
Lowest energy state : Ground state
Electrons cannot stand still therefore have an absolute minimum energyWhen a photon of the correct energy passes by it is absorbed and the electron goes to a higher energy level.
i.e. An Excited state
Absorption, Emission and Energy Levels
The electron can relax back to the ground state.
Upon relaxation it releases a photon, of the same energy it absorbed The energy of the photon absorbed or released has energy matching the difference between the energy levels involved
DE = Eex.s. – E g.s
Calculate the energy of a photon absorbed by a hydrogen atom when an electron jumps from the ground state to the n = 3 excited state?
Exercise
DE = Eex.s. – E g.s
DE = E(n=3) – E(n=1)
DE = -RhC/32 – (-RhC/12)
E=-RhC/n2
DE = -(RhC)(1/9 –1)=-(Ry)(-8/9)DE = (8/9) Ry
DE = (8/9) (2.179*10-18 J)
DE = 1.937*10-18 J
Ionization Limit
Notice that as you continually increase n the energy approaches 0 but does not get there.
The corresponding orbital radius would approach infinity.
What does this mean?
The electron is no longer in orbit i.e. The atom has ionized
The ionization energy is therefore the limit of DE as nex → ∞
IE = -RhC/∞2 – (-RhC/ninitial2)
IE = RhC/n2 = -E(n) > 0