1. LINE SPECTRA
New methods of splitting white light into its constituent wavelengths in the 19th century (the diffraction grating) revealed that the spectrum was not completely continuous.
Fraunhoffer discovered dark lines in the spectrum of the Sun

5. Increasing frequency
A single series of lines converges towards the high frequency end of the spectrum

6. BRIGHT LINES AND DARK LINES
It soon became apparent that these lines were associated with individual elements.
When gases were heated or stimulated electrically they shone with characteristic colours.
When their spectra were taken they were not continuous but consisted of a series of bright lines.
These bright line spectra were called
“emmision spectra”

7. Hydrogen gas at low pressure
High
Voltage
Diffraction grating

8. Absorption Spectra White light source
Diffraction grating
screen
Cool hydrogen gas
When white light is shone throough the cool gas, black lines appear in the continuous spectrum in exactly the same places as the bright lines appeared in the hot gas.

9. White Light
Hydrogen
Helium
Lithium
Mercury
Absorption spectrum of hydrogen

10. Explaining the lines for hydrogen
Hydrogen is the simplest atom. It has only one electron. Here the electron is in its lowest energy state: its “ground state”
More electron shells that the electron can occupy exist which are higher in energy.
The electron cannot occupy a position “in between” these “shells”.
An electron which occupies a higher energy state than the ground state will eventually fall back to the ground state
When the electron does this it will release a photon of fixed energy

11. The diagram shows an electron in
the second energy level.
In falling to the ground state it has to
lose photon of fixed energy
hf =E1-E2
Where E1 is the energy of the
level it falls
from and E2 is the energy of
the finishing level.

12. The electron gets to a higher level by absorbing a photon of exactly the right frequency to give it the energy to get there.
This process causes all the light at exactly that frequency to disappear from the spectrum. This is how dark absorption lines are produced

13. Emission spectra are produced when “excited” electrons which have been promoted to higher shells fall back
Photons of an exact frequency are produced for each possible transition

14. The lines converge because the energy difference between the outer orbitals becomes lower
We can see that the fall from a higher orbital produces a photon with greater energy and therefore a higher frequency

15. Emission spectra always consist of several series of lines becoming closer or “converging” at higher frequency
Emission spectrum showing a single series of lines
Lines converge towards the blue

16. The Balmer Series
One series of lines in the hydrogen spectrum, Converging in the near Ultra violet.
These lines are actually due to electrons falling to shell 2

18. Energy Levels in Hydrogen
An electron sitting just outside the influes of the nucleus (outside the atom) has zero energy.
If it comes under the influence of the nucleus it loses energy. This is the reason that each energy value is negative.
(The maximum energy the free electron could lose is 13.6eV if it falls to n=1).
n=6
 E6 = -   0.38 eV
n=5
 E5 = -   0.54 eV
n=4
E4 = -   0.85 eV
Increasing energy
n=3
E3 = -   1.51 eV
n=2
 E2 = -   3.40 eV
n=1
 E1 =   eV
  Energy Levels in Hydrogen

19. Energy Levels in Hydrogen
Increasing energy
n=3
E3 = -   1.51 eV
n=2
n=1
 E1 =   eV
An electron falling from n=3 to n=1 will lose the difference in energy between the two levels
(-13.60) eV = eV
  Energy Levels in Hydrogen

20. Energy Levels in Hydrogen
Increasing energy
n=3
n=2
 E2 = -   3.40 eV
n=1
 E1 =   eV
An electron moving from n=1 to n=2
will gain the difference in energy between the two levels
(-3.40) –(-13.60)= 10.20eV
  Energy Levels in Hydrogen

21. The formula
The energy change with an electron moving from any shell E2 another shell E1
ΔE= E1-E2
This change in energy is the energy of the photon that is emitted or absorbed in the process
so:
hf = E1-E2

22. An electron falling from n=3 to n=1 will lose -1.51 – (-13.6) eV = 12.09eV
That is it will lose 12.1 x 1.6 x 10-19J
This energy will be the energy of the photon produced
by:
E=hf