1. Matter and Particles of Light: Quantum Theory
Light (energy) and matter in motion behave both as waves and particles
Wave-Particle Duality - Quantum Theory
Particles of light are called photons: E = hf = hc/l
Photons of a specific wavelength l may be absorbed or emitted by atoms in matter
Matter is made of different natural elements: lightest Hydrogen (1 proton), heaviest Uranium (92 protons)
Smallest particle of an element is atom, made up of a nucleus (protons and neutrons), and orbiting electrons
Electrons and protons attract as opposite electrical charges, NOT gravitationally like planets and Sun

2. The simplest atom: Ordinary Hydrogen
Resemblance to
planets orbiting
the Sun is
superficial !
Electrons also
move both
as particles
and waves
p – positively charged
e – negatively “
One proton in the center (nucleus) and one electron in orbits of definite energy;
Ordinary H has no neutrons, but ‘heavy hydrogen’ has one neutron in the nucleus

3. Absorption and emission of photons by H-atom
An electron may absorb or emit light photons at specific wavelength
Wavelength (n = 3  n = 2): 6562 Angstroms (RED Color)
Energy of the photon must be exactly equal to the energy difference
between the two ‘orbits’

4. file:///E:/Univ7e/content/ch05/ html

5. Energy Level Diagram of 1H
Continuum
Energy Level Diagram of 1H
n=
n=5
n=4
n=3 (2nd excited state)
n=2 (1st excited state)
n=1 (Ground State)

6. Photons of all other energies (wavelengths) are
26
25
24
n=23
n=6
n=5
n=4
n=3 (2nd excited state)
n=2 (1st excited state)
Photons of all other energies (wavelengths) are
ignored and pass on by unabsorbed.
n=1 (Ground State)

7. Larger Jump = More Energy = Bluer Wavelength
62
52
42
n=32
n=6
n=5
n=4
n=3 (2nd excited state)
n=2 (1st excited state)
Larger Jump = More Energy = Bluer Wavelength
n=1 (Ground State)

8. Energy, Frequency, Wavelength
Light particles ‘photons’ have a unique wavelength
The more ‘energetic’ a wave, the higher its frequency, or lower its wavelength
Planck’s Law: Photon energy (‘quantum’) is
E = h f = h / l
‘h’ is the Planck’s constant
This ‘quantum’ of energy must be equal to the difference in energies between two electron orbits, for either absorption or emission by an atom

9. Spectrum of a Fluorescent Light
Mercury

10. Characteristic spectra of elements
Each element
has a unique
set of spectral
lines, thus
enabling its
identification in
the source.
Observations of spectra of different elements in a source
(planet, star, galaxy etc.) yields its chemical composition

11. Continuous, Absorption, and Emission spectra of a source
Continuous spectrum covers wavelengths in a given range; absorption or emission
spectrum consists of dark or bright lines respectively at definite wavelengths