Light and Electronic Transitions
The Big Questions
What is light?
 How is light emitted?
 What do electrons have to do with light?
 What are emission spectra?
 How do flame tests help identify metals?

The Electromagnetic Spectrum

All light is part of the EM spectrum.

Most is invisible:

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gamma, X-rays, UV, IR, microwaves, radio waves
Visible light: wavelength (w.l.) from 400 to 700
nm.
The EM Spectrum
EM Radiation

Light is a carrier of energy.
Energy is proportional to frequency.
 Frequency is inversely proportional to
wavelength.


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Longer wavelength = lower frequency = lower
energy.
Shorter wavelength = higher frequency = greater
energy.
Electrons and Quanta
Ground state – the lowest energy position
an e- can occupy.
 Excited state – a temporary high-energy
position.
 Quantum (pl. quanta) – the amount of
energy needed to move an e- to a higher
energy level.

Electrons and Quanta

If an atom absorbs exactly 1 quantum of
energy, an electron can be boosted from a
ground state to an excited state.


The electron is only in the excited state for a
very short period of time.
Soon the e- returns to its ground state and
emits the quantum of energy as light.

In some cases the emitted light is in the visible
spectrum.
Light and Electrons
Excited state
(E.S.) electron
Ground state
(G.S.) electron
Light and Electrons
Excited state
(E.S.) electron
1 quantum
Ground state
(G.S.) electron
Light and Electrons
Emission Spectrum

Emission spectrum – wavelengths of light
given off by an element when it is excited
(usu. by heat).

Every element has unique emission spectrum.
Emission Spectra
Hydrogen
Helium
Carbon
Flame Tests

Flame test – used to
ID some metals in
compounds.
Each metal gives a
flame a characteristic
color.
 Can identify metals
based on flame
colors.
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