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:

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. 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

Ground state (G.S.) electron

Excited state (E.S.) electron

1 quantum

Light and Electrons

Ground state (G.S.) electron

Excited state (E.S.) electron 1 quantum

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.