Chemical Ideas 6.1

Light and the electron.

Sometimes we use the wave model for light …

• λ (lambda)= wavelength

Wavelength increases, Frequency ?

C

decreases

c = λ ν

υcλ

c

c

νλ

c = speed of light = 3.00 x 108 m s-1

λ = wavelength m lambda

ν = frequency Hz ( or s-1 ) nu

c

Use m

for λ

C = λν problems….

Work out λ:

1. If ν is 3.00 m

2. If ν is 30.0 cm

3. If ν is 3.00 mm

Work out λ:

4. If ν

c = 3.00x108 m s-1

Sometimes we use the particle model …hE

• Packages of energy are called photons

• Light is a stream of photons

• E = 6.63 x10-34 x 4.5 x 1014 = 3.0 x 10-19 J

14105.4 x Hz

h = 6.63 x 10-34 J Hz -1

Planck constant

Rearranging again….

hE

h

E

E

h

E = hν problems …

• h = 6.63 x 10-34 J Hz -1

• Planck constant

5. If the frequency is 1.089 x 106 Hz, what is the energy of each photon?

6. If E = 3.65 x 10-20 J per photon, what is the frequency (ν) of the radiation?

Emission Spectrum of hydrogen.

• Black background

• Coloured lines.

Spectroscopy• Sample of

hydrogen

• High voltage

• Prism

• Electrons can only exist in fixed energy levels.

ΔE

h

E

• Electrons absorb energy and move to a higher energy level.

• The electrons drop back to a lower energy level and emit energy.

• The frequency of the radiation emitted depends on ΔE.

3 2 4 2

?

5

7 etc

1

2

34

6

Why no

• 7 2

• 8 2

• 9 2?

visible ultra violet

E increases

6

1

234

Balmer 2

E increasing

Lyman ?

1

234

1

Absorption Spectrum of hydrogen.

• Continuous spectrum

• Black lines.

How do the 2 compare?

What happens to electrons?

1

234

• Electrons absorb energy

• Electrons are excited

• Electrons move to higher energy level

• E = h v

Back to the Storyline….

• Photosphere of hot stars emit visible or UV light

• Particles in the chromosphere absorb some of the light

• Every element has a different absorption spectrum

• Have a look at assignment 5

H

He

Fe

X (g) X+(g) + e-Ionisation

energy

1

234

Energy needed to

remove one e- from 1 mole

of atoms of a gaseous element

Ground state electron?

7. Use ΔE = hν to calculate the energy per photon corresponding to this frequency (in J).

h = 6.63 x 10-34 J Hz -1.8. Ionisation energy has units of kJ mol-1. You

have calculated the energy required to ionise a single atom. Work out the ionisation energy for hydrogen. L = 6.02 x 1023 mol-1.

Convergence limit = 3.27 x 1015 Hz