CHAPTER 5

CHAPTER 5

The Structure

of the Atom

5.4 Light and Spectroscopy

2 5.4 Light and Spectroscopy

Today1808 1870 1897 1910 1925

DemocritusAtomism

460 – 370 BC

Dalton“Modern”

atomic theory

CrookesCathode rays

ThomsonDiscovery

of the electron

RutherfordDiscovery of the nucleus

PauliPauli exclusion principle


Today1808 1870 1897 1910 1925

DemocritusAtomism

460 – 370 BC

Dalton“Modern”

atomic theory

CrookesCathode rays

ThomsonDiscovery

of the electron




Today1808 1870 1897 1910

DemocritusAtomism

460 – 370 BC

Dalton“Modern”

atomic theory

CrookesCathode rays

ThomsonDiscovery

of the electron


1925



Today1808 1870 1897 1910

DemocritusAtomism

460 – 370 BC

Dalton“Modern”

atomic theory

CrookesCathode rays

ThomsonDiscovery

of the electron


1925


Do we have

evidence to

support these

claims?


Light is a form of electromagnetic energy that comes from electrons in atoms

The human eye can only detect a certain range of that energy: the visible spectrum.


Light is a form of

electromagnetic energy that

comes from electrons in atoms

The human eye can only detect

a certain range of that energy:

the visible spectrum.


White light from a lamp or the sun is not truly white!

Analyzing starlight with a prism(one of the first spectrometers)


Visible light is only a small range in the electromagnetic spectrum


We are surrounded by electromagnetic energy


Remember that light travels as bundles called photons

1 electron volt (eV) = 1.602 x 10–19 J.

A very small unit of energy

Energy of a photon

Calvin the photon (“light Calvin the photon (“light

particle”)particle”)


Wavelength and frequency are related


The wavelength of red laser light is 652 nm. What is its frequency? How much energy does a photon of this light have in electron volts?



Asked: Frequency and energy

Given:

Relationships:

9652 10 m ,c E h




Given:

Relationships:

Solve:

9652 10 m ,c E h

9

15 14

8 143 10 / 4.6 10

652 1

4.136 10 4.6 10 / 1.9

0

c m sc therefor

E h e

m

s

es

V s eV




Given:

Relationships:

Solve:

9652 10 m ,c E h

9

15 14

8 143 10 / 4.6 10

652 1

4.136 10 4.6 10 / 1.9

0

c m sc therefor

E h e

m

s

es

es VV




Given:

Relationships:

Solve:

Answer: Since 1 Hz = 1/s, the frequency is 4.6 x 1014 Hz and the energy is 1.9 eV.

9652 10 m ,c E h

8 14

9

15 14

3 10 / 4.6 10

652 10

4.136 10 4.6 10 .91 /

c m sc therefore

m

e

s

E h e VV s s


prism

electron

all possible energy levels

Light from an incandescent light bulb:


prism

electron

fixed energy levels

Light from pure hydrogen:


Hydrogen atoms can only absorb and emit light of very specific energies.


Why does the atom absorb only specific (discrete) energies?

Matter and light


Remember: only some energy levels

are allowed.

Why does the atom absorb only specific (discrete) energies?

Matter and light


Energy levels

Photon(energy)

Energy levels

If: The energy of the photon matches a gap between

levels

Then: Energy (light) is absorbed.

BUT if: The energy of the photon does not match a gap

between levels

Then: Energy (light) passes

through the atom.

Matter and light


Energy levels

Photon(energy)

Energy of the photon matches a gap between levels

Energy (light) is absorbed.

Just after the photon energy is absorbed:another photon is emitted

This new photon can be a specific color(wavelength)

Matter and light

A photon with a wavelength of 414 nm has energy Ephoton = 3.0 eV. Do you expect to see a spectral line with = 414 nm in the emission spectrum of the atom represented by this energy-level diagram? If so, what transition or transitions will emit it?


Each type of atom has a different electron structure.Each element has unique energy levels like a fingerprint.


Spectrum cards

How to read the

spectrum cards


Combinations of elements contain spectral lines from both.

Spectrum cards


Photon emitted

Photon absorbed

Energy levels

Photon(energy)

Energy of the photon matches a gap between levels

Reemission of light has two steps:


Energy levels

Photon(energy)

Emission spectrum

Absorption spectrum

There may be a range of energies, but only specific ones are absorbed


Visible light is only a small range of the electromagnetic spectrum.


Each type of atom has a different electron structure.Each element has unique energy levels like a fingerprint.

CHAPTER 5

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