CHAPTER 5 The Structure of the Atom 5.4 Light and Spectroscopy

460 – 370 BC 1808 1870 1897 1910 1925 Today Democritus Atomism Dalton “Modern” atomic theory Crookes Cathode rays Thomson Discovery of the electron Rutherford Discovery of the nucleus Pauli Pauli exclusion principle

460 – 370 BC 1808 1870 1897 1910 1925 Today Crookes Cathode rays Democritus Atomism Dalton “Modern” atomic theory Crookes Cathode rays Thomson Discovery of the electron Rutherford Discovery of the nucleus Pauli Pauli exclusion principle

Pauli exclusion principle 460 – 370 BC 1808 1870 1897 1910 1925 Today Democritus Atomism Dalton “Modern” atomic theory Crookes Cathode rays Thomson Discovery of the electron Rutherford Discovery of the nucleus Pauli Pauli exclusion principle

Do we have evidence to support these claims? 460 – 370 BC 1808 1870 1897 1910 1925 Today Democritus Atomism Dalton “Modern” atomic theory Crookes Cathode rays Thomson Discovery of the electron Rutherford Discovery of the nucleus Pauli Pauli exclusion principle 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) White light from a lamp or the sun is not truly white!

Visible light is only a small range in the electromagnetic spectrum

We are surrounded by electromagnetic energy

1 electron volt (eV) = 1.602 x 10–19 J. Energy of a photon Remember that light travels as bundles called photons A very small unit of energy 1 electron volt (eV) = 1.602 x 10–19 J.

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?

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:

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: Solve:

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: Solve:

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

Light from an incandescent light bulb: prism all possible energy levels electron

Light from pure hydrogen: prism fixed energy levels electron

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

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

Matter and light Why does the atom absorb only specific (discrete) energies? Remember: only some energy levels are allowed.

Energy (light) passes through the atom. Energy (light) is absorbed. Matter and light Energy levels Energy levels Photon (energy) Energy of the photon matches a gap between levels Energy of the photon does not match a gap between levels Energy (light) passes through the atom. Energy (light) is absorbed.

Energy (light) is absorbed. Matter and light Energy levels Photon (energy) another photon is emitted Energy of the photon matches a gap between levels specific color (wavelength) Energy (light) is absorbed.

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 Combinations of elements contain spectral lines from both.

Energy of the photon matches a gap between levels Reemission of light has two steps: Energy levels Photon (energy) Photon absorbed Photon emitted Energy of the photon matches a gap between levels

Emission spectrum Absorption spectrum Range of energies Emission spectrum Energy levels Photon (energy) Absorption spectrum

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.