Quantum Theory

Electromagnetic Spectrum

LOW

ENERGY

HIGH

ENERGY

g rays X-rays Ultraviolet Infrared Microwave

Radar

Radio waves

TVFM

ShortWave

LongWave

Visible spectrum

10-2nm 10-1nm 100nm 101nm 102nm 10-3cm 10-2cm 10-1cm 100cm 101cm 1cm 101m 102m 103m 104m

1019Hz 1018Hz 1017Hz 1016Hz 1014Hz 1013Hz 1012Hz 1011Hz 1010Hz 109Hz 100 MHz 10 MHz 1 MHz 100 KHz

RedOrangeYellowViolet Blue Green

700 nm600 nm500 nm400 nm

Wavelength, l

Frequency, n

103nm

1015Hz

Electromagnetic spectrum

WhiteLight

Davis, Frey, Sarquis, Sarquis, Modern Chemistry 2006, page 98

http://www.youtube.com/watch?v=bjOGNVH3D4Y

Photoelectric Effect

• Light is a form of energy • Light can hit a metal surface and cause the

metal to emit electrons– The photoelectric effect

• Light travels in waves • Light at any frequency can hit a metal surface

and cause the metal to emit electrons

Photoelectric Effect

http://phet.colorado.edu/en/simulation/photoelectric

Photoelectric Effect

A metal did not emit electrons when certain frequencies of light hit it

When red light strikes a metal surface, no electrons are ejected.

Light

MetalNucleus

Photoelectric Effect

There was a minimum frequency of light needed to get a metal to emit electrons

When green light strikes a metal surface, electrons are ejected.

Light

Electron

MetalNucleus

Planck’s Explanation • If electromagnetic radiation acted

as a wave, then it would emit energy continuously

• Instead, electromagnetic radiation is emitted in small specific amounts– Called quanta

• AKA: Things come in chunks • Quantum: the minimum energy

that can be lost or gained by an atom

Continuous vs. Quantized

A BZumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 330

Einstein’s Explanation • Built off of Planck’s ideas

– Agreed that electromagnetic radiation would emit energy continuously if it acted as a wave

– Agreed that electromagnetic radiation emits quanta

Einstein’s Explanation • Noticed that electromagnetic

radiation is sometimes continuous and sometimes quantized – Proposed a dual wave-particle

nature of electromagnetic radiation

– Light has wavelike properties – Light can be thought of as a

stream of particles

Einstein’s Explanation • Defined the term photon

– A particle of electromagnetic radiation (light) that has no mass and carries a quantum (bundle) of energy

• In order for the photoelectric effect to occur – The metal is struck by photons – Each photon must carry a certain

amount of energy in order to knock an electron loose from the metal

The Bohr Model

– Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus.

5.1

Bohr Model of Hydrogen

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 331

Nucleus

Possible electron orbits

e

e

The Bohr Model

• Each possible electron orbit in Bohr’s model has a fixed energy.– The fixed energies an electron can have are called

energy levels.– A quantum of energy is the amount of energy required

to move an electron from one energy level to another energy level.

5.1

The Bohr Model• Like the rungs of the

strange ladder, the energy levels in an atom are not equally spaced.

• The higher the energy level occupied by an electron, the less energy it takes to move from that energy level to the next higher energy level.

5.1

The Quantum Mechanical Model• The propeller blade has the same probability of

being anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller.

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The Quantum Mechanical Model• In the quantum mechanical model, the probability

of finding an electron within a certain volume of space surrounding the nucleus can be represented as a fuzzy cloud. The cloud is more dense where the probability of finding the electron is high.

5.1

Atomic Orbitals

• An atomic orbital is often thought of as a region of space in which there is a high probability of finding an electron.– Each energy sublevel corresponds to an orbital of a

different shape, which describes where the electron is likely to be found.

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Atomic Orbitals

• Different atomic orbitals are denoted by letters. The s orbitals are spherical, and p orbitals are dumbbell-shaped.

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Atomic Orbitals

• Four of the five d orbitals have the same shape but different orientations in space.

5.1

Vocabulary• Orbital

– The space around a nucleus that has a high probability of finding an electron

– Simply a probability graph of where we can find an electron • Quantum numbers: tell us the properties of atomic

orbitals and the properties of electrons in the orbitals • Principle quantum number

– Symbol: n – The energy level that an electron occupies

• Angular momentum quantum number– Symbol: l– The shape of the orbital

• Spin quantum number – +1/2 or -1/2 – Spin state of an electron