ELECTROMAGNETIC ELECTROMAGNETIC RADIATIONRADIATION

ELECTROMAGNETIC ELECTROMAGNETIC RADIATIONRADIATION

The Wave Nature of Light

Much of our present understanding of

the electronic structure of atoms has

come from analysis of

the light emitted or absorbed

by substances

Electromagnetic Radiation

Radiant energy which carries energy through space.

All types of electromagnetic radiation move through a vacuum at

a speed of 3.00 x 108 m/s

Wave-like Nature of Electromagnetic Radiation

Electromagnetic radiation is measured in wavelenghts.

Electromagnetic RadiationElectromagnetic Radiation

wavelengthVisible light

wavelengthUltaviolet radiation

Amplitude

Node

Since all electromagntic radiation travels at the same velocity in vacuum, c,

its frequency, , is inversely proportional to

its wavelength,

= c

Electromagnetic RadiationElectromagnetic Radiation

• Waves have a frequencyWaves have a frequency

• Use the Greek letter “nu”, Use the Greek letter “nu”, , for frequency, , for frequency, and units are “cycles per sec”and units are “cycles per sec”

• All radiation: All radiation: • • = c = c

• where c = velocity of light = 3.00 x 10where c = velocity of light = 3.00 x 1088 m/sec m/sec

• Note that long wavelength = small frequencyNote that long wavelength = small frequency

• Short wavelength = high frequencyShort wavelength = high frequency

Electromagnetic Spectrum

Indicates the wavelenghts of

electromagnetic radiation

characteristic of various regions of the

electromagnetic spectrum

Electromagnetic RadiationElectromagnetic RadiationNote that long wavelength = small frequencyNote that long wavelength = small frequency

Short wavelength = high frequencyShort wavelength = high frequency

increasing increasing wavelengthwavelength

increasing increasing frequencyfrequency

See Screen 7.4See Screen 7.4

Atomic Line Spectra and Atomic Line Spectra and Niels BohrNiels BohrBohr’s greatest contribution to Bohr’s greatest contribution to

science was in building a science was in building a simple model of the atom. It simple model of the atom. It was based on an was based on an understanding of the understanding of the SHARP LINE SPECTRASHARP LINE SPECTRA of excited atoms.of excited atoms.

Niels BohrNiels Bohr

(1885-1962)(1885-1962)

Bohr’s Model of the Hydrogen Atom

Line Spectra

Produced when gases are placed under reduced pressure in a tube and a high voltage

is applied

- colored lines, separated by black regions are produced

Line Spectra

In 1885, Johann Balmer observed that

the four lines of the hydrogen spectrum

fit a formula

Visible lines in H atom spectrum are called the BALMER Visible lines in H atom spectrum are called the BALMER series. series.

High EHigh EShort Short High High

Low ELow ELong Long Low Low

Line Spectra Line Spectra of Excited Atomsof Excited Atoms

= C( 1/22 -1/n2 ) n = 3,4,5,6

C = 3.29 x 1015 s-1

Predicts the frequency of each line of the hydrogen line spectra

Bohr also assumed the electron could “jump” from one allowed energy state

to another.

• Energy is absorbed when electron moves to a higher energy state.

• Energy is emitted when when electron moves from higher to a lower energy state

Orbital Energies

En = (-RH)(1/n2) n = 1,2,3,4….

RH = Rydberg constant

(2.18 x 10-18 J)

n = principle quantun number

Line Spectra Line Spectra of Excited Atomsof Excited Atoms

• Excited atoms emit light of only certain wavelengths

• The wavelengths of emitted light depend on the element.

Atomic Spectra and BohrAtomic Spectra and BohrAtomic Spectra and BohrAtomic Spectra and Bohr

1.1. Any orbit should be Any orbit should be possible and so is any energy.possible and so is any energy.

2.2. But a charged particle But a charged particle moving in an electric field moving in an electric field should emit energy. should emit energy.

+Electronorbit

One view of atomic structure in early 20th One view of atomic structure in early 20th century was that an electron (e-) traveled century was that an electron (e-) traveled about the nucleus in an orbit.about the nucleus in an orbit.

Bohr Model stated thatBohr Model stated thatBohr Model stated thatBohr Model stated that

electrons can only exist in certain electrons can only exist in certain discrete orbits — called discrete orbits — called

stationary statesstationary states. .

Each electron is restricted to Each electron is restricted to QUANTIZED energy states.QUANTIZED energy states.

n = quantum no. = 1, 2, 3, 4, ....n = quantum no. = 1, 2, 3, 4, ....

Atomic Spectra and BohrAtomic Spectra and BohrAtomic Spectra and BohrAtomic Spectra and Bohr

• Only orbits where n = integral no. are Only orbits where n = integral no. are permitted.permitted.

• Results can be used to explain atomic Results can be used to explain atomic spectra.spectra.

Atomic Spectra and BohrAtomic Spectra and BohrAtomic Spectra and BohrAtomic Spectra and Bohr

If electrons are in quantized energy If electrons are in quantized energy states, then states, then E of states can have only E of states can have only

certain values. certain values.

This explain sharp line spectraThis explain sharp line spectra..

n = 1

n = 2E = -C (1/2 2)

E = -C (1/1 2)n = 1

n = 2E = -C (1/2 2)

E = -C (1/1 2)

Calculate Calculate E for an electron “falling” from E for an electron “falling” from

high energy level (n = 2) to low energy high energy level (n = 2) to low energy

level (n = 1). level (n = 1).

E = EE = Efinalfinal - E - Einitialinitial = -C[(1/1 = -C[(1/122) - (1/2)) - (1/2)22]]

E = -(3/4)CE = -(3/4)C

.

n = 1

n = 2E = -C (1/ 22)

E = -C (1/ 12)E N E R G Y

.

n = 1

n = 2E = -C (1/ 22)

E = -C (1/ 12)E N E R G Y

E = -(3/4)CE = -(3/4)C

C has been found from experiment (and is now C has been found from experiment (and is now called R, the Rydberg constant)called R, the Rydberg constant)

R (= C) = 1312 kJ/mol or 3.29 x 10R (= C) = 1312 kJ/mol or 3.29 x 101515 cycles/sec cycles/sec

so, E of emitted lightso, E of emitted light

= (3/4)R = 2.47 x 10= (3/4)R = 2.47 x 101515 sec sec-1-1

and l = c/n = 121.6 nmand l = c/n = 121.6 nm

.

n = 1

n = 2E = -C (1/ 22)

E = -C (1/ 12)E N E R G Y

.

n = 1

n = 2E = -C (1/ 22)

E = -C (1/ 12)E N E R G Y

Atomic Line Spectra and Atomic Line Spectra and Niels BohrNiels Bohr

Atomic Line Spectra and Atomic Line Spectra and Niels BohrNiels Bohr

Bohr’s theory was a great Bohr’s theory was a great accomplishment.accomplishment.

Rec’d Nobel Prize, 1922Rec’d Nobel Prize, 1922

Problems with theory —Problems with theory —• theory only successful for H.theory only successful for H.• introduced quantum idea introduced quantum idea

artificially.artificially.Niels BohrNiels Bohr

(1885-1962)(1885-1962)