Waves & Particles Ch. 4 - Electrons in Atoms. Properties of Light Different types of electromagnetic radiation (x-rays, radio waves, microwaves, etc…)

Waves & ParticlesCh. 4 - Electrons in Atoms

Properties of Light

• Different types of electromagnetic radiation (x-rays, radio waves, microwaves, etc…) SEEM to be very different from one another. Yet they share certain fundamental characteristics.

• 3.0 x 108 m/sec is the speed of light!

A. Wavelength

• Wavelength – distance between identical points on successive waves

• Usually measured in nanometers

B. Frequency

• Frequency – the number of complete wave cycles that pass a given point in one second: the unit is cycles/second but is written as sec-1, or Hertz.

• One wave in one second equals one Hertz!

Amplitude

• Amplitude is the height of a wave. It is also measured in length units like nanometers, angstroms, etc.

A. Waves

A

greater amplitude

(intensity)

greater frequency

(color)

crest

origin

trough

A

EM Spectrum

• Frequency & wavelength are inversely proportional

c = f c: speed of light (3.00 108 m/s): wavelength (m, nm, etc.)f: frequency (Hz)

EM Spectrum

c = f

•Longer Wavelength means Lower Frequency

•Shorter Wavelength means Higher Frequency

•Higher Frequency means Higher Energy

•Lower Frequency means Lower Energy

Range

• The range of visible light is from 400 to 700 nanometers. Low energy is colored red while high energy is colored violet.

• Violet:   400 - 420 nm • Indigo:   420 - 440 nm • Blue:   440 - 490 nm • Green:   490 - 570 nm • Yellow:   570 - 585 nm • Orange:   585 - 620 nm • Red:   620 - 780 nm

•Ephoton = hf

E : engery of a photon (J)

h: Plank’s constant (6.63 x 10-34 J . sec)

f: frequency (Hz or sec-1)

EM Spectrum

LOW

ENERGY

HIGH

ENERGY

EM Spectrum

LOW

ENERGY

HIGH

ENERGY

R O Y G. B I V

red orange yellow green blue indigo violet

Examples

• 2. What is the wavelength of radiation whose frequency is 6.24 x l013 sec-1?

Given: Work:

f = 6.24 x l013 Hz

c = 3.00 108 m/s

λ = ?

λ = c/f

λ = 3.00 108

6.24 x l013

Answer

• 4.81 x 10-6 m

Examples• 3. What is the frequency of radiation whose

wavelength is 2.20 x l0-6 nm? (1 m = 1,000,000,000 nm)

Given: Work:

c = 3.00 108 m/s

λ = 2.20 x 10-6 nm(you have to change nm to m)

f = ?

f = c/ λ

f = 3.00 108

2.20 x 10-15

Answer

• 1.36 x 1023 Hz

Hydrogen Atom Line Emission Spectra

• When investigators passed electric current through a vacuum tube containing hydrogen gas at low pressure, they observed the emission of a characteristic pinkish glow. When a narrow beam of the emitted light was shined through a prism, it was separated into a series of specific frequencies (and therefore specific wavelengths, c =) of visible light. The bands of light were part of what is known as hydrogen’s LINE-EMISSION SPECTRUM. (page 95)

• The lowest energy state of an atom is its ground state.

• A state in which an atom has a higher amount of energy is an excited state. When an excited atom returns to its ground state, it gives off energy.

B. Bohr Model

• e- exist only in orbits with specific amounts of energy called energy levels

• Therefore…

– e- can only gain or lose certain amounts of energy

– only certain photons are produced

B. Bohr Model

1

23

456 • Energy of photon depends on the difference in energy levels

• Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom

C. Other Elements

• Each element has a unique bright-line emission spectrum.

– “Atomic Fingerprint”

Helium

Bohr’s calculations only worked for hydrogen!