Unanswered Questions

• Rutherford’s work on the atom was important but left big questions: – How are electrons arranged around the atom?

– Why aren’t electrons pulled into the positive nucleus?

– What accounts for the difference in chemical behavior among atoms?

Unraveling Chemical Behavior

• In the early 1900’s chemists found that certain elements emitted visible light when heated in a flame (now called atomic emission spectra).

• They found that this and all of an element’s chemical behavior is related to the arrangement of the electrons in its atoms.

Electromagnetic Radiation

• A form of energy that exhibits wavelike behavior as it travels through space.

• Includes visible light, ultraviolet, infrared, TV, radio, X-ray, microwaves & gamma • All e-m waves travel at the speed of light (c) which is 3.00 X10 8 m/s.

• All e-m waves have a wavelength, frequency and amplitude.

• This is the wave model of light.

Electromagnetic Spectrum

C = λ x v • C = speed of light in m/s and λ = wavelength • Wavelength is given in meters and frequency is given in hertz or s -1 .

• Wavelength and frequency have an inverse relationship (one goes up the other goes down).

• Greater frequency = greater energy

Try It 1

A helium-neon laser emits light with a wavelength of 633 nm. What is the frequency of this light?

Try It 2

What is the wavelength of X rays having a frequency of 4.80 x 10 17 Hz?

Need for a new model

• The wave model of light cannot explain why heated objects emit only certain frequencies (colors) of light at a given temperature • Or why some metals emit electrons when colored light of a specific frequency shines on them.

Planck’s Work

• Found that heated metals emit specific colors (frequencies) of light (flame tests).

• As objects get hotter and possess more energy, the metal emits different colors (frequencies) of light.

• Found matter can only gain or lose energy in small, specific amounts (a quantum)

Quantum

• The minimum amount of energy that can be gained or lost by an atom at a time.

• E quantum = hv • Since C = λ x v, then E = hc / λ • h = Planck’s constant: 6.626 x 10 -34 Joules .

seconds or J .

s

More quantum ideas

• According to Planck’s theory, matter can have only certain amounts of energy — quantities of energy between these values do not exist. • So this was a big deal at the time!

Try It 3

Calculate the energy of a gamma ray photon whose frequency is 5.02 x 10 20 Hz.

= (6.626 x 10 -34 J .

s) (5.02 x 10 20 Hz) = 3.33 x10 -13 J

Photoelectric Effect Increase light’s intensity and more electrons are ejected.

Increase light’s frequency and ejected electrons move faster.

Photoelectric Effect • Each metal has a minimum frequency necessary for this to happen.

• Varying intensity and frequency changes the number of electrons ejected & their speed.

• Led to development of photoelectric cells used in street lights, solar calculators, etc.

Einstein’s Proposal • The wave model couldn’t explain the photoelectric effect.

• So in 1905 Einstein proposed the “particle nature” of light.

• Says that light is both a wave and particle • Light is really a stream of tiny particles or bundles of energy called photons.

Photons

• Bundles of energy that make up all EM waves like light.

• Have no mass but each photon carries a quantum of energy so E quantum = E photon = hv

Atomic Emission Spectra • If a gas is heated, it absorbs energy which can cause some electrons to be “excited” or bumped up to the next energy level.

• These excited electrons are unstable, so emit or lose the energy and drop back to their original energy level.

• The emitted energy is seen as different colors of light.

• The process continues over & over as long as energy is put into the gas.

• Each element has a unique emission spectra that consists of certain frequencies or colors.

• Elements can be identified by their spectra.

Try It 4

• An FM radio station broadcasts at a frequency of 98.5 MHz. What is the wavelength of the station’s broadcast signal? 98.5 MHz x 10 6 Hz / 1 MHz = 98.5 x 10 6 Hz = (3.00 x 10 8 m/s) / 98.5 x 10 6 Hz =3.05 meters

Try It 5

Calculate the energy of a photon of ultraviolet light that has a wavelength of 49.0 nm. E = (6.626 x 10 -34 J .

s) (3.00 x 10 8 m/s) / (49.0 x10 -9 m) E = 4.06 X 10 -18 J