Chemistry is in the electrons - College of · PDF fileChemistry is in the electrons ... of the light not absorbed White absorbs ... Quantization and the photoelectric effect Light

Chemistry is in the electrons

Electronic structure –arrangement of electrons in atom

Two parameters:– Energy

– Position

The popular image of the atom is incorrect: electrons are not miniature planets orbiting a nuclear sun

Learning objectives

Describe properties of waves and calculate basic properties like wavelength and frequency

Calculate quantities using the photon model of light

Describe the basic principles of the Bohr model

Distinguish between the “classical” view and the “quantum” view of matter

Describe Heisenberg Uncertainty principle and deBroglie wave-particle duality

Calculate wavelengths of particles

Rutherford’s epic experiment revealed the

positive nucleus with the electrons occupying the

vast void around it

Problem: Why don’t the electrons

collapse into the nucleus?

The planetary idea

A planet in a stable orbit circulates indefinitely

Orbiting charged particles emit energy - spiral

into the nucleus emitting energy as it does so

Conventional explanations don’t work

Yet... atoms exist and

electrons are stable

outside the nucleus

Familiar symbol of

planetary atom is

incorrect

There must be

another explanation...

Unlocking the mystery of light

Earth’s energy

provider

Chief nourisher of

life’s feast

Inspiration for

romance

Symbol of good

Object of worship

Source of wisdom

Light and colour

White light contains many colours

They can be separated by a prism (rainbow)

Absorption determines colour

Objects are the colour

of the light not

absorbed

White absorbs

nothing

Black absorbs

everything

Complementary

colors:

– Green absorbs red

– Violet absorbs yellow

Let there be light – properties of

waves

Electromagnetic waves

are characterized by:

– Wavelength – distance

between two peaks

– Frequency - number of

wave peaks that pass a

fixed point per unit time

– Amplitude - height of the

peak measured from the

center line

– Velocity – speed of the

crest

Wavelength and colour

Different “colours” of electromagnetic radiation are waves with different wavelengths and frequencies.

All electromagnetic radiation has the same velocity: the speed of light – 3 x 108 m/s

Velocity (c/ms-1) = wavelength (λ/m) x frequency (ν/s-1)

Wavelength proportional to 1/frequency - as λ ↑ ν ↓

c

Continuous range of wavelengths and frequencies:– Radio waves at low-frequency end

– Gamma rays at high-frequency end

– Visible region is a small slice near the middle

Waves in X-ray region have wavelength approximately same as atomic diameter (10–10 m)

Radiation becomes more dangerous as frequency increases

Harmful radiation

The electromagnetic spectrum

Atoms emit and absorb radiation at

specific wavelengthsAbsorption is light removed by the atom from incident light

Emission is light given out by an energetically excited atom

The absorption and emission lines are at the same wavelengths

The lines from the H atom form a neat series.

Do these spectra have anything to do with the electronic structure?

Each element has a unique spectrum

Electronic structures

of each element are

different

Spectra can be used

to identify elements –

even in very remote

locations

Empirical classification of the

spectra of the hydrogen atom

All of the lines in the H atom spectrum can be fit

to an equation

– m and n are integers (n > m)

– R is the Rydberg constant

The Balmer–Rydberg equation.

22

111

nmR

Inner shell

Outer shell

So, everything is cool with waves?

NOT

Observations of radiation from heated

bodies could not be described using

classical methods – the ultraviolet

“catastrophe”

Shortcomings with waves...

Blackbody radiation and the

ultraviolet catastropheObservations of radiation from heated bodies could not be described using classical methods – Intensity tends to infinity at shorter wavelength!

Observed radiation exhibits a maximum that depends on the temperature of the body– As T ↑, λmax ↓

Chunky energy and the Planck

equation

Blackbody radiation explained if energy is

emitted in discrete amounts (quanta)

instead of changing continuously

Quantization of energy

E = hν

h is Planck’s constant = 6.626 x 10-34 Js

Ele

ctr

ical

cu

rren

t

Frequency

Quantization and the photoelectric

effect

Light incident on a metal surface causes

electrons to be emitted.

Below threshold frequency nothing happens

Above threshold, current increases with intensity

No

current

flows

Current

flows

Photons: light as particle and wave

In photoelectric effect, light behaves like a particle

Energy of electron given by:

Energy is “quantized” into packets - photons

Photon energy depends on frequency:

E = hν

As frequency increases photon energy increases

e eKE h

Photon energy and the

electromagnetic spectrumAs frequency increases, photon energy increases

“Dangerous” radiation has high photon energy – UV

light, X-rays, gamma rays (ionizing radiation)

Harmless radiation has low photon energy – IR,

radiowaves

Light: particle or wave or both?

Newton advanced a corpuscular theory of light, even as he discovered the refraction of light in a prism

Huygens developed a wave theory of light

Maxwell equations cemented mathematical description of electromagnetic waves

Discovery of light interference and its description by wave theory made the latter triumphant in the 19th

century

Ultraviolet catastrophe and the photoelectric effect establish the photon

So what is light exactly? The enduring mystery...