Electrons and the

EM Spectrum

Models of the Atom So far, the model of the

atom consists of protons and neutrons making up a nucleus surrounded by electrons. After performing the gold foil experiment, Rutherford hypothesized a model of the atom that looked much like the one below.

electron

neutron

proton

What about the electrons??• Rutherford didn’t know

exactly where the electrons were located in the atom, just that they surrounded it, or why chemical bonding occurred.

• Bohr figured out that electrons orbit in energy levels around the atom.

The Bohr Model

• Niels Bohr (1852-1962) was a student of Rutherford and believed the model needed improvement.

• Bohr proposed that an

electron is found only in

specific circular paths, or

orbits, around the nucleus.

Models of the Atom

However, this model could not explain the chemical and physical properties of the elements.

Models of the Atom

For example it could not explain:

1. why metals give off certain colors when heated in a flame

-or- 2 .why objects heated to high temperatures first glow dull red, then yellow, then white

So what is light?

Light is all about the Electrons!

• Light is a form of electromagnetic radiation that travels like a wave through space as PHOTONS.

• When electrons get excited, they jump up to higher energy levels and then fall back down

• Depending on how high they jump, they will give off a different color of LIGHT

Atomic Emission Spectra

ground state

excited state

ENERGY IN PHOTON OUT

GA

IN e

nerg

y

LOS

E energy

Emission Spectrum

Atomic Emission Spectra

• Every element has a UNIQUE emission spectrum. The colors that you see represent the element’s electrons jumping through the energy levels!

• LONG JUMPS are represented by HIGH energy colors (violet & blue)

• SHORT JUMPS are represented by LOW energy colors (red).

C. Johannesson

EM Spectrum

LOW

ENERGY

HIGH

ENERGY

Properties of Light

• Movement of excited electrons to lower energy levels, and the subsequent release of energy, is seen as light! Before 1900, scientists thought light behaved solely as a wave. This belief changed when it was later discovered that light also has particle-like characteristics. This is called the wave-particle duality of light.

First, Let’s look at the WAVE nature of light

Light as a WAVE

Wave Properties

A

Lesser frequency

greater frequency

crest

origin

trough

Properties of Light

• The significant feature of wave motion is its repetitive nature, which can be characterized by the measurable properties of wavelength and frequency.

Waves

• Wavelength () - length of one complete wave

• Frequency (f) - # of waves that pass a point during a certain time period– hertz (Hz) = 1/s

Properties of Light

• Electromagnetic radiation is a form of energy that exhibits wavelike behavior (wavelength, frequency, ect) as it travels through space. Together all forms of electromagnetic radiation form the electromagnetic spectrum.

Now let’s turn back to the first page of notes today…

• It’s time to draw the electromagnetic spectrum!

EM Spectrum

LOW

ENERGY

HIGH

ENERGY

R O Y G. B I V

red orange yellow green blue indigo violet

On the electromagnetic spectrum, the lowest energy waves (longest wavelength and lowest frequency) are radio waves. The highest energy waves (shortest wavelength and highest frequency) are gamma rays.

• FREQUENCY and WAVELENGTH are INVERSELY proportional. (f ↑ ↓)

• ENERGY and FREQUENCY are DIRECTLY proportional. (E↑ f ↑)

Now, let’s look at the PARTICLE nature of light

Light as a PARTICLE

Properties of Light

• In the early 1900’s, scientists conducted experiments involving interactions of light and matter that could not be explained by the wave theory of light.

Properties of Light

• One experiment involved a phenomenon known as the photoelectric effect. The discovery of the photoelectric effect led to the description of light as having both wave and particle properties.

• A Quantum of light energy is called a PHOTON.

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)

C = latin for celeritas (speed)

Example 1

If the frequency of a wave is 500 hz, what is the wavelength?

C = λf

C = 3.0 x 108 m/s

f = 500 hz

λ = ?

3.0 x 108 = 500 x λ λ = 600,000 m

One sig fig or 6 x 105

Quantum Theory

E: energy (J, joules)h: Planck’s constant (6.626 10-34 J·s)f: frequency (Hz)

E = hf

The energy of a photon is proportional to its frequency.

Example 2

What is the energy of a wave if the frequency is 300. hz?

E = hf

f= 300. hz

h = 6.626 x 10-34

E = 300. x 6.626 x 10-34

E = 1.99 x 10-31 3 sig figs

Example 3

• If the energy of a wave is 9.00 x 10-19 J, find frequency and wavelength

E = hf

E= 9.00 x 10-19 hz

h = 6.626 x 10-34

9.00x 10-19 = 6.626 x 10-34 x f

f = 1.36 x 1015 hz 3 sig figs

If the energy of a wave is 9.00 x 10-19 J, find frequency and

wavelength C = λf

If f = 1.36 x 1015 hz

C = 3.00 x 108 m/s

3.00 x 108 m/s = λ x 1.36 x 1015

λ = 2.21 x 10 -7m 3 sig figs.

We are all connected!!!

• http://www.youtube.com/watch?v=XGK84Poeynk