Ch. 6Electronic Structure

and the Periodic Table

Part 1: Light, Photon Energies, and Emission Spectra

• Compare the wave and particle natures of light.

radiation: the rays and particles —alpha particles, beta particles, and gamma rays—that are emitted by radioactive material

• Define a quantum of energy, and explain how it is related to an energy change of matter.

• Contrast continuous electromagnetic spectra and atomic emission spectra.

electromagnetic radiation

wavelength

frequency

amplitude

electromagnetic spectrum

Light, a form of electronic radiation, has characteristics of both a wave and a particle.

quantum

Planck's constant

photoelectric effect

photon

atomic emission spectrum

Review

We left off with Rutherford and Chadwick discovering nucleus and neutrons

This proved that J.J. Thomson’s “plum pudding” model of the atom was incorrect.

So where do we go next?

Rutherford’s model of the atom

Rutherford Model: Problems Nucleus surrounded by electrons.

How did e- fill up space surrounding a (+) nucleus?

What prevented the electrons from being pulled right into the nucleus?

To answer this, must understand relationship of light and electrons

The Atom and Unanswered Questions (cont.)

• In the early 1900s, scientists observed certain elements emitted visible light when heated in a flame.

• Analysis of the emitted light revealed that an element’s chemical behavior is related to the arrangement of the electrons in its atoms.

At Rutherford Model

Electrons pictured as particles

Light pictured as waves

Discovered electrons have wave-like properties, and light has particle-like properties

What do we do? Describe electrons as having

dual wave-particle nature (or properties)Sometimes it acts like a particleSometimes it acts like a wave

Has stood up against many experiments to prove it wrong

Explains how electron isn’t pulled into nucleus.

Electromagnetic (EM) radiation Light as a wave

Form of Energy that exhibits wavelike behavior as it travels

Speed = 3.00 x 108 m/s (speed of light through air)

The Wave Nature of Light (cont.)

• The wavelength (λ) is the shortest distance between equivalent points on a continuous wave.

• The frequency (ν) is the number of waves that pass a given point per second.

• The amplitude is the wave’s height from the origin to a crest.

The Wave Nature of Light (cont.)

Wavelength

Wavelength : distance between corresponding points on a wave

λ = wavelength

λ is the Greek letter lambda

Wavelength is usually in nanometers (nm) or meters

Wavelength

http://en.wikipedia.org/wiki/Wavelength

Frequency

(ν), the Greek letter nu (not Vee)

Number of waves that pass a given point in a specific amount of time

Frequency units are in Hertz (Hz) or 1/ seconds ( /s )

Relationship between wavelength and frequency c = λν

Where c = speed of light

Correlation? As wavelength decreases, frequency increases

As wavelength increases, frequency decreases

This is an inverse relationship

The Wave Nature of Light (cont.)

• The speed of light (3.00 108 m/s) is the product of it’s wavelength and frequency c = λν.

What is the frequency of a wave with wavelength of 100 nm?

The Particle Nature of Light

• The wave model of light cannot explain all of light’s characteristics.

• Matter can gain or lose energy only in small, specific amounts called quanta.

• A quantum is the minimum amount of energy that can be gained or lost by an atom.

Max Planck (1900) Described light as having particle-like

properties

When hot object loses energy, it doesn’t do it continuously as it would if it were a wave

Loses energy in form of a quanta

Quanta?

Quantum – finite quantity of energy that can be gained or lost by an atomSpecific: if it costs $1.25 to get a soda from

machine, and you give it $1.00, do you get a a soda?

Photon – individual quantum of light

The Particle Nature of Light (cont.)

• The photoelectric effect is when electrons are emitted from a metal’s surface when light of a certain frequency shines on it.

Einstein

In 1905, said Planck’s work applied to all EM. Explains photoelectric effect –

must absorb photon with specific energy to dislodge an electron

When electron is dislodged, it must be in the form of a particle

But as it moves, (we see it as color), it is in the form of a wave

Shows dual nature of light (wave and particle)

The Particle Nature of Light (cont.)

• Albert Einstein proposed in 1905 that light has a dual nature.

• A beam of light has wavelike and particle-like properties.

• A photon is a particle of electromagnetic radiation with no mass that carries a quantum of energy.

Ephoton = h Ephoton represents energy.h is Planck's constant. represents frequency.

Energy of a Photon (or any wave of energy)

E = h ν

E = energy ( in joules, j) v = frequency h = Planck's constant

6.63 x 10 -34 J * s (Joule Seconds)

As frequency goes up, what happens to the energy?

What is the energy with a wave of frequency 1 x 1016 Hz?

Light through a prism

Continuous spectrumAll wavelengths in a given range are includedWhy we see rainbowsSeparated by wavelength

Electromagnetic spectrumConsists of all electromagnetic radiation,

arranged by increasing wavelengths

Light through a prism

http://spaceplace.nasa.gov/en/kids/misrsky/misr_sky.shtml

Electromagnetic Spectrum

Hydrogen Atom Spectrum

Pass high voltage through Hydrogen gas

Gas glows, and you can pass this light through prism

Creates a bright line spectrum or atomic emission spectra

Atomic Emission Spectra (cont.)

Atomic Emission Spectra (cont.)

• The atomic emission spectrum of an element is the set of frequencies of the electromagnetic waves emitted by the atoms of the element.

• Each element’s atomic emission spectrum is unique.

Hydrogen’s atomic emission spectra

Each line caused by light of a different wavelength

What causes the light?

Electrons, man

Electrons get boosted by voltage from ground state (or normal state) to excited state.

When they relax back down to ground (almost immediately), they give off certain amounts of energy

Line spectrum: produced when an electron drops from a higher energy orbit to a lower energy orbit

Ground State vs. Excited State

Ground StateThe state of lowest energy of an atom

Excited StateA state in which an atom has a higher

potential energy than its ground state

What causes the lines?

Each line = energy from electron as it drops from excited state to ground state

Energy of photon = difference in energy between ground and excited state.