JOURNAL #37

What is the Bohr model? Refer to your textbook as needed.

INTRODUCTION TO ATOMIC STRUCTURE

TODAY’S LEARNING GOAL:

We will describe the Bohr model.

LIGHT AND SOUND In 1905 Einstein derived an equation

relating mass and energy. You should be familiar with this equation:E = mc2

This equation has been changed a bit since, but a relationship has now, for the first time in history, been established between matter and energy and between physics and chemistry

LIGHT AND SOUND Because Einstein was able to prove a

relationship between matter and energy, we today can understand more about matter by learning all about energy.

We can see this relationship between energy and matter specifically when we look at some of the unusual properties of the wave nature of energy

THE NATURE OF LIGHT: WAVE OR PARTICLE? The nature of light has been debated

for thousands of years.

In the 1600’s Newton argued that light was a stream of particles. Huygens countered that it was a wave. Both had good arguments, but neither could prove it

YOUNG’S DOUBLE SLIT EXPERIMENT In 1801, Thomas Young settled the

argument with his Double Slit Experiment.

We will take a closer look at the results of this experiment, but first we need to understand waves…

http://video.mit.edu/watch/thomas-youngs-double-slit-experiment-8432/

YOUNG’S DOUBLE SLIT EXPERIMENT Young tested to see if light was a wave by

seeing if it created an interference pattern when it went through the 2 slits, like a wave would.

The double slit experiment relies on 2 properties of waves:Diffraction Interference

Each slit generates a new wave due to diffraction. Those waves then either constructively or destructively interfere on a far away screen.

LET’S REVIEW WHAT WE’VE LEARNED:

What principle is responsible for light spreading as it passes through a narrow slit?A. DiffractionB. PolarizationC. DispersionD. interference

Answer: A

DOUBLE SLIT MAXIMA AND MINIMA Interference occurs because each

point on the screen is not the same distance from both slits. Depending on the path length distance, the wave can interfere constructively or destructively

Bright lines- Maxima Dark lines -Minima

LET’S REVIEW WHAT WE’VE LEARNED:

What principle is responsible for alternating light and dark bands when light passes through 2 or more narrow slits?A. DiffractionB. PolarizationC. DispersionD. interference

Answer: D

IF LIGHT IS A WAVE…WHAT IS WAVING? In sound waves, we know it’s the

pressure in the air. In any simple harmonic motion there

has to be 2 forms of energy and a means to move between them.

But what does that mean for light?

ACCELERATING CHARGES CREATE E-M WAVES A great way to start this up is to make a

charge (like an electron) accelerate. That creates a changing electric field

which creates a changing magnetic field Which creates a changing electric field…

which creates a changing magnetic field Which creates a changing electric field…

which creates a changing magnetic field Which creates a changing electric field…

which creates a changing magnetic field

CREATING ELECTROMAGNETIC WAVES

In physics we learned that changing magnetic field produces an electric field.

Changing an electric field produces a magnetic field as well

Once these changing fields are first started up, they keep creating each other…and travel on their own.

These traveling fields are called electromagnetic waves.

LET’S REVIEW WHAT WE’VE LEARNED:

An electric field is produced by a A. Constant magnetic fieldB. Changing magnetic fieldC. Either a constant or a

changing magnetic fieldD.Gravitation

Answer: B

LET’S REVIEW WHAT WE’VE LEARNED:

A changing electric field will produce aA. CurrentB. Gravitation fieldC. Magnetic field

Answer: C

LIGHT IS AN ELECTROMAGNETIC WAVE

Young showed that light is a wave. Electromagnetic waves exist and travel at

the speed of light Light was shown to be an electromagnetic

wave The frequency of an electromagnetic wave

is related to its wavelength. For electromagnetic waves, in a vacuumC = λ VC = speed of light, λ = wavelength (m) V = frequency

ELECTROMAGNETIC SPECTRUM

All electromagnetic radiation travels at the same velocity: the speed of light ©

C= 3.00 x 108 m/s

LET’S REVIEW WHAT WE’VE LEARNED:

All electromagnetic waves travel through a vacuum at A. Same speedB. Speeds that are proportional

to their frequencyC. Speeds that are inversely

proportional to their frequency

D. Speeds too slow to measureAnswer: A

WHY DOES THIS ALL MATTER? Light behaves like a wave and so does matter! Electrons fired on at a time towards two slits

show the same interference pattern when they land on a distant screen.

Since all matter and energy are now understood they share certain properties (wavelength for example) the interaction of matter with light has allowed us to probe the nature of matter itself, from the structure of the atom to the unique behavior of molecules. The structure and behavior of matter is the domain of the chemist!!

QUANTUM A quantum of energy is the minimum

quantity of energy that can be lost or gained by an atom

The relationship between a quantum of energy and the frequency of radiation is E=hv E= energy (joules), V= frequency and h= is a

funamental physical constant (planck’s constant) 6.626 x 10-34J.s

THE HYDROGEN ATOM LINE EMISSION SPECTRUM

When current is passed through a gas at low pressure, the potential energy of some of the gas atoms increases.

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

A state in which an atom has a higher potential energy than it has in its ground stat is an excited state.

When an excited atom returns to its ground state, it gives off the energy it gained in the form of electromagnetic radiation. (ex: neon lights)

THE HYDROGEN ATOM LINE EMISSION SPECTRUM

When a narrow beam of emitted light was shined through a prism, it was separated into four specific colors of the visible spectrum.

The four bands of light were part of what is known as Hydrogen’s line-emission spectrum.

Attempts to explain why hydrogen atoms gave off only specific frequencies of light is called quantum theory.

THE HYDROGEN ATOM LINE EMISSION SPECTRUM

When an excited Hydrogen atom falls to its ground state or to a lower energy excited state, it emits a photon of radiation.

The energy of this photon (E=hv) is equal to the difference in energy between the atoms initial state and its final state.

The fact that hydrogen atoms emit only specific frequencies of light indicated that the energy differences between the atom’s energy states were fixed. (hydrogen atoms exists only in very specific energy states)

BOHR MODEL The puzzle of the hydrogen atom spectrum

was solved by Niels Bohr. The electron can circle the nucleus only in

allowed paths, or orbits. When the electron is in one of these orbits, the

atom has a definite, fixed energy. The electron is in its lowest energy state when

it is in the orbit closest to the nucleus. This orbit is separated from the nucleus by a

large empty space where the electron cannot exist.

The energy is higher when the electron is in orbits that are farther from the nucleus.

BOHR MODEL Example:

When standing on a ladder, the higher up you go, the more potential energy you have.

Your energy cannot correspond to standing between 2 steps because you cannot stand in midair.

The same way for electrons. They can be in one orbit or another, but not in between.

BOHR MODEL How does this explain the observed

spectral lines? While in a given orbit, the electron is

neither gaining nor losing energy However, it can move to a higher energy

orbit by gaining an amount of energy equal to the difference in energy between the higher energy orbit and initial lower energy orbit.

BOHR MODEL When a H atom is in excited state, its

electron is in one of the higher energy level orbits.

When the electron falls to a lower energy level, a photon is emitted – called emission.

Absorption is the process in which energy must be added to an atom in order to move an electron from a lower energy level to a higher energy.

YOUR ASSIGNMENT Chapter 4 section 1 review (pg 103) Do problems 1-5.