Unit 3 - Atomic StructureUnit 3 - Atomic Structure

Chapter 5 Chapter 5

Addison/Wesley TextbookAddison/Wesley Textbook

Earliest Model

450 BC – Democritis, a Greek philosopher, first uses the word “atomos” which means indivisible

Definition of atom today – Smallest particle of an element that still retains properties of that element

Late 1700’s

Lavoisier – Law of Conservation of Matter

Proust – Law of Constant Composition

This says that the same compound from any source always contains the same elements in the same proportion by mass

First Atomic Theory

John DaltonProposed in 1803Compilation of other people’s

work and a little of his ownStill true except for one partGood biography:

http://www.slcc.edu/schools/hum_sci/physics/whatis/biography/dalton.html

Dalton’s Atomic Theory

Each element is composed of tiny, indivisible atoms

Each element’s atoms are the same and unique

Atoms are only rearranged in any chemical reaction

A compound has the same number and kind of atom.

The Atom Today

Since 1981 we have “seen” atoms with a scanning tunneling microscope.

Uses a fine tip and a stable environment to trace the electronic field and image it on a computer

Lots of galleries on the Web:http://physics.nist.gov/GenInt/STM/

fig1.html

A Good Example

Discovery Atomic Structure

Early research comes from physicists’ work on electricity

“Electricity” is property of “electron”, which is amber

In ancient Greece, pieces of amber were rubbed and static electricity discharged

Ben Franklin did early research in late 1700’s

Great Experimenter

His work

Discovered two kinds of charges, positive and negative

Opposite charges attractLike charges repelObjects pick up chargesThey discharge when touched to ground

Lightning

New research all the timeGreat photography – Check YouTubehttp://www.pbs.org/wgbh/nova/sciencenow

/3214/02.html

http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.lightning/

Electricity Research after Franklin

Physicists liked to zap things in the mid 1800’s

Cathode ray tube was device used by many (diagram)

Same device used as TV screen

Cathode Ray TubeCathode Ray Tube

How it Works

Metal is electrified in an evacuated tubeAll metals gave a greenish ray going to the

positive electrodeRay could be attracted by a positive

charge, repelled by a negative charge.It could actually make a paddle wheel

move - particle

Discovery of the Electron

JJ Thomson – Cavendish Lab - 1896Used cathode ray tube to determine amount of

deflectionDetermined that particle has a negative chargeDetermined the charge to mass ratio of the

particleAnimation:

http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#

Finding the Charge of an Electron

American physicist – Robert MillikanFamous Oil Drop Experiment (handout)See animation

Explanation

Drops of oil are sprayed into a chamber X-rays cause electrons to be formed they cling to oil (in varying numbers) Drops pass through a set of electric plates which have a charge Millikan adjusted charge to balance the charge on each drop Found the greatest common factor

Conclusion

Charge on an electron is 1.60 X 10-19 Coulombs

Mass of an electron is 9.11 X 10-19 gramsVirtually without mass

Discovery of Radiation

Henri Becquerel accidentally discovered radiation in 1896

Photographic plate wrapped and put in drawer for weekend gets exposed

Rock was “radiating” somethingRock was pitchblende which contains

radium

Characteristics of radiationSpontaneously emitted by some elementsStudied by Marie and Pierre CurieThey discovered several elements,

including uranium and poloniumAtom emits radiation and then changesThis gave clues to what atom is actually

made of

Marie and Pierre Curie

Good sitehttp://www.aip.org/hist

ory/curie/

Further Research on Radiation

Ernst Rutherford is brought to Cavendish Lab in early 1900’s

Studied radioactivityAnalyzed nature of radiationHandout

Magic Bullet

Alpha Particle chosenRight sizeCould be detected afterwardsHelium nucleus – 2 protons and 2

neutrons+2 charge

Gold Foil Experiment

Rutherford got grad students to design set up

Geiger and MarsdenWanted to confirm

Thomson’s “Plum Pudding” model of the atom – electrons stuck in positive pudding

Handout

Explanation

Find a source of alpha particlesAim them at a piece of gold foilCheck to see where they come out by

counting fluorescent spots

Results

Most went throughVery small number were deflected almost

straight backOnly explanation was that all matter was

concentrated into a dense nucleus Nucleus had a positive chargeElectrons traveled in empty space around

the nucleusMovie: Empty Space

Results

Most went throughVery small number were deflected almost

straight backOnly explanation was that all matter was

concentrated into a dense nucleus Nucleus had a positive chargeElectrons traveled in empty space around

the nucleusMovie: Empty Space – next slide

Atom is Empty SpaceFrom NOVA

Modern Atomic TheoryThere are 3 major subatomic particles

(protons, neutrons, and electrons).

There are basic particles that make these up but we will not discuss them

The proton also came from the cathode ray tube

The neutron was discovered by Chadwick, a student of Rutherford in 1935.

Summary of Particles

OUTSIDE

NUCLEUS

NUCLEUSNUCLEUS

VERY SMALLLARGELARGE

-1NONE+1

ELECTRONNEUTRONPROTON

Planetary Model

Proposed by RutherfordElectrons orbit nucleus like planets around sunAtoms are neutral so

#protons = #electronsCharge on electron: 1.602 X 10-19 C or “1”Mass of proton: 1.67 X 10-24 g or 1 amu (atomic mass unit)

Atomic Number

Defined by Henry Mosely (1887-1915)Student of RutherfordUnique for each elementNumber of protons in the nucleusWhat is atomic number of

nitrogen? Uranium?

Isotopes

Means “type or form”All atoms of the same element have the

same number of protonsThere may be different types of the same

elements, called isotopesVary in number of neutrons, massTry Carbon-12 and Carbon-14

Characteristics of Isotopes

Varying massesSame chemical and

physical propertiesSome may be

unstable, and therefore radioactive

Symbol

Carbon-12 12 is mass number,

# protons + # neutronsAlso written

126C

Mass # - Atomic # = # of neutrons

Atomic Mass

Mass of an isotope in amu’s is simply the

Mass numberMost elements have several common

isotopesMass on periodic table must reflect this,

that is why there are decimalsWeighted average calculation (like grades)

Calculation

Multiply the mass of each isotope by its abundance as a decimal

Add each of these to get weighted average

Try one

Mass Spectrometer

Inject gaseous form of elementStrip electrons (positive charge)Sort by size with a magnetic fieldComputer counts the isotope and gives

a readout

Animation

http://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.html