Cathode Rays Anode

Unit 3 - Atomic Structure Chapter 5 Prof.devender singh Roorkee

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 Dalton Proposed in 1803 Compilation of other people’s work and a little of his own Still true except for one part Good biography: http://www.slcc.edu/schools/h um_sci/physics/whatis/biogra phy/dalton.html

Dalton’s Atomic Theory Each element is composed of tiny 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:

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 attract Like charges repel Objects pick up charges They discharge when touched to ground

Lightning He felt that lightning was static electricity on a larger scale. http://news.nationalgeograp hic.com/news/2006/05/0504 _060504_lightning_video.ht ml

Electricity Research after Franklin Physicists liked to zap things in the mid1800’s Cathode Ray Tube was device used by many (diagram) Same device used as TV screen

Cathode Ray Tube

How it Works Metal is electrified in an evacuated tube All metals gave a greenish ray going to the positive electrode Ray 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 - 1896 Used cathode ray tube to determine amount of deflection Determined that particle has a negative charge Determined the charge to mass ratio of the particle Animation: http://highered.mcgrawhill.com/sites/0072512644/student_view0/chapte r2/animations_center.html#

Finding the Charge of an Electron American physicist – Robert Millikan Famous Oil Drop Experiment (handout) See animation

Millikan Oil Drop Experiment

Explanation Drops of oil are sprayed into a chamber X-rays cause electrons to be formed and they cling to oil (in varying numbers) Drops pass through a set of electric plates which can have a charge put on them 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 grams Virtually 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” something Rock was pitchblende which contains radium

Characteristics of radiation Spontaneously emitted by some elements Studied by Marie and Pierre Curie They discovered several elements (uranium, radium, and polonium) Atom emits radiation and then changes This gave clues to what atom is actually made of

Marie and Pierre Curie Good site http://www.aip.org/history/curie/

Further Research on Radiation Ernst Rutherford is brought to Cavendish Lab in early 1900’s Studied radioactivity Analyzed nature of radiation Handout

Three Kinds of Radiation

Magic Bullet Alpha Particle chosen Right size Could be detected afterwards Helium nucleus – 2 protons and 2 neutrons +2 charge

Gold Foil Experiment Rutherford got grad students to design set up Geiger and Marsden Wanted to confirm Thomson’s “Plum Pudding” model of the atom – electrons stuck in positive pudding Handout

Plum Pudding Model

Rutherford’s Experiment

Explanation Find a source of alpha particles Aim them at thin gold foil (like bullets at tissue paper) Check to see where they come out by counting fluorescent spots

Results Most went through Very small number were deflected almost straight back Only explanation was that all matter was concentrated into a dense nucleus Nucleus had a positive charge Electrons traveled in empty space around the nucleus

Modern Atomic Theory Three major particles – protons, electrons, and neutrons Now there are many subatomic particles (nuclear physics) Proton also discovered with the cathode ray tube in 1900 Neutron discovered last by Chadwick in 1935

Summary of Particles PROTON

NEUTRON

ELECTRON

+1

NONE

-1

LARGE

LARGE

VERY SMALL

NUCLEUS

NUCLEUS

OUTSIDE NUCLEUS

Planetary Model Proposed by Rutherford Electron orbits nucleus like a planet around the sun Atoms are neutral so #protons = #electrons Charge on electron: 1.602 X 10-19 C is simplied to “1” Mass of proton: 1.67 X 10-24 g is simplified to 1 amu (atomic mass unit)

Atomic Number Defined by Henry Mosely (1887-1915) Student of Rutherford Unique for each element Number of protons in the nucleus What is atomic number of nitrogen? Uranium?

Isotopes Means “type or form” All atoms of the same element have the same number of protons There may be different types of the same elements, called isotopes Vary in number of neutrons, mass Try Carbon-12 and Carbon-14

Characteristics of Isotopes Varying masses Same chemical and physical properties Some may be unstable, and therefore radioactive

Symbol Carbon-12 12 is mass number, # protons + # neutrons Also written 12

C

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Mass # - Atomic # = # of neutrons

Atomic Mass Mass of an isotope in amu’s is simply the mass number Most elements have several common isotopes Mass on periodic table must reflect this, that is why there are decimals Weighted 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 element Strip electrons (positive charge) Sort by size with a magnetic field Computer counts the isotope and gives a readout

Mass Spectrometer

Animation

http://www.colby.edu/chemistry/OChem/DEMO

Computer Printout