Unit 3 - Atomic Structure Chapter 5 Addison/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 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/biograp hy/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 attract Like charges repel Objects pick up charges They discharge when touched to ground

Lightning New research all the time Great photography – Check YouTube http://www.pbs.org/wgbh/nova/sciencenow /3214/02.html http://www.teachersdomain.org/resource/p hy03.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 Tube Figure 02-03 Title: Cathode-ray tube. Caption: (a) In a cathode-ray tube, electrons move from the negative electrode (cathode) to the positive electrode (anode). (b) A photo of a cathode-ray tube containing a fluorescent screen to show the path of the cathode rays. (c) The path of the cathode rays is deflected by the presence of a magnet. Notes: Keywords: Cathode Ray Tube

Figure 02-04 Title: Cathode-ray tube with perpendicular magnetic and electric fields. Caption: The cathode rays (electrons) originate from the negative plate on the left and are accelerated toward the positive plate on the right, which has a hole in its center. A beam of electrons passes through the hole and is then deflected by the magnetic and electric fields. The three paths result from different strengths of the magnetic and electric fields. The charge-to-mass ratio of the electron can be determined by measuring the effects that the magnetic and electric fields have on the direction of the beam. Notes: Keywords:

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.mcgraw- hill.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

Figure 02-05 Title: Millikan's oil-drop experiment. Caption: A representation of the apparatus Millikan used to measure the charge of the electron. Small drops of oil, which had picked up extra electrons, were allowed to fall between two electrically charged plates. Millikan monitored the drops, measuring how the voltage on the plates affected their rate of fall. From these data he calculated the charges on the drops. His experiment showed that the charges were always integral multiples of 1.602 x 10-19 C, which he deduced was the charge of a single electron. Notes: Keywords:

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 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, including uranium 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/hist ory/curie/

Figure 02-06 Title: Marie Sklodowska Curie (1867-1934). Caption: When M. Curie presented her doctoral thesis, it was described as the greatest single contribution of any doctoral thesis in the history of science. Among other things, Curie discovered two new elements, polonium and radium. In 1903 Henri Becquerel, M. Curie, and her husband, Pierre, were jointly awarded the Nobel Prize in physics. In 1911 M. Curie won a second Nobel Prize, this time in chemistry. Notes: Keywords:

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

Figure 02-08 Title: Behavior of alpha (α), beta (β), and gamma (γ) rays in an electric field. Caption: The α rays consist of positively charged particles and are therefore attracted to the negatively charged plate. The β rays consist of negatively charged particles and are attracted to the positively charged plate. The γ rays, which carry no charge, are unaffected by the electric field. Notes: Keywords:

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

Figure 02-09 Title: J. J. Thomson's "plum-pudding" model of the atom. Caption: Thomson pictured the small electrons to be embedded in the atom much like raisins in a pudding or seeds in a watermelon. Ernest Rutherford proved this model wrong. Notes: Keywords:

Figure 02-10 Title: Rutherford's experiment on the scattering of α particles. Caption: The red lines represent the paths of the α particles. When the incoming beam strikes the gold foil, most particles pass straight through the foil, but some are scattered. Notes: Keywords:

Explanation Find a source of alpha particles Aim them at a piece of gold foil 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 Movie: Empty Space

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 Movie: Empty Space – next slide

Atom is Empty Space From NOVA

Figure 02-11 Title: Rutherford's model explaining the scattering of α particles. Caption: The gold foil is several thousand atoms thick. Because most of the volume of each atom is empty space, most α particles pass through the foil without deflection. When an α particle passes very close to a gold nucleus, however, it is repelled, causing its path to be altered. Notes: Keywords:

Modern Atomic Theory There 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 VERY SMALL LARGE -1 NONE +1 ELECTRON NEUTRON PROTON

Planetary Model Proposed by Rutherford Electrons orbit nucleus like planets around sun Atoms are neutral so #protons = #electrons Charge 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 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 126C 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

Figure 02-13 Title: A mass spectrometer. Caption: Cl atoms are introduced on the left side of the spectrometer and are ionized to form Cl+ ions, which are then directed through a magnetic field. The paths of the ions of the two isotopes of Cl diverge as they pass through the magnetic field. As drawn, the spectrometer is tuned to detect 35Cl+ ions. The heavier 37Cl+ ions are not deflected enough for them to reach the detector. Notes: Keywords:

Animation http://www.colby.edu/chemistry/OChem/D EMOS/MassSpec.html

Figure 02-14 Title: Mass spectrum of atomic chlorine. Caption: The fractional abundances of the 35Cl and 37Cl isotopes of chlorine are indicated by the relative signal intensities of the beams reaching the detector of the mass spectrometer. Notes: Keywords: