2Catalyst (5 min) What does the atomic number of each atom represent? What does the mass number of each atom represent?How many protons and neutrons are in the nuclei of the following elements?phosphorus (P)cadmium (Cd)lead (Pb)
4``Nothing exists except atoms and empty space; everything else is opinion.'' Democritus
5Democritus 460 – 370 B.C.the first to propose that matter was not infinitely divisibleBelieved that matter was made up of tiny indivisible particles called atomosEverything is composed of small atoms moving in a voidIdeas rejected by leading philosophers because they did not believe in empty space
7John Dalton 1766-1844 English School Teacher Introduced his ideas in 1803Used experiments that studied the ratio that elements combine in chemical reactions to transform Democritus’s ideas on atoms into a scientific theoryBecame the head of his quaker school at 12, was also reading newton’s principia at that time. French chemist pelletier traveled to meet the famous scientist and was shocked to see him teaching arithmetic to elementary boys.
8Dalton’s Atomic Theory All elements are composed of tiny indivisible particles called atomsAtoms of the same element are identical. The atoms of any one element are different from those of any other element.Atoms of different elements can physically mix together or chemically combine to form compoundsIn a chemical reaction, atoms are separated, combined or rerranged. Atoms of one element are never changed into atoms of another element.Atoms cannot be created, divided into smaller particles or destroyed.
10But if the scientists couldn’t SEE the atom, how did they even know it existed?!
11``To the electron: may it never be of any use!'' J.J. Thomson
12J.J. ThomsonHypothesized that the structure of an atom was like ‘plum pudding’Discovered the electron in the Cathode Ray ExperimentDiscovered that this subatomic particle has a negative chargeDiscovered isotopes 1913
13Cathode Ray Tube Experiment Identified the first subatomic particle, the electronDetermined the charge to mass ratio of the electronMass of the electron is MUCH less then the mass of a hydrogen atomMeant there are particles smaller than atoms!
15Cathode Ray Experiment Altering the gas in the tube and the material used for the cathode have no effectProves: the ray’s negative particles (electrons) are found in all forms of matterCathode Ray is deflected toward the positively charged plate by an electric fieldProves: the particles in the ray must have a negative charge
17“Fullness of knowledge always means some understanding of the depths of our ignorance; and that is always conducive to humility and reverence. “Robert Millikan
18Robert A. MillikanConduced oil drop experiment to determine the mass and quantity of an electrons charge
19“You should never bet against anything in science at odds of more than about 1012 to 1.” Ernest Rutherford
20Ernest Rutherford 1871-1937 Student of J.J. Thomson Conducted the gold foil experimentDiscovered that an atom consists of mostly empty space through which the electrons moveConcluded there is a tiny dense region called the nucleus which is located in the center of the atoms and contain all its massElectrons are held in the atom by attraction with positively charged protons.Wasn’t particularly great at mathematics, in lectures would often get lost in his own equations and ask his students to finish. According to colleague James chadwick, also wasn’t particularly clever at experimentation. He was simply tenacious and open minded. He was prepared to work longer and harder then most. His greatest discovery came when he spent tedious hours counting alpha particles – work that would normally be farmed out.
21Gold Foil Experiment Shot alpha particles at a thin piece of gold foil Most of particles passed through foil which indicated that atoms are composed of mostly empty spaceAlpha particle sometimes bounced back which indicated they were deflected from the nucleus
26But what keeps the electrons from spiraling into the nucleus?
27``An expert is a person who has made all the mistakes that can be made in a very narrow field.'' Enter niels bohrNiels Bohr
28Niels BohrDeveloped theory that electrons orbited the nucleus in neat, planet-like orbitsPlanetary Model 1913Nucleus surrounded by orbiting electrons at different energy levelsElectrons have definite orbitsUtilized Planck’s Quantum Energy theoryWorked on the Manhattan Project (US atomic bomb)Bohr was a student of Rutherford’s, who postponed his honeymoon when he first came up with the idea that electrons had specific orbits. One the nobel prize for his work the year after einstein
30How can the protons bundle inside the nucleus without blowing the atom apart?
31James Chadwick 1932showed that the nucleus contained a neutral particle called the neutron.Enter James chadwick, a colleague of rutherfords at cambridge who spent 11 years of his life to finding the neutron
32Quantum Model (learn next class) Developed from wave equation by Erwin Schrodinger (1926)
33Fill in the chart using your notes (5 min) NameDemocritusDaltonThomsonRutherfordBohrTime FrameKey PointsModel
34Who Did It? (5 min)Six scientists were sitting around a table discussing their contributions to Atomic Theory. Use the clues to determine where they were sitting and what contribution they made.
35Subatomic Particles Table Summary In your notes fill in the table comparing the relative charge, mass and location of each of the subatomic particles (3 min)
36In your notes, make a timeline graph showing the development of modern atomic theory (5 min)
37You Do:Complete the following book problems on a separate sheet of paper: p.104 #1, 4 p.105 #13, 15 p.122 #39, 62
38ClosingHow did the concept of the atom change over time and what key experiments and scientists led to our current understanding of the atom?
39HomeworkDescribe the location of the electrons in Thompson’s ‘plum pudding’ model of the atomIn the Rutherford atomic model, which subatomic particles are located in the nucleus?Describe Thomson’s and Millikan’s contributions to atomic theory.