2 Atomic Theories Atomic Theory – A Short History Fifth Century, BCE DemocritusBelieved matter was composed of very small, individual particles that were indestructibleHe called them “atomos” (meaning uncuttable)His ideas persisted for centuries even though there was no experimental proof
3 Atomic Theories JOHN DALTON - 1808 Revised early Greek ideas into testable scientific theoryBased his Atomic Theory onthree important concepts:1. Law of Conservation of Mass2. Law of Multiple Proportions3. Law of Definite Proportions
4 Atomic Theories Law of Conservation of Mass States that mass cannot be created or destroyedthe mass of the reactants equals the mass of the products
5 Atomic Theories Law of Multiple Proportions (pg 77) States that when two elements combine to form two or more compounds, the mass of one element that combines with a given mass of the other is in the ratio of small, whole numbers
6 Atomic Theories Law of Definite Proportions States that a chemical compound always contains the same elements in exactly the same proportions by massExample: water
7 Atomic Theories Dalton’s Principles All matter is composed of extremely small particles called atoms which cannot be subdivided, created or destroyedAtoms of a given element are identical in their physical and chemical propertiesExample: all water molecules freeze at 0 deg C and react with explosively with sodium
8 Atomic Theories Dalton’s Principles (continued) 3. All atoms of one element are different from those of any other element4. Atoms combine in simple, whole-numbered ratios to form compoundsBased on the Laws of Definite and Multiple Proportions
9 Atomic Theories Dalton’s Principles (continued) 5. In chemical reactions, atoms are combined, separated or rearranged but NEVER created, destroyed or changedBased on The Law of Conservation of Mass
10 Atomic Theories Dalton’s Principles (continued) 6.Atoms of one element are never changed into atoms of another element in a chemical reaction
11 (Protons, Neutrons, Electrons) Atomic TheoriesDalton, however, did all this work in the early 1800’s without ever knowing about subatomic particles!(Protons, Neutrons, Electrons)
12 MMMMM…..That plum pudding looks delicious! AtomsThe Adventures of J.J. Thomson, Plum Pudding and the Electron!MMMMM…..That plum pudding looks delicious!
13 Atoms Chapter One: The Discovery of the First Subatomic Particle: The ElectronJJ Thomson was an EnglishPhysicist who was studyingelectricity. He decided to builda glass tube which contained twometal plates at either end. Hethen pumped all the air out of the tube and attached a voltagesource to either end. A glowing beam came out of the cathode and struck theanode and the walls of the glass tube. He called this a cathode ray.
14 Atoms JJ Thomson’s Cathode Ray Tube (CRT) Anode – attached to the positive terminal of thevoltage sourceCathode – attached to theNegative end of the voltagesource
15 AtomsTo test this, he placed a magnet near the tube and the beam was deflected away from it, proving it was negatively charged.He also placed a small paddlewheel in the tube, which turned when hit by the beam. That meant the particles had mass.The particles are called “electrons.”
16 Atoms Where did the beam come from? Because most of the air had been removed from the tube, and the beam originated at the negatively charged cathode, Thompson reasoned the ray must be negatively charged.
18 AtomsAtoms have no charge, yet they gave off negatively charged electrons. The scientists hypothesized that there must be positive charges also included in the atom.JJ Thomson proposed the “Plum Pudding” model of the atom, which states that electrons were embedded in amass of positively charged matter.
19 AtomsIn 1909, one of his students, Ernest Rutherford, disproved the “Plum Pudding” model by doing is famous “Gold Foil” experiment.BLING!
20 Atoms Atomic Nuclei History 1911: Ernest Rutherford does his famous experiment“Gold Foil Experiment”He shot radioactive alpha particles at an extremely thin gold foilMost particles went right through to the other sideand were detected on thescreen behind it.Only 1 in 8000 bouncedback.
24 Atoms Rutherford’s Conclusions: Most atoms are made of empty space All of an atom’s positive charge and almost all of its mass is contained in an extremely smallareaHe called this a “nucleus”Eventually, other scientists later determined more details about the nuclei of atoms.
25 Atoms Rutherford’s Model of the Atom: Electrons orbit the nucleus just as planetsorbit the sun. However, he could not explainwhy the negatively charged electrons did notcrash into the positive charged nucleus.
26 AtomsTwo years later, Danish physicist, Niels Bohr, proposed the Bohr Model of the atom
27 Atoms Bohr’s Model has the following characteristics: Electrons are located certain distances from the nucleusEach distance is a certain quantity of energy that the electron can haveElectrons closest to the nucleus have the lowest energy, while the ones further away are in higher energy levelsThe difference between two energy levels is called a quantum of energy.Electrons can be only in an energy level, NOT between levels.Electrons do not give off energy while they are in an energy level.(Page 91)
28 Atoms Various Scientists: Created the quantum mechanical model: Electrons are not located in specific, fixed, circular orbitsElectrons have certain allowed energies and one can determine how likely it is to find an electron with that particular amount of energyLooks at probabilities of locating an electron