History of the Atomic Theory Unit 3 L.1 The Atom

Aristotle & other Early Thinkers…. Thought all things (matter) was made up of the 4 “elements” – earth, water, air, & fire

Democritus, 440 B.C. Greek Philosopher

First Theory Proposed – Explain the quote: “The material cause of all things that exist is the coming together of atoms and void.”

Democritus proposed that: Atoms are the “uncuttable” particle. Name comes from “atomos” (Greek for “Indivisible”) Atoms are always moving (~ Brownian motion)

Why his ideas were not accepted Aristotle did not accept atoms because you could not observe them No experiments! It took over 2000 years for this idea to re-emerge

1. John Dalton (1808) (The Age of Enlightenment!) Other scientists had shown that matter is conserved(Lavoisier), and that substances combine in a set ratio(Proust): e.g. water = 2 H’s and 1 O always Dalton asked “WHY?” He also came up with the Law of Multiple Proportions, as well a Dalton’s Atomic Theory. He experimented, & produced empirical data!

Dalton’s Atomic Theory All substances are made of atoms.

2. Atoms cannot be created, divided nor destroyed. 3. Atoms of the same element are exactly alike 4. Atoms join with other atoms to make new substances

Dalton’ Atom model. Basically, a tiny solid ball, different sizes for the different elements.

"Daltons symbols". Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Daltons_symbols.gif#/media/File:Daltons_symbols.gif

2. J. J. Thomson-1897 Worked with a Cathode Ray Tube

Cathode Ray(Crookes) Tube "JJ Thomson Crookes Tube Replica" by Kurzon - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:JJ_Thomson_Crookes_Tube_Replica.jpg#/media/File:JJ_Thomson_Crookes_Tube_Replica.jpg The cathode rays (blue) were emitted from the cathode, sharpened to a beam by the slits, then deflected as they passed between the two electrified plates.

Thomson’s Conclusions Atoms are not the smallest particle! They are divisible! They are made up of negatively charged particles(corpuscules) embedded in positively charged matrix(the pudding). Atoms are neutral. All elements used showed the same results!

Thomson’s Atomic Theory The “Plum Pudding” Model The atom is mostly positively charged matrix with small, negatively charged particles, corpuscles, embedded throughout

The Plum Pudding Model Or negative corpuscles Matrix

Or like Chocolate chip ice-cream….(yum!)

Homework  Copy labeled diagrams of Thomson’s Experiment and Atom Model.

Homework  Copy and Answer the following in yr ntbk What discovery demonstrated that atoms are not the smallest particles? What did Dalton do in developing his theory that Democritus did not do? Analyzing Methods. Why was it important for Thompson to repeat his experiment?

E.R. Ernest Rutherford (1909) About the Gold Foil Experiment, “It was quite the most incredible event that has ever happened to me in my life.” E.R.

Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge. Note that the image is not to scale; in reality the nucleus is vastly smaller than the electron shell.

Rutherford’s Conclusions The atom is mostly empty space! (The void!) There is a positively charged center to the atom

Rutherford's Atom Model

Compare Thompson’s & Rutherford’s Models "Geiger-Marsden experiment expectation and result" by Kurzon - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Geiger-Marsden_experiment_expectation_and_result.svg#/media/File:Geiger-Marsden_experiment_expectation_and_result.svg

James Chadwick - Discovered Neutrons – neutral particles in the nucleus(center) Was Rutherford’s student.

The Bohr (Planetary) Model Electrons travel around the nucleus in definite paths, or orbits Each path/orbit is located in levels at certain distances Electrons can jump from one path(energy level) to the next), but can’t stay in between paths Like a ladder!

Bohr’s Planetary Model

The Modern Theory: Think of bees around a beehive……! Electrons do not follow defined paths, but move in ‘regions’ around the nucleus Exact path and location of an electron is not known. Regions are where electron is “likely” to be found These regions are called “electron clouds” Think of bees around a beehive……!

"Helium atom QM" by User:Yzmo - Own work. Licensed under CC BY-SA 3 "Helium atom QM" by User:Yzmo - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Helium_atom_QM.svg#/media/File:Helium_atom_QM.svg

The Schrödinger and Heisenberg MODERN Cloud Model

Classwork  Copy and answer in notebook In what part of an atom is most of its mass located? What are 2 differences between the atomic theory described by Thomson and that described by Rutherford? Comparing Concepts. Identify the difference in how Bohr’s theory and the Modern theory describe the location of electrons.

Give 2 similarities and 2 differences between Compare and Contrast Give 2 similarities and 2 differences between J.J. Thompson’s Model and The Modern Model of the Atom

Classwork and Homework  Do all questions in text p158-166 Not the Lesson Review.