The Answer

Atomic Structure Chemistry Fall 2012

Democritus Key idea: atom as a physically indivisible unit Key idea: atom as a physically indivisible unit “Atom” literally means “uncuttable” or “unable to be divided further” “Atom” literally means “uncuttable” or “unable to be divided further” Question: How did Democritus come to the atomic hypothesis? Question: How did Democritus come to the atomic hypothesis?

J.J. Thomson and the Electron Experiment: Used a cathode ray tube to show that negatively-charged “cathode rays” come from the negative plate Experiment: Used a cathode ray tube to show that negatively-charged “cathode rays” come from the negative plate Key idea: cathode rays were made of negatively charged electrons Key idea: cathode rays were made of negatively charged electrons

Thomson’s Plum Pudding Model Individual electrons arranged in a positively charged body Individual electrons arranged in a positively charged body Question: What was Thomson’s contribution to the atomic model? Question: What was Thomson’s contribution to the atomic model?

Ernest Rutherford and the Gold Foil Experiment Experiment Shoot positively charged alpha particles at gold foil Shoot positively charged alpha particles at gold foil With Plum Pudding model, alpha particles should go right through With Plum Pudding model, alpha particles should go right through Actually, some bounce back! Actually, some bounce back!

Rutherford’s Reaction “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you… It was then that I had the idea of an atom with a minute massive centre, carrying a charge.” Key idea: central nucleus Key idea: central nucleus

Rutherford’s Planetary Model Planetary model: Central, dense, and positively charged nucleus, orbited by negatively charged electrons Planetary model: Central, dense, and positively charged nucleus, orbited by negatively charged electrons Question: What was Rutherford’s contribution to the atomic model? Question: What was Rutherford’s contribution to the atomic model?

Niels Bohr and the Bohr Model The problem: Rutherford’s planetary model was physically impossible because it was not stable. The problem: Rutherford’s planetary model was physically impossible because it was not stable. Bohr’s idea: Electrons are allowed to be in special orbits where they are stable. Bohr’s idea: Electrons are allowed to be in special orbits where they are stable.

Bohr Model’s Successes and Failures The good: successful properties predicted for hydrogen The good: successful properties predicted for hydrogen The bad: failure for all other elements The bad: failure for all other elements Question: What was Bohr’s contribution to the atomic model? Question: What was Bohr’s contribution to the atomic model?

Recap Democritus: atoms as indivisible units Democritus: atoms as indivisible units Thomson: negatively-charged electrons Thomson: negatively-charged electrons Rutherford: positively-charged nucleus Rutherford: positively-charged nucleus Bohr: special electron orbits Bohr: special electron orbits Summary: Write a paragraph of at least five sentences answering the questions from the lecture. Summary: Write a paragraph of at least five sentences answering the questions from the lecture.

Summary Prompt How has the concept of the atom changed over time? How has the concept of the atom changed over time? What was the contribution of technology to discoveries made about the atom? What was the contribution of technology to discoveries made about the atom?

Summary Prompt