Today’s Catalyst 1. What is a chemical change? 2. Which of the following substances can NOT be broken down by a chemical change? Why? A. ammonia B. propane C. carbon D. water

Today’s Catalyst 3. Sketch a timeline of the atom including what we have learned so far (Dalton and Thomson). Include a drawing of the atom, the year of the discovery, and a description of the atom. (DO AT THE END OF CLASS TODAY!) 4. Correct the following statement to make it true… “All atoms of the same element are identical.”

2 Hmmmmm….. Wonder what these mean?? Coming soon!! 8 18

Modern Atomic Theory Three more dead guys

By the end of the class period today I will be able to… Understand Rutherford’s alpha particle experiment and the discovery of the atomic nucleus. Identify Thomson’s Plum Pudding idea as a model for atomic structure. Draw a representation of Bohr’s nuclear atom. Understand why Bohr’s model is fundamentally incorrect

Thomson’s Electrons Who was J.J. Thomson? Thomson experimented (in 1897) with a cathode-ray tube like the one shown on the next slide. He discovered negatively charged particles that are now known as electrons. Like Plums in Pudding After learning that atoms contain electrons, Thomson proposed a new model of the atom. Thomson thought that electrons were mixed throughout an atom, like plums in a pudding.

The Cathode-Ray Tube

Lightning Fast Review What is an example of one of Dalton’s postulates? 2) Which of Dalton’s postulates is no longer considered true? Why? 3) How did Thomson picture the atom? What was the analogy he used?

Ernest Rutherford New Zealander In 1911 performed his gold foil experiment

The Gold Foil Experiment

Observations from gold foil experiment: Most of the alpha particles passed through the gold foil un-deflected Very few bounced back (great deflection)

Actual Results Expected Results Modern Atomic Theory Actual Results Expected Results

Rutherford’s Conclusion: 1) The nucleus is small, dense, and has a positive charge 2) The nucleus is composed of protons and neutrons

Summarize the gold foil experiment Summarize the gold foil experiment. What model did Rutherford’s experiment disprove?

Sample Summary Alpha Particles (2 protons/2 neutrons) were shot at a piece of gold foil. Part of the particles were deflected (bounced off), while others passed straight through. Thereby convincing Rutherford that the center of an atom (the nucleus) is positively charged. Rutherford’s experiment disproved Thomson’s plum pudding idea. If the plum pudding idea were true, all the alpha particles would have passed straight through.

Niel Bohr 1912 Convinced atom was small positive nucleus with electrons orbiting around it

Constructed model of the hydrogen atom with quantized energy levels (electrons can only exist on certain energy levels/rings) Bohr Model

Pros and Cons of the Bohr Model +The model gives us a clear visual of the atom +Accurate model for Hydrogen Con: -Electrons DO NOT move around the nucleus in circular orbits like planets orbiting the sun

Rules for drawing Bohr Models 1) We will be working with neutral atoms, so we can expect the number of electrons in each element to be equal to that element’s number of protons! (#electrons = #protons) 2) Draw each electron energy level with a ring. 3) Electrons fill energy levels in the following way: 2 8 18

Carbon 6 Protons 6 Electrons Bohr Model

Nitrogen 7 protons 7 electrons Bohr Model

Sodium ? protons ? electrons Bohr Model

Silicon ? protons ? electrons Bohr Model

Finish the 2nd Catalyst Question AND complete the EXIT TICKET Today’s Homework Finish the 2nd Catalyst Question AND complete the EXIT TICKET