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.” Hmmmmm….. Wonder what these 2 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
1) 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:
1) Most of the alpha particles passed through the gold foil un-deflected
2) Very few bounced back (great deflection) 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. 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
Pros: Con: +The model gives -Electrons DO NOT us a clear visual move around the of the atom nucleus in circular orbits like planets +Accurate model orbiting the sun for Hydrogen 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 Bohr Model 6 Electrons
Nitrogen
7 protons 7 electrons Bohr Model
Sodium
? protons ? electrons Bohr Model
Today’s Homework Finish the 2nd Catalyst Question AND complete the EXIT TICKET