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.”
Today’s Catalyst
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
By the end of the class period
today I will be able to…
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.
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?
1) Most of the alpha particles passed through the gold foil un-deflected
2) Very few bounced back (great deflection)
Observations from gold foil experiment:
1) The nucleus is small, dense, and has a positive charge
2) The nucleus is composed of protons and neutrons
Rutherford’s Conclusion:
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.
Sample Summary
Bohr Model
• Constructed model of the hydrogen atom with quantized energy levels (electrons can only exist on certain energy levels/rings)
Pros:
+The model gives us a clear visual of the atom
+Accurate model for Hydrogen
Pros and Cons of the Bohr Model
Con:
-Electrons DO NOT move around the nucleus in circular orbits like planets orbiting the sun
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
Rules for drawing Bohr Models