Do Now 8/29/14  A student in the lab wanted to classify an unknown substance as an element, compound, homogeneous mixture, or heterogeneous mixture. The.

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Development of the Atomic Theory

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Presentation transcript:

Do Now 8/29/14  A student in the lab wanted to classify an unknown substance as an element, compound, homogeneous mixture, or heterogeneous mixture. The student took the liquid substance and put it on top of a Bunsen burner. As the liquid heated up, she was able to separate two substances from each other through evaporation. How would you classify this substance? Why?

Objective By the end of this class period I will be able to…. describe the modern structure of the atom and summarize the experiments that led to its development.

This has a few pieces to it…  SWBAT describe and summarize Dalton’s 5 postulates  SWBAT describe and summarize the design of: J.J. Thomson’s cathode-ray experiment, Rutherford’s alpha particle experiment and the discovery of the atomic nucleus.  SWBAT describe and draw a representation of Bohr’s nuclear atom.

Modern Atomic Theory 4 Dead White Guys Some people just like proving they’re better than you…

Turn and Talk: 2 min.  Why might this peach have been used to explain the structure of the atom to kids?  What do the parts of the peach represent?  How would you describe an atom, what is it, what’s in it, how does it work?

Oldest mention of the atom… Democritus suggested that if you kept splitting an object in half, you would eventually get to a tiny, fundamental piece that can’t be broken down any further.

Atom The smallest particle of an element that has the chemical properties of that element

Key Players in Modern Atomic Theory

John Dalton English school teacher turned Chemist Proposed his atomic theory in 1808

Reading  Read the following passage about John Dalton and his ideas about the atom.  What were his postulates?  After you have found them all, decide: which are still accepted views and which are outdated?

Postulate 1 All matter is composed of tiny, indivisible particles called atoms

Postulate 2 Atoms of a given element are identical

Postulate 3 Atoms of a given element are different from those of any other element

Postulate 4 Atoms of different elements combine in simple whole-number ratios to form chemical compounds

Examples: CO 2 Cu(NO 3 ) 2

H2OH2OH2OH2O

H 2 O 2 Hydrogen 1 Oxygen

Fe(NO 3 ) 2

Fe(NO 3 ) 2 1 Iron 2 Nitrogen 6 Oxygen

Let’s do our last postulate…

Postulate 5 Atoms cannot be created or destroyed. In chemical reactions, atoms are combined, separated, or rearranged

Where Dalton was Wrong! 2 of Dalton’s postulates are not exactly correct. Which are they? Turn and Talk (2 min.)

Where Dalton’s Wrong and Right Dalton was wrong about… But, He was right about…

Every atom of a given element always has one thing that is the same. What is it? The same number of protons

J.J. Thomson’s Cathode Ray Experiment British Physicist Performed his experiments in the late 1800s Until this time, it was not believed that the atom was composed of various parts

J.J. Thomson Discovered atoms contain electrons in 1897 by using a cathode ray tube

Cathode Ray Tube

Plum Pudding Model Thomson believed that electrons were like raisins embedded in a positively charged ‘pudding’ (plum pudding model)

Plum Pudding Model Thomson believed the mass of the atom was uniformly distributed throughout the atom

Stop, Read and Jot  Alternate reading in pairs the short selection on your table. When done, discuss and answer the following question in your notes:

JJ Thomson discovered the electron in 1897, which of Dalton’s postulates did this discovery change? Postulate #1: Matter is made of tiny, indivisible particles ACTUALLY, electrons are smaller than atoms and are in every atom

What tool did JJ Thomson use to discover the electron?

How did Thomson know that atoms must include a negatively charged particle?

Dear John, ROLE: J.J. Thomson AUDIENCE: John Dalton FORMAT: Letter TOPIC: You just discovered the electron! Write a letter to John Dalton telling him how you have just updated his theory

Quick Break…

Model of the atom in 1897 With the person next to you, brainstorm a way to test if this is the correct structure of the atom.

New Zealander In 1911 performed his gold foil experiment Ernest Rutherford

The Gold Foil Experiment

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:

Modern Atomic Theory Expected Results Actual Results

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

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

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

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: Rules for drawing Bohr Models

Carbon Bohr Model 6 Protons 6 Electrons

Nitrogen Bohr Model 7 protons 7 electrons

Sodium Bohr Model ? protons ? electrons

Create/complete the timeline of the atomic theory Independent Practice

 All 4 Scientists associated with the atomic theory  The atomic model each scientist developed (include dates)  The subatomic particles each scientist discovered  The experiments that led to each discovery/ model What must be included?