1. History of Atomic Structure

2. Democritus

3. Ancient Philosophy Who: Democritus When: 400 B.C. Where: Greece
What: Democritus believed that matter was made of small, indivisible particles he named “atoms”.
Why: Democritus used observation and inference to explain the existence of everything.

4. “What you see is what you get.”
At this time in history, science was:
“What you see is what you get.”

5. John Dalton

6. Particle Theory Who: John Dalton When: 1803-1808 Where: England
What: Described atoms as tiny particles that could not be divided (like billiard balls). Thought each atom of a given substance were identical.
Why: Building on the ideas of Democritus in ancient Greece.
Solid Spheres

7. Dalton’s Atomic Theory
1. All matter is composed of atoms 2. Atoms of same element are identical AND atoms of different elements are different 3. Atoms can’t be divided, created, or destroyed 4. Atoms of different elements combine to form compounds 5. In chemical reactions, atoms are combined, separated, or rearranged.

8. Does Dalton’s Atomic Theory Still Hold True Today???

9. Dalton didn’t know about isotopes
Dalton didn’t know about isotopes. All identical atoms are NOT identical because we have different forms of them.
Dalton didn’t know about parts of an atom! They CAN be divided into parts! We now know about proton, neutron, electrons, nucleus etc.

10. J. J. Thompson

11. Discovery of Electrons
Who: J. J. Thompson
When: 1904
Where: England
What: Thompson used Cathode Rays to discover that electrons were smaller particles of an atom and were negatively charged.
Why: theorized that the neutral particles called atoms have no charge because it is actually made up of two types of charged parts, one positive and one negative so the charge cancels out.

12. J.J. Thompson’s Plum Pudding Model
NOTICE:
NO nucleus!

13. Honors Chemistry: Atomic Theory
Cathode Ray Tube
J.J. Thomson
Discovered mass/charge of e-
                 
1906 Nobel Prize in Physics

14. Ernest Rutherford

15. Atomic Structure I Who: Ernest Rutherford When: 1909 Where: England
What: Conducted a Gold Foil experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.
Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

16. Rutherford’s Results:
Most of the large positive particles went straight through! (hit nothing)
2. Some were totally bounced back, hit something very hard and dense.
3. Some were deflected. In entirely different directions from each other!
Meant there were positive
AND negative particles/areas
inside!

17. The Nuclear Model: Extremely small, dense and positively
charged nucleus surrounded by empty
space sparsely occupied by electrons.

18. Niels Bohr

19. Atomic Structure II Who: Niels Bohr When: 1913 Where: England
What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus.
Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

20. Planetary Model
Electrons travel around the nucleus in orbits of that have a set energy level
Scientists cannot predict where they will be at any given moment.

21. Planetary Model of Bohr’s atom

22. Bohr’s Atom
electrons in orbits
nucleus

23. James Chadwick

24. Who: James Chadwick
When: 1932
Where: England
What: Identified third subatomic particle= the neutron
Why: The earlier atomic models were missing mass