1. Unit 2: Atomic Theory & Structure
WARM UP
Unit 2: Atomic Theory & Structure
Mr. Proton says “I live in the nucleus.”
Mr. Electron asks, “Are you sure?”.
Mr. Proton replies, “Yes, I’m _________.
Mr. Neutron was asked if he supported more gun control or the right to bear arms.
He replied,“_____________________________”
What do you do with dead elements?
Barium (Ba, element # 56)
positive
IDK, on most issues I’m neutral.

2. Unit 2: Atomic Theory & Structure
Section 1 – Early Models of the Atom

3. Early Models of the Atom
Democritus (c. 400 B.C.)
- Philosopher who thought about splitting a piece of matter in half many times until it could not be split any more.
- proposed that “atomos” (atoms) are the smallest particle of matter and are indivisible and indestructable.
Democritus' ideas were limited because they didn’t explain chemical behavior and they lacked experimental support

4. Early Models of the Atom
(1803) John Dalton studied numerous chemical reactions and determined the mass ratios of the elements involved.
Democritus' ideas were limited because they didn’t explain chemical behavior and they lacked experimental support

5. Dalton’s Atomic Theory
1) All matter is composed of tiny, indivisible atoms.
In size, mass, & prop’s atoms of the same element are identical,
atoms of different element are different

6. Dalton’s Atomic Theory
3) Atoms chemically combine, separate, or rearrange.
4) Atoms combine in simple whole-number ratios to form compounds.
(Law of Definite Proportions)
(Law of Conservation of Mass)

7. Cathode Ray Tube Experiment
(1904) J.J. Thomson
Cathode Ray Tube Experiment
Passing electricity through a cathode ray tube filled with gas at low pressure causes a glowing beam to travel from one end to the other.
In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron.
Thomson performed experiments that involved passing electric current through gases at low pressure.
The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.

8. Cathode Ray Tube Experiment
(1904) J.J. Thomson
Cathode Ray Tube Experiment
A magnet causes the beam to deflect
In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron.
Thomson performed experiments that involved passing electric current through gases at low pressure.
The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.

9. Cathode Ray Tube Experiment
(1904) J.J. Thomson
Cathode Ray Tube Experiment
cathode rays deflect toward a positive plate and away from a negative plate
In a cathode-ray tube, electrons travel as a ray from the cathode (-) to the anode (+). A television tube is a specialized type of cathode-ray tube.

10. (1904) J.J. Thomson
Cathode Ray Tube Experiment
Conclusion:
atoms contain negatively charged particles called electrons
Thomson’s
Plum Pudding Model
atoms are positively charged material with negative electrons evenly distributed

11. (1911) Ernest Rutherford Gold Foil Experiment
Positively charged alpha particles were beamed at a thin sheet of gold foil

12. (1911) Ernest Rutherford
Gold Foil Experiment
(expected)

13. (1911) Ernest Rutherford Gold Foil Experiment
1/10,000 alpha particles deflect from the gold foil
b) Rutherford concluded that most of the alpha particles pass through the gold foil because the atom is mostly empty space. The mass and positive charge are concentrated in a small region of the atom. Rutherford called this region the nucleus. Particles that approach the nucleus closely are greatly deflected.

14. (1911) Ernest Rutherford
Gold Foil Experiment
Conclusion:
atoms are mostly empty space filled with e–
the positive charge and mass are in a tiny central region
nucleus:
tiny, dense, central, positive core of atoms containing protons and neutrons

15. (1 trillionth of the atom)
the atomic nucleus
(1 trillionth of the atom)
1 cm diameter lump of nuclei = 1 billion tons

16. The Atomic Nucleus
3 subatomic particles:
≈ 0

17. 4.2

18. Quick Quiz!
1. The ancient Greek philosopher Democritus is credited with suggesting that ____.
atoms react in whole number ratios
atoms can change into other elements
all matter is made of indivisible atoms
atoms are stupid anyway

19. Quick Quiz.
2. Dalton's atomic theory improved earlier ideas about atoms by ___.
teaching that all matter is composed of tiny particles called atoms
theorizing that all atoms of the same element are identical
using experimental methods to establish a scientific theory
not relating atoms to chemical change

20. Quick Quiz.
3. Which of the following is NOT part of Dalton’s atomic theory?
All elements are made of atoms.
Atoms of the same element are identical.
Different elements can mix or combine in simple whole-number ratios called compounds.
Atoms can be changed into other atoms through chemical reactions.

21. Quick Quiz.
4. Which of the following is TRUE about subatomic particles?
protons and neutrons are positively charged
electrons have practically no mass
protons and electrons are in the nucleus
neutrons and electrons have no charge

22. Quick Quiz.
5. Most of the volume of the atom is occupied by the ________________.
electrons
protons
neutrons
protons and neutrons