1. Unit 2:
Atomic Theory
and Structure

2. Dalton’s Atomic Theory
Early Models of the Atom
(c. 400 B.C.) Democritus proposed that “atomos” are indivisible.
atom: smallest particle of matter that retains its properties.
(1803) Dalton’s experiments developed a scientific theory of atoms.
Democritus' ideas were limited because they didn’t explain chemical behavior and they lacked experimental support
Dalton’s Atomic Theory
(4 conclusions)

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

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

5. conservation of mass - the total mass of any material system is neither increased nor diminished by reactions Attributed to French chemist Antoine Lavoisier in the late 18th century.

6. Cathode Ray Tube Experiment
(1904) J.J. Thomson
Cathode Ray Tube Experiment
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.
Cathode Ray Tube

7. WHY? (1904) J.J. Thomson Cathode Ray Tube Experiment
cathode rays deflect toward a positive plate
WHY?
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.
negatively charged

8. Cathode Ray Tube Experiment
(1904) J.J. Thomson
Cathode Ray Tube Experiment
Conclusion:
atoms contain negatively charged particles called electrons
Thomson’s
Plum Pudding Model
because bare atoms are neutrally charged Thomson proposed that they are + charged material with – charged electrons evenly distributed

9. (1911) Ernest Rutherford Gold Foil Experiment
1908 Nobel Prize for work with radiation (+ alpha particles)
(expected)

10. (1911) Ernest Rutherford Gold Foil Experiment +
1/10, 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. +

11. (1911) Ernest Rutherford
Gold Foil Experiment
Conclusion:
atoms are mostly empty space filled with e–
the + charge and mass are in a tiny central nucleus
nucleus:
tiny, massive, core of atoms containing + protons and (neutral) neutrons

12. Atom (electron) radius 10-10 m
the atomic nucleus
Nucleus radius m
Atom (electron) radius m
Distance comparison like a marble to the distance to Houligan’s (as the bird flys)
1 cm diameter lump of nuclei = 1 billion tons

13. 17.7 The Atomic Nucleus
3 subatomic particles:

14. 4.2

15. 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

16. 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

17. 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.

18. 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

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