1. Atomic Theory & Structure

2. Early Greek Theories
Democritus
400 B.C. – Democritus thought matter could not be divided infinitely
Called nature’s basic particle the atom.
This led to the idea of atoms in a void
350 B.C. – Aristotle said matter was continuous
Modified an earlier theory that matter was made of four “elements”: Earth, air, fire, water
He was wrong, but his opinion was accepted for 2000 years
Neither view was supported by evidence
Aristotle

3. Foundation of the Atomic Theory
In the 1790s, scientists began to focus on quantitative analysis, which led to several discoveries:
Law of Conservation of Mass = Mass is neither created nor destroyed during chemical reactions or physical changes.
Law of Definite Proportions = compounds contain the same elements in the same proportions, regardless of sample size

4. Dalton’s Atomic Theory
1808 – John Dalton proposed an explanation based on experimentation
All matter is made of atoms
Atoms of an element are identical
Each element has different atoms
Atoms of different elements combine in constant ratios to form compounds
Atoms are rearranged in chemical reactions

5. Thomson Model
Scientists observed that rays were deflected away from a negatively charged object.
This is the Cathode Ray Experiment
Thomson noted that these negative particles were a part of all atoms.

6. Adding Electrons to the Atomic Model
Dalton’s “Billiard ball” model ( )
Atoms are solid and indivisible.
Thomson “Plum pudding” model (1900)
Negative electrons in a positive framework.
The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.

7. Radioactive substance
Ernest Rutherford (1911)
Shot alpha (α) particles at a sheet of gold foil
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive substance

8. Rutherford Most particles passed through. So, atoms are mostly empty.
Some positive -particles deflected or bounced back!
Thus, a “nucleus” is positive & holds most of an atom’s mass.

9. Early 1900-1913 What was happening in the world? Speed limits invented
President McKinley is shot.
Queen Victoria dies.
First vacuum cleaner created.
Nobel Prize is created.
Post office begins working.
Serbia and Greece declare war on Bulgaria.
Titanic sinks 1912.

10. Neils Bohr (1913) Electrons orbit the nucleus in “shells”.
Neils Bohr proposed a model in which electrons surround the nucleus like the planets do the sun.
Bohr Model

11. Cloud Model
1920’s
Electrons don’t orbit, they surround nucleus in a cloud.
This is where electrons are likely to be found—the more energy an electron has, the farther from the nucleus it is.

12. To Summarize 1. Democritus—Ball Model
2. Dalton—Ball Model, 5 key points
3. Thomson—Plum Pudding Model, Found Electrons
4. Rutherford—Gold Foil Experiment, Found Nucleus
5. Bohr—Orbitals, Found that electrons travel around nucleus
6. Current Theory—Cloud Model

13. Draw the Model Progression…

14. Structure of the Atomic Nucleus
All atomic nuclei are composed of protons (+) and neutrons (neutral) and electrons (-)
Subatomic particle masses:
Electron = x kg
Proton = x kg
Neutron = x kg

15. Atomic Nucleus Nuclei are held together by nuclear forces
Short-range forces that hold the nucleus together

16. The Sizes of Atoms
Atomic radius = the distance from the center of the nucleus to the outer portion of its electron cloud
Expressed as picometers (pm)
1pm = m
Atomic radii range from 40 to 270 pm
Nuclear radius is about pm

17. Observing & Collecting Data
Observing = using the senses to get information
Data collection may be:
Qualitative  descriptive data
Color, texture, odor, etc
Quantitative  numerical
Measurements of mass, distance, volume, temperature, etc