1. Introduction to Atoms
Chapter 14
Section 1

2. History of Atom All atoms share the same basic structure
During past 200 years, scientists have proposed different models

3. Dalton’s Model
Based on experiments, Dalton developed a theory of structure of matter
4 main concepts:
All matter is composed of tiny, indivisible particles called atoms
Atoms of each element are exactly alike
Atoms of different elements have different masses
Atoms of different elements can join to form compounds

4. Dalton’s Model

5. Thomson’s Model End of 1800s
Thomson discovered that atoms were not simple, solid spheres
Atoms contained subatomic particles
Very small, negatively charged
Called them electrons

6. Thomson’s Model Also knew that atoms were electrically neutral
Must contain enough positive charge to balance negative charge of electrons
Developed model where electrons were stuck into a positively charged sphere
Like chocolate chips in cookie dough

7. Thomson’s Model

8. Rutherford’s Model
By early 1900s, scientists knew that positive charge of atom comes from subatomic particles called protons
1911—Rutherford begins to test theory
His experiments led him to believe that protons are concentrated in a small area at center of atom
Called this area the nucleus

9. Rutherford’s Model
Rutherford’s model describes an atom as mostly empty space, with a center nucleus that contains nearly all the mass
Like the pit in a peach

10. Bohr’s Model Modified Rutherford’s model in 1913
Proposed that each electron has a certain amount of energy
Helped electron move around nucleus
Electrons move around nucleus in region called energy levels
Energy levels surround nucleus in rings, like layers of onion

11. Bohr’s Model Has been called planetary model
Energy levels occupied by electrons are like orbits of planets at different distances from the sun (nucleus)

12. Electron Cloud Model Model accepted today
Electrons dart around in an energy level
Rapid, random motion creates a “cloud” of negative charge around nucleus
Electron cloud gives atom its size and shape

13. Electron Cloud Model