1. The evolution of the atom
What is an Atom

2. What is an atom
An atom is the smallest particle of an element that retains its identity in a chemical reaction
The atom has three basic parts
Protons – positively charged particles
Neutrons – particles with zero charge
Both found in the Nucleus
Electrons – negatively charged particles

3. To the starting line Democritus (470-380 B.C.)
Proposed matter could not be divided into smaller and smaller pieces forever
Called small, hard particles “atomos”
Father of modern atomic thought

4. Several hundred years later…
John Dalton 1808
An English teacher
Created first atomic theory
Viewed atoms as tiny, solid spheres

5. Dalton Says Atoms are tiny, invisible particles
Atoms of one “element” are the same
Atoms of different elements are different
Compounds form by combining atoms in simple whole number ratios
Dalton Say’s stand on one foot
Dalton Say’s start jumping
Stop!

6. It’s Electric! J.J. Thomson 1897 Cathode Ray Experiment
Proved existence of smaller particles
Proposed a (-) particle

7. ELECTRON!!! Time For Pudding
Thomson developed the “plum-pudding” model
A negative charge particle is found inside a positive dough
ELECTRON!!!

8. PROTONS!!! Let’s get nuclear Ernest Rutherford 1911
Showed atoms have (+) particles in the center and are mostly empty space
Later calls the center the nucleus
PROTONS!!!

9. The experiment

10. Now enter bohr Niels Bohr 1913 Improved Rutherford’s design
Proposed electrons move in shells at a specific energy level

11. Where’s the beef? James Chadwick 1932
Discovers neutrons with Rutherford
Particles have NO charge (hence neutral = neutrons) also found in the nucleus

12. How about now? The Modern Model 1932 – present
Electrons move in a cloud
It is impossible to know where any one electron is at a given time