1. Historical Development of an Atom

2. Democritus B.C
Democritus was a Greek philosopher ( B.C.) who is the father of modern atomic thought.
He proposed that matter could NOT be divided into smaller pieces forever (uncuttable)
He believed that matter was made of small, hard, particles, that he called “atomos”
Thought atoms are made of a single material formed into different shapes and sizes
Only had a mental model or the atom – nothing recorded

3. John Dalton 1807 He built upon Democritus’s atomic theory.
Matter is made up of atoms
Atoms cannot be divided into smaller pieces.
All the atoms of an element are exactly the same
Different elements have different kinds of atoms
Compounds form by combining atoms/elements (experiments)

4. John Dalton 1807
Model Drawing
Solid throughout
Same throughout

5. J.J. Thompson 1898 Particles smaller than the atom exists
Discovered the electron, which is negatively charged
If atoms have negative particles, then atoms must also have positive particles.
Cathode ray experiment
“plum pudding model” – said the atom was like plum pudding with pieces of plums dispersed throughout

6. J.J. Thompson 1898
Model Drawing
Positive and negative particles

7. Ernest Rutherford 1911 Rutherford performed the gold foil experiment.
In his experiment a radio active beam emitted (+) alpha particles toward gold foil.
Most particles went through, some were deflected, and others bounced back!
This showed that atoms have a very dense (+) center.

8. Ernest Rutherford 1911
Rutherford showed that atoms have (+) particles in the center and are mostly empty space.
He called the center of the atoms the nucleus.
All the mass of an atom and its positive charge are located in the nucleus.

9. Ernest Rutherford 1911 Model Drawing Positive nucleus
Negative particles surround nucleus

10. Niels Bohr 1913 Niels Bohr improved on Rutherford’s model.
He proposed that electrons move around the nucleus in specific layers or shells/energy levels
Every atom has a specific number of electron shells.
Said electrons can jump levels

11. Niels Bohr 1913 Electrons move in orbits around the nucleus
1st energy level = 2 electrons
2nd energy level = 8 electrons
3rd energy level = 18 electrons
4th energy level = 32 electrons

12. Niels Bohr 1913 Model Drawing
Electrons orbit the positive nucleus on energy levels

13. James Chadwick 1932 Chadwick discovered neutrons
He worked with Rutherford and discovered particles with no charge
He called those particles neutrons
Neutrons are found in the nucleus of an atom.

14. Chadwick 1932
p=protons n = neutrons
P n n n
Pnnnnppp

15. Electron Cloud Theory Erwin Schrodinger - 1925
Electrons do not follow fixed orbits, but tend to occur more frequently in certain areas around the nucleus
it is impossible to know where an electron is at any given time
*this is the theory we use today!

16. Electron Cloud Model Current
Model Drawing
Electrons are located in clouds