1. Atoms: Development of the Atomic Theory1

2. Democritus
460 BC - Greek philosopher proposes the existence of the atom
He pounded materials until he made them into smaller and smaller parts
He called them “atoma” which is Greek for “indivisible”.

3. Democritus His Theory: All atoms: Are small hard particles
Are made of a single material formed into different shapes and sizes
Are always moving, and they form different materials by joining together

4. John Dalton 1803 - British chemist
Elements combine in specific proportions to form compounds
Solid Sphere Model or Billiard Ball Model
Proposed by John Dalton

5. John Dalton His Theory:
All substances are made of atoms that cannot be created, divided, or destroyed.
Atoms join with other atoms to make new substances.
Atoms of the same element are exactly alike, and atoms of different elements are different in mass and size.

6. Plum Pudding Model or Cookie Dough Model
J.J. Thomson
English chemist and physicist
Discovered 1st subatomic particles
Plum Pudding Model or Cookie Dough Model
Proposed by J.J. Thomson

7. J.J. Thomson His Theory: Used Cathode Ray Tube to conduct experiments.
Atoms contain negatively charged particles called electrons and positively charged matter.
Created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in

8. 8

9. Proposed by Ernest Rutherford
New Zealand physicist
Discovered the nucleus
Nuclear Model
Proposed by Ernest Rutherford

10. 10

11. Ernest Rutherford His Theory:
Used Gold Foil Experiment for his research.
Small, dense, positively charged particle present in nucleus called a proton
Electrons travel around the nucleus, but their exact places cannot be described.

12. Bohr Model or Planetary Model
Niels Bohr
Danish physicist
Discovered energy levels
Bohr Model or Planetary Model
Proposed by Niels Bohr

13. Nucleus
Energy
levels
Electron-
negative charge 13

14. Niels Bohr
His Theory:
Electrons travel around the nucleus in definite paths and fixed distances.
Electrons can jump from one level to a path in another level.

15. Proposed by Erwin Schrodinger
Erwin Shrodinger
Austrian physicist
Developed the electron cloud model
Electron Cloud Model
Proposed by Erwin Schrodinger

16. Erwin Shrodinger His Theory:
The exact path of electrons cannot be predicted.
The region referred to as the electron cloud, is an area where electrons can likely be found.

17. Modern Theory of the Atom
Atoms are composed of three main subatomic particles: the electron, proton, and neutron.
Most of the mass of the atom is concentrated in the nucleus of the atom.

18. Modern Theory of the Atom
The protons and neutrons are located within the nucleus, while the electrons exist outside of the nucleus.
In neutral atoms, the number of protons is equal to the number of electrons.

19. Modern Theory of the Atom
The type of atom is determined by the number of protons it has.
The number of protons in an atom is equal to the atomic number.

20. Modern Theory of the Atom
The sum of the number of protons and neutrons in a particular atom is called the mass number.
Valence electrons are the outermost electrons.