1. DEVELOPMENT OF THE ATOMIC THEORY ATOMS

2. 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”. Democritus

3. 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 Democritus

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

5. 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. John Dalton

6. 1897 - English chemist and physicist; discovered 1 st subatomic particles J. J. Thomson Plum Pudding Model or Raisin Bun Model Proposed by J.J. Thomson

7. His Theory: 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. J. J. Thomson

8. 1912 - New Zealand physicist discovered the nucleus Ernest Rutherford Nuclear Model Proposed by Ernest Rutherford

9. Gold Foil Experiment

10. His Theory:  Small, dense, positively charged particle present in nucleus called a proton  Electrons travel around the nucleus, but their exact places cannot be described. Ernest Rutherford

11. 1913 - Danish physicist; discovered energy levels Niels Bohr

12. 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. Niels Bohr

13. 1924 - Austrian physicist; developed the electron cloud model Erwin Shrodinger Electron Cloud Model Proposed by Erwin Schrodinger

14. 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. Erwin Shrodinger

15. 1932 - English physicist; discovered neutrons His Theory: NNeutrons have no electrical charge. NNeutrons have a mass nearly equal to the mass of a proton. UUnit of measurement for subatomic particles is the atomic mass unit (amu). James Chadwick Neutrons in Nucleus

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

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

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

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