1. TITLE The Development of Atomic Theory

2. Early Ideas Democritus—Greece 400 B.C. His idea:  There was a limit to how far you could divide matter. You would eventually end up with a piece of matter that could not be cut.  Called particles atoms--Greek for “indivisible”

3. His theory:  all atoms are small hard particles  made of a single material formed into different shapes and sizes  always moving, and that they form different materials by joining together

4. Early Ideas Aristotle--Greece 384- 322 B.C. –All matter was continuous, that matter could be divided into smaller and smaller pieces forever — did not believe in atoms –Opinion accepted for 2000 years

5. John Dalton 1803 He performed many experiments to study how elements join together to form new substances. He found that they combine in specific ratios and he supposed it was because the elements are made of atoms. 2000 Years Later

6. His ideas:  All matter is made of very small particles called atoms.  Atoms of one element are identical in size, mass, and properties; atoms of different elements differ in size, mass, and properties

7.  Atoms cannot be subdivided, created, or destroyed.  Atoms of different elements combine in simple whole-number ratios to form compounds.  Atoms are combined, separated, or rearranged in chemical reactions.

8. J.J. Thomson 1897 English chemist and physicist discovered 1 st subatomic particles

9. His theory:  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  referred to it as the plum pudding model

10. He proposed the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream. His ideas

11. the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream. The Plum Pudding Model Negatively charged Positively charged

12. Ernest Rutherford 1911 New Zealand physicist discovered the nucleus Was a student of J.J. Thomson but disagreed with the “Plum Pudding Model” Devised an experiment to investigate the structure of positive and negative charges in the atom.

13. Gold Foil Experiment

14. What did most of the particles shot at the gold foil do?  Most of the particles traveled straight through the gold foil What was the surprising behavior of a few of the particles?  A few of the particles were deflected and some even bounced back

15. 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

16.  most of the matter of the atom is found in a very small part of the atom. This is called the nucleus of the atom. It is very tiny and extremely dense

17. Niels Bohr 1913  Danish physicist discovered energy levels  proposed that electrons move in paths at certain distances around the nucleus.  Electrons can jump from a path on one level to a path on another level

18. Bohr’s Model

19. James Chadwick 1932 English physicist discovered neutrons

20. His theory  neutrons have no electrical charge  neutrons have a mass nearly equal to the mass of a proton  unit of measurement for subatomic particles is the atomic mass unit (amu)

21. Modern Theory  Atoms are composed of three main subatomic particles: electron, proton, and neutron.  Most of the mass of the atom is concentrated in the nucleus of the atom.  The protons and neutrons are located within the nucleus while the electrons exist outside of the nucleus. 21

22.  In stable atoms, the number of protons is equal to the number of electrons.  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 sum of the number of protons and neutrons in a particular atom is called the atomic mass