2. Atomic Theory  Atoms were thought to be the smallest particles of matter (Democritus)  Every type of matter was made of a different atom  This idea that all matter is made up of fundamental particles is called Atomic Theory

3. Conservation of Matter  In 1782, Antoine Lavoisier observed that the mass of particles before and after a chemical reaction were the same.  He concluded that mass is neither created or destroyed. This is called the Law of Conservation of Matter.

4. Conservation of Matter  John Dalton then added that the atoms were only rearranged.  What this means is that the chemical composition before and after a chemical reaction are different, because the arrangement of the atoms is different.

5. Hypothesis, Theory, Law  Hypothesis – a testable prediction to explain an observation  Theory – an explanation based on many observations, that has been supported by many experiements  Law – a fact of nature that is observed so often that it becomes accepted as truth

6. Atomic Structure  Dalton thought that the atom was the smallest piece of matter, and it could not be simplified any more.  He was ALMOST right.  There are three subatomic particles. sub – meaning under, below, or beneath

7. Electron  The first subatomic particle discovered was the electron.  It was discovered by J.J. Thomson in 1897 by the cathode ray tube experiment.  Through this experiment it was determined that electrons have a negative charge.

8. Electron  Cathode Ray Tube Experiment Video

9. Subatomic Particles  Thanks to Thomson’s discovery it was now known that atoms were not just neutral  So the search for more subatomic particles began

10. Subatomic Particles  Why did scientists know that there must be more subatomic particles after the discovery of the electron?  Atom are neutral because of a balance of positive and negative charges

11. Protons  Protons have a positive charge.  The amount of charge on a proton is equal to that on an electron.  Protons were discovered when it was observed that the rays of a cathode ray tube comes from both sides, positive and negative

12. Neutrons  Neutrons have a neutral charge, or no charge.  Once again we can thank Thomson for the neutron. While he did not discover the first neutron he found that Neon had two different weights in 1910.

13. Neutrons  This observation of an element having two different masses contradicted the current atomic theory.  Isotopes are two, or more, atoms of the same element that have a different mass.

14. Neutrons  Because it was known that the mass came from the protons there had to be another subatomic particle adding to the mass, but not effecting the charge.  The neutron was confirmed in the early 1930s.

15. Atomic Structure  While the discoveries of the different subatomic particles was happening discoveries of the structure were also occurring.  In 1909, Ernest Rutherford carried out his gold foil experiment.

16. Atomic Structure  Gold Foil Experiment Video

17. Atomic Structure  The gold foil experiment proved that the protons and neutrons, almost all the atom’s mass, was in a very small nucleus in the center of the atom.  Therefore, almost all of the atom is empty space.

18. Atomic Structure  But where are the electrons?  Electrons are located in what is called the electron cloud that surrounds the nucleus.

19. Atomic Structure  The electron cloud is broken into different layers called energy levels.  The first level holds two electrons and the rest hold eight electrons.  This holds true until larger masses are reached

20. Numbers  Atomic number is the number of protons an atom has.  Mass number is the number of protons and neutrons that gives an atom its mass.

21. Numbers  Each isotope has a different mass number.  Atomic mass is an average of the mass of each isotope and how common it is.

22. Numbers  Chlorine has two major isotopes Cl - 35 and Cl – 37 Cl – 35 has an abundancy of 75% Cl – 37 has an abundancy of 25%  So what is the atomic mass of Chlorine?

23. Numbers  Chlorine has two major isotopes Cl - 35 and Cl – 37 Cl – 35 has an abundancy of 75% Cl – 37 has an abundancy of 25%  So what is the atomic mass of Chlorine?  A: 35.5 amu (atomic mass units)