1. Chapter 4

2. Atom Smallest particle of an element that retains its chemical identity Atoms of the same element cannot be separated chemically because they share identical chemical properties

3. Daltons Atomic Theory Dalton have four postulates (characteristics) of atoms First postulate Second postulate Third postulate Fourth postulate

4. First Postulate All elements are composed of tiny indivisible particles called atoms – Atoms cannot be divided into smaller pieces

5. Second Postulate Atoms of the same element are identical. – Atoms of the same element have identical chemical and physical properties. This is why they cannot be separated from one another by chemical means

6. Third Postulate Atoms of different elements can physically mix together or can chemically combine in simple whole number ratios to form compounds – Remember when elements mix they do not have to form compounds

7. Fourth Postulate Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms, however, never change during a chemical reactions

8. Revision of Daltons First Postulate Daltons first postulate was later revised to read – Atoms cannot be divided by chemical means No chemical reaction can divide atoms. However, nuclear weapons, and nuclear power have shown how atoms can be divided by nuclear processes

9. Subatomic particles Three sub atomic particles exist Protons Electrons Neutrons

10. Protons Discovered by Eugen Goldstein Positively charged particle found inside the nucleus of an atom Protons have a mass of approximately 1 amu

11. Neutrons Neutral particles (carry no charge) Found in the nucleus of an atom Neutrons have a mass of approximately 1 amu

12. Electrons Discovered by J. J. Thompson with the Cathode Ray Tube experiment Carry a negative charge Found outside the nucleus of the atom Have very little mass (approximately 1/1840 the mass of a proton)

13. Cathode Ray Tube Experiment Passed a “beam of electricity” called a cathode ray through a gas in a cathode ray tube The beam responded to magnetic and electric forces which indicates it was charged particles Those particles were later called electrons

14. Cathode Ray Tube Experiment Because the cathode ray tube experiment could be repeated with many types of anodes and cathodes it was found that electrons resided in all materials. Hence all elements contained electrons

15. Rutherford Gold Foil Experiment Discovered atoms had a nucleus Fired alpha particles into gold foil Results

16. Most particles went straight through the gold foil – Atoms are mostly space Some particles bounced back – Atoms have dense tightly packed core