1. Chemistry Chapter 4 Notes #1 09/10/10

2. Democritus Matter is composed of empty space where atoms move Atoms are solid, homogenous, indivisible, and indestructible Different types of atoms have different sizes and shapes Differing properties of matter are due to the size, shape, and mvmt of atoms Apparent changes in matter result from changes in the groupings of atoms and not in changes in the atoms themselves

3. Democritus 1 st person to believe that matter was not infinitely divisible Just ideas – no science (proof) 460 – 370 BC Completely rejected by Aristotle (did not believe in atoms)

4. John Dalton 1766-1844 English School Teacher Used science to prove Democritus’ ideas Beginning of modern atomic theory

5. John Dalton All matter is composed of very small particles (atoms) All atoms of a given element are identical (having same mass, size, and properties) Atoms of a specific element are different from those of any other element Atoms cannot be created or destroyed or divided into smaller particles Different atoms combine in simple whole number ratios to form compounds In a chemical reaction, atoms are separated, combined, or rearranged

6. Since Dalton…. Modern atomic theory has been refined “Atom” Smallest particle of an element that retains the properties of that element

7. Sir William Crookes Cathode ray – typical set up used to study mass and charge of particles Crookes noticed flashes of light (radiation) He tested and proved Cathode rays were a steam of charged particles The particles had a negative charge After changing the electrodes and the gas in the tubes, the ray was still present, indicating that these particles were in all matter

8. JJ Thomson & Robert Millikan Continued with Crookes work Found that the mass of these particles was less than that of Hydrogen What did this mean? Something smaller than the atom! Atoms are divisible Millikan did work at the same time – said these particles were negative – called them electrons Thomson Proposed the Plum Pudding Model Positive atom with evenly dispersed negative

9. Ernest Rutherford Disproved the plum pudding model with the gold foil experiment Rutherford set forth the idea of the nuclear atom (with a dense center) He called it a “nucleus” It has a positive change Almost all of the atom’s mass Negatives charges surround the nucleus – held together by attraction between pos. & neg. 8 years later = protons (1920) 1932 – his coworker (Chadwick) discovered neutrons

10. Neils Bohr In atomic physics, the Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus — similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity.atomic physicsatom nucleuselectronssolar system electrostatic forcesgravity