1. Atomic Theory Chapter 11, Section 1

2. Ancient Greeks AristotleDemocritus (440 BC)  Break things apart forever and keep their identity  All things were composed of 4 ‘elements’ (Earth, Air, Fire, Water)  Keep breaking in half - eventually get to something that can’t be broken down (indivisible particle)  “THE ATOM” Democritus was a PHILOSOPHER, not a SCIENTIST. He used reasoning, not experiments to come up with his idea. What are all things made of? The debate about the nature of matter!

3. Ancient Greeks AristotleDemocritus (440 BC)  Break things apart forever and keep their identity  All things were composed of 4 ‘elements’ (Earth, Air, Fire, Water)  Keep breaking in half  Eventually get to something that can’t be broken down (indivisible particle)  “THE ATOM” Atoms are small, hard particles made of a single material and formed into different shapes and sizes. Atoms are always moving. Atoms form different materials by joining together. What are all things made of? The debate about the nature of matter! Although Democritus is closer to being correct, Aristotle's ideas were more widely accepted. 1800’s  experimental data began to support Democritus Although Democritus is closer to being correct, Aristotle's ideas were more widely accepted. 1800’s  experimental data began to support Democritus

4. Dalton’s Atomic Theory (1803) 1 st to provide experimental evidence of atoms. Experimented with combining elements Noticed that elements combined in specific proportions

5. Dalton’s Atomic Theory (1803) DALTON SUMMARIZED 1.Atoms are the smallest particles 2.Can’t Be Divided (nothing inside atom) 3.Atoms of the same type make ELEMENTS 4.ELEMENTS join to make new substances (COMPOUNDS) –2 H + O  H 2 O (Water is made of two types of atoms)

6. Thomson’s Experiment (1897) Used a cathode-ray tube. –Invisible beam would “bend” towards positive side  beam made of negative particles –Discovered electrons

7. Thomson’s Model (1897) Discovered atoms are made of smaller parts. Since atoms are neutral, they must also have positive particles (to balance the negative ones he discovered). Proposed Plum-Pudding model or ‘choc chip ice-cream’ model Ice cream = Positive charges (protons) Chocolate Chips = Negative charges (electrons)

8. Rutherford (1911) Gold foil experiment – aimed beam of positively charged particles at thin sheet of gold foil –If atoms were “soft blobs” the particles should pass straight through

9. Rutherford (1911) Only a few particles bounced back  Atoms have dense positively charged centers (nucleus) Most particles passed straight through the foil  Atoms have lots of empty space.

10. Bohr Model (1913) Used crazy math tricks to build on Rutherford’s model Electrons travel in paths (around nucleus) and are found in specific energy levels Atoms can jump between levels from path to path We will use the Bohr model to represent atoms in here AKA “Solar System model”

11. Modern Theory – Twentieth Century Scientists Electrons move unpredictably but spend more time in certain regions than others. Electron clouds – where electrons are most likely to be found