1. Structure of the Atom - Models Chapter 19 Section 1 – Part 2 Pages 580-583

2. Models – Tools for Scientists Models are used to represent things that are difficult to visualize / see, or picture in the mind. Models must accurately represent all information known about what is being modeled.

3. The Changing Atomic Model Figure 5, page 582 - Models of the atom have changed through time. 1. Democritus (~400 B.C.)  Greek philosopher  Three main ideas about matter  Three main ideas about matter a. Universe made of invisible units, called them “atomos”  Atomos – means uncuttable  Atomos – means uncuttable b. Movements of these atoms caused the observed changes in matter. observed changes in matter. c. Theory explains some of his observations - Lacked evidence

4. The Changing Atomic Model Figure 5, page 582 2. In the 1700’s emphasis was placed on careful and repeated measurements.

5. The Changing Atomic Model Figure 5, page 582 3. John Dalton (1808) – English school teacher English school teacher - Widely accepted atomic theory due to supporting evidence. - 3 main points a. Every element is made of tiny unique particles tiny unique particles (atoms) that cannot be (atoms) that cannot be divided. divided. b. Atoms of the same element are exactly alike. element are exactly alike. c. Atoms of different elements can join to form elements can join to form molecules. molecules.

6. The Changing Atomic Model Figure 5, page 582 4. Joseph John Thomson (1904) – English physicist - Inferred by experimentation that atoms contain negatively charged particles contain negatively charged particles  electrons  electrons - Thought electrons were - Thought electrons were evenly embedded in evenly embedded in positively charged sphere. positively charged sphere.  Like chocolate chip cookie dough  Like chocolate chip cookie dough

7. The Changing Atomic Model Figure 5, page 582 5. Ernest Rutherford (1911) – British physicist - Proposed most of an atom’s mass (all the positive charge) positive charge) is in the nucleus is in the nucleus - Nucleus surrounded by electrons by electrons - “Gold Foil” experiment

8. The Changing Atomic Model Figure 5, page 582 6. Neils Bohr (1913) – Danish physicist - Proposed that electrons orbit electrons orbit nucleus in fixed nucleus in fixed paths paths - Like planets orbiting the sun orbiting the sun

9. The Changing Atomic Model Figure 5, page 582 7. James Chadwick (1913) – Rutherford’s student - Nucleus contains neutral neutrons and positive protons

10. The Changing Atomic Model Figure 5, page 582 8. Electron Cloud model – (1926) the current model - Electrons do not follow fixed orbits - Tend to occur more frequently in certain areas around nucleus areas around nucleus  Looks like a fan blade spinning - This model basically describes a cloud-like region - This model basically describes a cloud-like region where the electron is likely to be found. where the electron is likely to be found.

11. Electron Cloud Model / Modern Atomic Model - Impossible to accurately locate an electron at any given time. electron at any given time.