1. Structure of Atoms The History and Models of Atoms

2. Scientific Theory  Definition: scientific principle to explain phenomena: a set of facts, propositions, or principles analyzed in their relation to one another and used, especially in science, to explain phenomena. Microsoft® Encarta® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.  Simply put, it is the best explanation there is to a series of events.  Calling it “just” a theory implies there is no evidence; a theory in science is always supported by all the evidence available at the time.  Example: Geocentric Theory vs. Heliocentric Theory

3. Scientific Theory Observations (are made) Inferences (explanations) Hypothesis (“if”-based on an assumption-; “then”, -prediction of outcome) Theory (best explanation and supported by evidence)  Even though a theory is supported by evidence, as new evidence is collected, it is subject to changes.

4. Studying Atoms  Scientists have been studying atoms since the 1800s.  If atoms are too small to be seen, how do we know about them?  House:

5. Studying Atoms  Scientists have been studying atoms since the 1800s.  If atoms are too small to be seen, how do we know about them?  If atoms are too small to be seen, how do we study them?  Scientists use models to study atoms.  Sometimes scientists can tell what things look like by studying how they act.  Example: Wind

6. Models  In science, a model is an idea, a picture, or an object that is built to explain how something else looks or works.  The model might not look exactly like the object it is built to describe, but it helps people understand the way the object acts.

7. History of the Atom  Democritus, a Greek philosopher, first used the term atom (atomos) around 400 BC.  Both stated that if matter is cut several times, eventually it could not be cut anymore.  In 1803, John Dalton showed that materials were made of atoms.

8. History of the Atom  In the late 1800s we discovered that the atom has many smaller parts.  In the late 1800s, English physicist J. J. Thomson discovered the electron.

9. History of the Atom  In 1901 Ernest Rutherford discovered the nucleus, or center of the atom.

10. History of the Atom  In 1917 Niels Bohr, working as Ernest Rutherford’s student, discovered how electrons orbit the nucleus.  His model of the atom is the current base for the modern atomic model.

11. History of the Atom  The electron cloud is the region around the nucleus occupied by electrons.  Atoms are much, much denser near the nucleus.