The History and Models of Atoms Atomic Theory The History and Models of Atoms

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

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.

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:

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

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.

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.

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.

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

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.

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