1. History of the Atom and Periodic Table

2.  Around 400 B. C., Democritus claimed all matter was made of tiny, indivisible particles called “atoms”.  Aristotle disagreed and claimed matter was made of earth, wind, water, and fire. Democritus and Aristotle

3.  Mendeleev was responsible for creating the 1 st periodic table. Elements were arranged by atomic mass. Dmitri Mendeleev

4.  Henry Moseley created the Modern Periodic Table of Elements that we use today. It is arranged by atomic number rather than mass. Henry Moseley

5.  In the early 1800s, Dalton created his atomic theory about matter. John Dalton

6.  Thomson discovered the first subatomic particle called the electron.  Electrons are negatively charged and orbit the nucleus.  He later discovered a subatomic, positively charged particle in the nucleus called the proton. J. J. Thomson

7.  Rutherford realized that atoms had a nucleus in the center that is orbited by electrons. Between the two is a lot of empty space. Ernest Rutherford

8.  Bohr expanding on the atomic model by placing electrons on separate energy levels. Niels Bohr

9.  Chadwick discovered the last subatomic particle that had a neutral charge and it is called the neutron.  His discovery made us realize isotopes existed.  Isotopes are atoms of the same element with a different number of neutrons.  Proved Dalton’s Atomic theory was incorrect again by showing atoms of the same element can be different. James Chadwick