1. History of the Atomic Theory

2. Democritus  Greek Philosopher 400 B.C.  His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained.  He named the smallest piece of matter “atomos,” meaning “not to be cut.

3.  This theory was ignored and forgotten for more than 2000 years!

4. John Dalton Early 1800s - He came up with the 4 principles of matter…. 1. All elements are made up of “atoms” 2. Atoms of the same element are exactly alike. Atoms of different elements are different. all Au atoms are “the same” all Al atoms are “the same” but Al and Au are different 3. Compounds are formed by the joining of atoms of two or more elements. 4. Atoms can be rearranged to form compounds, but they are never changed.

5. Dimitri Mendeleev  By the mid 1800’s there were about 70 elements known.  Mendeleev came up with a way to organize them, based on their atomic mass.  He essentially created the 1 st periodic table

8. JJ Thomson  Thomson passed a current through a cathode ray tube. The current moved towards the positive charge.  It had a negative charge – Why?

9.  He suggested that an atom is made of even smaller particles.  He proposed a model of the atom called the “Plum Pudding” model.  Atoms made up of a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

10. Neils Bohr  1913  Electrons revolve around the nucleus like planets revolve around the sun.  Electrons are located in different energy levels further and further from the nucleus.  We now know this model isn’t accurate, but it is simple and often still used.

11. Ernest Rutherford  Early 1920’s  He fired positively charged particles at a thin sheet of gold foil  Most of the positively charged particles passed right through the gold atoms in the sheet of gold foil without changing course at all.  Some of the positively charged particles however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

12. An atom is mostly empty space. If a nucleus were the size of a grape seed and sat in the middle of Mile High Stadium, the closest electron would be in the nose bleed seats.

13. James Chadwick  1930  Did a similar experiment to Thomson, using a cathode ray tube and discovered the neutron.

14. Modern Electron Configuration Model Also Called the Wave Model  Electrons do not move about an atom in a definite path, like the planets around the sun, but they are not moving in random patterns either.  Their possible location depends upon the amount of energy the electron has.