Atomic History and Theories

The History of Atomic Theory They were WRONG….ALOT! The History of Atomic Theory

Remember! A LAW describes what happens in nature, but doesn’t explain why. A THEORY is an explanation of what happens in nature based on repeated observations or experiments.

The History of Atomic Theory

Democritus Greek philosopher, not scientist 1st to suggest matter is made of tiny particles called atoms “atomos” “atomos” = Greek for “indivisible” Atomic Model: atoms were small, hard particles that were different shapes and sizes

John Dalton First to propose an ATOMIC THEORY based on science: 1. Each element is composed atoms. 2. All atoms of a given element are identical*; the atoms of different elements are different. 3. Atoms are not created or destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine. 5. In a chemical rxns elements are combined, separated or rearranged.

*Asterisk The Asterisk by number 2 in Dalton’s Atomic Theory reminds us that it was later found to be incorrect. We now know that all atoms of a given element are NOT identical due to the discovery of isotopes. Remember ISOTOPES are an atom with the same number of protons, but DIFFERENT number of neutrons!

Dalton’s atomic model: a tiny sphere that is indivisible

J.J. Thomson 1st to suggest that there were particles SMALLER than the atom!! Discovered the 1st subatomic particle…….. the ELECTRON! Experiment: Cathode Ray Cathode ray can be deflected by a magnetic field because electrons have negative charge Developed the “plum pudding model.”

Thomson’s “plum-pudding” model: the atom is composed of electrons surrounded by a cloud of positive charge to balance the electron's negative charge

Remember! Thomson vs. Dalton Thompsons model proved Daltons Atomic Theory was wrong…. Atoms are divisible into smaller subatomic particles

Ernest Rutherford gold foil experiment discovered the NUCLEUS, and realized that it was very dense and contained positively charged PROTONS suggested that most of the atom is empty space

Gold Foil Experiment: Rutherford shot tiny positively-charged alpha particles through a thin sheet of gold foil What Rutherford expected: What really happened:

Gold Foil Experiment: Most of the particle passed straight though, BUT some of the alpha particles bounced back. Some of the alpha particles had run into concentrations of + charge and, since like charges repel, they had been hurled straight back by them.

Rutherford’s atomic model: All of the positive charge is crammed inside a tiny, massive nucleus about ten thousand times smaller than the atom as a whole. The atom is mostly empty space.

Scientists were looking for missing mass Scientists were looking for missing mass. The protons and electrons in the atom didn’t weigh enough to account for the mass of the atom

James Chadwick Rutherford’s co-worker proved the existence of NEUTRONS, neutral particle in the atom’s nucleus.

Chadwick’s model of the atom Protons and neutrons are in the nucleus, which only takes up one millionth of a billionth of the volume of the atom. The electrons are in the electron cloud, which constitutes almost all of the volume of the atom.

Neils Bohr Proposed that electrons are made up of different shells

Subatomic Particles to Know Location Mass Charge Proton Nucleus 1 amu +1 Neutron Electron Cloud Practically zero -1

Foldable Outside Democritus Dalton Thomson Rutherford Chadwick Bohr Inside Include a description of each scientists contribution to the atom. Draw a picture of each model. Example on the board