HISTORY OF THE ATOM

Aristotle 400 BC 400 BC - Claimed that there was no smallest part of matter - Claimed that there was no smallest part of matter - Thought different substances were made up of proportions of fire, air, earth, and water.

Democritus 400 BC 400 BC - Matter could be subdivided into smaller and smaller particles until a basic part of matter could not be divided further - He called these particles atoms, which means “indivisible” in Greek.

John Dalton Dalton’s Atomic Theory 1. All matter is made of atoms. Atoms are indivisible and indestructible. 2. All atoms of a given element are identical in mass and properties 3. Compounds are formed by a combination of two or more different kinds of atoms. 4. A chemical reaction is a rearrangement of atoms

Sir William Crookes ’s - Studied the effects of magnetic and electric fields on the cathode ray using a cathode ray tube (CRT)

He discovered cathode rays had the following properties: He discovered cathode rays had the following properties: 1. Travel in straight lines from the cathode 2. Cause glass to glow 3. Transmit a negative charge to objects they strike 4. Are deflected by electric fields and magnets to suggest a negative charge 5. Cause pinwheels in their path to spin indicating they have mass

What is a CRT? 1. Consists of a gas-filled glass tube with 2 metal plates a. 1 negatively charged (the cathode) and the other positively charged (the anode) cathodeanodecathodeanode 2.When a very large voltage is placed across the electrodes a. the neutral gas inside the tube will ionize into a plasma plasma b. The current will flow as electrons travel from the cathode (-) to the anode (+) called a cathode ray.

J.J. Thomson Discovered the electron using the CRT Found that the cathode ray was attracted to a positively charged metal plate Found that the cathode ray was attracted to a positively charged metal plate Cathode ray must be negative because opposites attract. Cathode ray must be negative because opposites attract.

Thomson’s Plum Pudding Model Atoms are neutrally charged (0) Atoms are neutrally charged (0) - there must be a positive charge in the atom, too if there are electrons (-) Made a speculation at what an atom might look like: Made a speculation at what an atom might look like: - a cloud of positive electricity with “electrons” stuck in the cloud (chocolate chip cookie!)

R.A. Millikan ( ) Oil drop experiment measured the charge of an electron - Electron’s charge = x coulombs - Electron’s mass = 9.11 x grams

Ernest Rutherford Used a gold foil experiment to determine Used a gold foil experiment to determine - the tiny center of the atom is dense and positively charged - Discovered: the + nucleus The proton (+) the nucleus has almost all of the mass of an atom most of the atom is empty space assumed electrons were outside the nucleus and create most of the volume of the atom assumed electrons were outside the nucleus and create most of the volume of the atom

The Gold Foil Experiment "It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Niels Bohr Bohr expanded upon Rutherford’s theory by proposing that 1. electrons travel only in certain successively larger orbits. 2. the outer orbits could hold more electrons than the inner ones, and that these outer 3. the orbits determine the atom's chemical properties

The Bohr Model

James Chadwick : Discovered - the neutron in the nucleus (with the + proton) - the neutron has mass, but no (0) charge

Louis de Broglie Discovered: 1. All particles, including light & electrons show both wave behavior & particle behavior 2. Electrons do not travel in any specific pattern - The exact location cannot be determined - There is a high probability of finding electrons in regions (orbitals)

Werner Heisenberg Determined the “Uncertainty Principle” - Any effort to locate an electron will cause the electron to move, therefore, it is impossible to locate electrons