History of the Atom SWBAT #1: Describe the development of the model of the atom through history SWBAT #2: Identify the main components of the nuclear atom

History of Atom Video

Democritus – 400BC Greek philosopher Was first to suggest the existence of atoms Believed all matter was made up of atoms and that atoms were indivisible and indestructible Atoms are indivisible and indestructible No internal Structure

Dalton’s Atomic Theory Using experimental methods Dalton (1766-1844) developed an atomic theory All elements are composed of tiny indivisible particles called atoms Atoms of the same element are identical AND atoms of any one element are different from those of other elements Atoms of different elements can physically mix together to form mixtures or can chemically combine to form compounds Chemical reactions occur when atoms are separated, joined, or rearranged

Dalton’s Atomic Model - 1803 No internal structure

What’s in an Atom Throughout history several scientists have performed experiments to figure out what exactly makes up atoms The two we will be studying are Thomson’s Cathode Ray experiment and Rutherford’s Gold Foil experiment

Thomson Model -1897 Cathode Ray Experiment Discovered electrons Internal Structure Ball of positive charge containing electrons Often referred as the plum pudding model

Cathode Ray At both ends of the sealed tube are metal discs which are connected to electricity In the tube is gas Pass electric current through the gas The cathode ray that showed up when the power was turned on was attracted to a positive plate

Wait….. Let’s think about this If Thomson discovered that atoms have negative particles called electrons…… AND atoms are electrically neutral What else might they have in the same number of electrons? Another scientist named Golstein observed rays traveling in opposite direction of the cathode rays. He called them canal rays. We call them protons! Atoms have protons. Positive charged particles in the same number as electrons.

Rutherford Model - 1911 1. Gold Foil Experiment 2. Discovered the nucleus 3. Concluded that an atom is mostly empty space with the nucleus in the center. AND The nucleus contains almost all the mass of the atom with protons and neutrons inside.

Gold Foil Experiment Due to Thomson scientists believed atoms are just balls of positive and negative particles If you shoot radiation at the atom it should pass strait through. Rutherford sent alpha radiation at gold foil expecting the radiation to pass through Most of it did…….. Some of the radiation bounced back!!! What was the radiation hitting?

Gold Foil Experiment

Rutherford Model - 1911 Internal Structure Nucleus contains protons and neutrons Electrons float around the nucleus

Comparison Thomson - 1897 Rutherford - 1911

The Nuclear Atom Has…. Atoms have a tiny nucleus The nucleus contains most of the mass of the atom Protons and neutrons are found in the nucleus Electrons are outside the nucleus Particle Symbol Charge Mass Electron e- 1- 1/1840 amu Proton p+ 1+ 1 amu Neutron n0

Something to Think About If the atom was the size of a football stadium How big would the nucleus be? The size of a marble (with all the mass of the stadium in it)

Bohr Model - 1913 Internal Structure Nucleus contains protons and neutrons Electrons travel in fixed circular orbits around the nucleus

Comparison Rutherford - 1911 Bohr - 1913

Quantum Mechanical Model - 1926 Internal Structure Nucleus contains protons and neutrons Physicists decide that the atom has no definite shape and electrons don’t have precise orbits Instead electrons are found in regions called atomic orbitals Electron Cloud Draw a circle around the atomic orbitals, high probability of finding all electrons

Comparison Bohr - 1913 Quantum Mechanical Model - 1926

Dalton to Quantum Mechanical Model – The history of the model of the Atom