1. History of Atoms Notes Use the information on each slide to fill in the blanks on the student notes.

2. History of Atoms Democritus Who first thought of the atom? The Greek philosopher Democritus (about 400 BC) – he thought if you cut an object over and over and over again, there would eventually be a piece that you couldn’t cut anymore. He called that piece an atom. In Greek, atom means “indivisible”. His idea of an atom was a round, solid mass. Democritus by Agostino Carracci (1557-1602) Round, Solid Atom

3. History of Atoms John Dalton The Atomic Theory John Dalton was an English Scientist. In 1803 he proposed the atomic theory that says: -Atoms cannot be divided -Atoms are invisible -All elements are made of atoms -Atoms of the same element are the same -Atoms of different elements are different -Molecules are formed by two or more atoms joined together Round, Solid Atom Billiard Ball Model

4. History of Atoms Joseph J. Thomson What are atoms made of? In 1897 a scientist named Joseph J. Thomson was experimenting with gases and electricity and discovered that atoms have negative and positive charged particles. The negative charged particles are called electrons and the positive charged particles are called protons. He imagined an atom to be similar to plum pudding with lumpy, negative electrons in the smooth, proton pudding.

5. History of Atoms Ernest Rutherford What else are atoms made of? Ernest Rutherford discovered in 1908 that atoms are mostly made of empty space while doing his now famous GOLD FOIL experiment. In the center of the atom is a dense nucleus of protons surrounded by electrons. In fact, if the nucleus of an atom was the size of a pea, the atom would be as big as a football stadium! The electrons would be flying around the outside of the stadium and the rest of the atom would be empty space! Rutherford’s electrons orbit the center of the atom.

6. History of Atoms Ernest Rutherford Imagine one of these in the middle of there. All the rest of the stadium is empty space and the electrons are flying around the outside.

7. Ernest Rutherford’s Gold Foil Experiment Ernest Rutherford’s biggest achievement in atomic theory (empty space) was made during his GOLD FOIL experiment. He shot a beam of alpha particles (radioactive) through a thin gold foil sheet, expecting to have them pass through the plum pudding model of the atom. However, a small number of positive particles were repelled at very high angles while most were not deflected at all, passing right through. This showed there was a solid center to the atom that the particles bounced off of. Rutherford's gold foil experiment helped describe the nuclear structure of the atom by proving the existence of positive particles lumped together in the center of the atom – later called the nucleus. Top: Expected results - alpha particles passing right through the plum pudding model of the atom undisturbed. Bottom: Observed results - small portion of the particles were deflected, and some by very large angles, indicating a small, concentrated positive charge. Rutherford concluded that the positive charge must be all in the center of the atom.

8. History of Atoms Niels Bohr What do atoms look like? Niels Bohr in 1913 proposed that the atoms look like a solar system, with the protons in the nucleus acting like the sun, and the electrons flying around the nucleus like planets orbiting the sun. Electrons orbit the nucleus in shells. When drawn this way, it is called a Bohr diagram of an atom. Bohr Diagram: electrons are drawn in shells around the nucleus Bohr believed the electrons orbit the nucleus

9. History of Atoms Werner Heisenberg What do we currently believe? Werner Heisenberg in 1926, proposed that electrons don’t really fly all around the atom. He said that electrons had certain spots they floated around in called orbitals (sort of like balloons). He created Quantum Mechanics (mathematical description of energy and matter) and the Uncertainty Principle which says you cannot know the exact position and speed of an electron at any given moment. Heisenberg’s atomic model uses an electron cloud. The electron cloud maps the possible location of electrons in an atom instead of specific locations. Heisenberg’s Model: Electron Cloud (dots are possible spots electrons may be found)

10. History of Atoms James Chadwick The Last Piece of the Puzzle James Chadwick in 1932, discovered that atoms also contain neutrons. Neutrons do not have a charge, they are “neutral”. They have the same mass as a proton. The neutrons and the protons make up the nucleus, or the center of the atom. Chadwick discovered neutrons in the nucleus of an atom

11. History of Atoms Scientist Mnemonic Device Did Democritus Dumbledore Dalton Try Thomsen Repelling Rutherford Behind Bohr HogwartsHeisenberg Castle?Chadwick