Atomic Theory

Mr. Tollefson

Chemistry Class

Introduction

• Students will be introduced to the atom and the development of the atomic theory from ancient times to the present day. It is your job to take notes on the pertinent information in this presentation.

The Atom

• Democritus, an Ancient Greek philosopher, first purposed the word ‘Atom’ around 375 BC. The atom was defined as a small round indivisible particle. Democritus described the atom by talking about the constant division of an object until the object could no longer be divided. The idea of the Atom had to wait for 2200 years to be proven.

Dalton and Atomic Theory

• John Dalton’s (d1844) Atomic Theory consists of 4 parts. It was the first departure from the ancient idea of the atom. Most of the elements of this theory are still true today.

Dalton and Atomic Theory

– All elements are composed of indivisible particles called atoms.

– Atoms of the same element are identical. Atoms of element are different from others.

– Atoms can mix together in whole number ratios– Chemical reactions occur when atoms combine,

separate or rearrange

Electron’s: Discovery 1897

• The English physicist Sir JJ Thompson discovered the electron in 1897. He experimented with sealed tubes with gases in them. He passed high voltage electric current through the tubes. The glowing beam created is called a cathode ray.

Cathode Rays

• Thompson thus created CRT’s or Cathode Ray Tubes, the basis for televisions and computer monitors.

• Thompson found that the rays were attracted to metal plates that carry positive charges. Thompson proposed that a cathode ray is composed of tiny negatively charged particles about 1/2000 the mass of the hydrogen atom. He called them electrons.

Plum Pudding

• Thompson also worked with modified cathode ray tubes that gave rays traveling in the opposite direction. These particles were found to have the same amount of charge as electrons, just the opposite sign. These were later called protons.

PLUM PUDDING

• From what Thompson learned about protons and electrons he proposed his ‘Plum Pudding’ model of the atom. Protons and electrons are mixed together in a big blob with no particular order.

Nucleus

• Ernst Rutherford made a major advance in atomic theory with a simple experiment with complex results (1912). Sometimes called the ‘gold foil experiment’ it is the foundation of the modern atom.

Radioactive Source

Gold Foil

Phosphorescent

Screen

Alpha

Radiation

Diagram for gold foil experiment

Deflected radiation

GOLD FOIL EXPERIMENT

• Using a radioactive source Rutherford fired alpha particles at a very thin piece of gold foil. He expected the alpha particle to easily pass through the foil and strike the screen on the opposite side. He found that some of the particles were deflected at small angles and some were reflected straight back.

Nucleus

• The results of Rutherford’s experiment was his proposal of the nuclear atom. He proposed that in the center of an atom was a very small and dense ‘nucleus’ and that the nucleus was composed of protons.

Rutherford Atom

• The electrons in the Rutherford atom hold no specific place in the atom, they are free to move about. So, the Rutherford model of the atom has a small dense nucleus with free roaming electrons.

BOAR? BOOR? BOHR? BAOR

• Neils Bohr worked with Lord Rutherford for a short while and came up with his own hypothesis for the structure of an atom. It is called the planetary model of the atom.

Planetary Atom

• The only difference between the planetary atom and the Rutherford atom is the placement of the electrons in orbitals. Electrons can only occupy specific orbits at specific energy levels around an atom. The farther away an electron is from the nucleus the higher it’s energy state.

Planetary atom

Electrons in orbit, nucleus at center.

Electrons = 2n2

The Modern Atom

• Today’s atom is very similar to the Bohr atom. The modern atom is sometimes called the quantum mechanical atom. That is because the theory was based on a type of physics called quantum mechanics.

Quantum Mechanics

• Todays Atom is only different from Bohr’s Atom in that it does not actually occupy orbits. It has a probability of occupying certain places around the atom and 90% of the time it spends in the ‘Bohr Orbits’. The position of the electron is determined by it’s energy.

Schrodinger

• The main mathematics for the quantum atom were performed by Erwin Schrodinger in the 20’s and 30’s. The equation that describes the position of an electron around an atom is called Schrodinger’s Equation and deals in differential calculus.

Neutron

• The Final sub-atomic particle was found in the 1930’s by James Chadwick. He proposed this particle had no charge but was as massive as the proton and also resided in the nucleus. He called it the neutron, after it’s neutral charge.

Today

• Today we think there are sub-sub-atomic particles called ‘quarks’. Each Proton, neutron and electron is made up of 3 quarks. Quarks come in many different types and have partial charges of 2/3 and -1/3. Quarks also have other properties named, strangeness, charm, flavor, and color.