Detailed History and Theory of the Atom

Foundations of Atomic Theory

• Definition of the Atom: The smallest particle of an element that retains the properties of that element.

• Democritus (400 B.C.): He was the first individual to mention the concept of the atom.

• Key Scientists in Atomic History:

  • John Dalton

  • J.J. Thomson

  • Robert Millikan

  • Ernest Rutherford

  • James Chadwick

  • Niels Bohr

John Dalton's Atomic Theory (1766–1844)

• Model Name: Billiard Ball Model, which posits that there is nothing inside the atom.

• Five Principles of Dalton's Theory:

  1. Every element is made up of tiny particles called atoms.

  2. All atoms are indivisible and indestructible.

     • Revision Note: This is considered an incorrect fact because atoms actually contain protons, neutrons, and electrons.

  3. All atoms of a given element are identical to one another and different from atoms of other elements.

     • Revision Note: This is considered an incorrect fact because atoms of the same element with a different number of neutrons are known as isotopes.

  4. Atoms of two or more different elements combine to form compounds in the same ratios each time.

     • Example 1: Oxygen ($O$) and Hydrogen ($H$) combine to form $H_2O$.

     • Example 2: Sodium ($Na$) and Chlorine ($Cl$) combine to form $NaCl$.

  5. A chemical reaction involves the rearrangement, separation, or combination of atoms. Atoms are NEVER created or destroyed in a chemical reaction.

     • Reaction Example: $2H_2 + O_2 \rightarrow 2H_2O$

Subatomic Particles and Properties

• Components of an Atom: Atomic particles are smaller particles within an atom including neutrons, protons, and electrons.

• Proton Properties:

  • Charge: Positive charge ($+$)

  • Mass: $1\,a.m.u.$

  • Location: Inside the nucleus

• Neutron Properties:

  • Charge: Neutral, no charge ($0$)

  • Mass: $1\,a.m.u.$

  • Location: Inside the nucleus

• Electron Properties:

  • Charge: Negative charge ($-$)

  • Mass: Negligible mass ($0\,a.m.u.$)

  • Location: Outside the nucleus

J.J. Thomson and the Plum Pudding Model

• Discovery Year: 1897

• Discovery: Discovered the electron using a cathode ray tube.

• The Model: The Plum Pudding Model describes electrons embedded within a positively charged atom.

Robert Millikan and Electron Properties

• Year: 1913

• Contribution: Calculated the charge of electrons in Coulombs.

• Mass Calculation: Calculated the mass of electrons.

Ernest Rutherford and the Gold Foil Experiment

• Discovery Year: 1911

• Experiment: The gold foil experiment. In this experiment, a source fired particles at gold foil. While most particles went right through the foil, some were deflected.

• Rutherford's Conclusions:

  1. The atom is mostly empty space: The evidence for this was that most alpha particles went straight through the gold foil.

  2. The atom has a very small, dense nucleus: The evidence for this was that very few particles were deflected away from the foil.

  3. The nucleus is positive: The evidence for this was that the alpha particle was deflected away and was not attracted to the nucleus.

James Chadwick and the Neutron

• Discovery Year: 1913

• Contribution: Discovered the neutron.

• Function: The neutron is located in the nucleus and helps hold the nucleus together.

Niels Bohr and the Planetary Model

• Year: 1913

• Model Name: Planetary Model.

• Structure: Electrons orbit around the nucleus in fixed paths, similar to the way planets orbit around the sun.