History of ChemistryDiscoveries and Atoms

Early GreeksDemocritus all matter is made of small, indivisible particles called atomosAristotle matter is continuous and NOT made of smaller particles

Robert Boyle (1600s)1st true chemist

Discovered a relationship between pressure and volume (Boyles Law)

Antoine LavoisierMatter cannot be created or destroyed Law of Conservation of Mass

Joseph ProustFound that a given compound always contains exactly the same proportion of elements by massLaw of Definite Proportions

John Dalton (1800s)The ratios of the masses of elements in a compound can always be reduced to small whole numbers

Law of Multiple Proportions

Daltons Atomic Theory1) all matter is composed of tiny particles called atoms2) the atoms of an element are always identical while the atoms of different elements are different3) compounds form when atoms combine; atoms combine in small whole number ratios4) reactions involve reorganization of atoms; the atoms themselves do not change

DaltonProposed the Billiard-ball model of the atom

Joseph Gay-Lussac (1809)Measured the volumes of gases that reacted with one another to develop theLaw of Combining Volumes of Gases

Amadeo Avogadro at the same temperature and pressure, equal volumes of gases contain the same number of particlesAvogadros hypothesis

J.J. ThomsonProduced a cathode ray which was deflected by a negative electric field

Thus the ray must be made of negative particles (electrons)

J.J. ThomsonSince atoms are neutral, they must also have a positive area

Plum pudding model

J.J. ThomsonProtons were found to be 1836 X the mass of an electronCharge of proton is +1

Robert MillikanOil drop experiment to determine the magnitude of the electrons charge

which is now known as -1

James ChadwickDiscovered high energy particles with no charge and the same mass as the proton the neutron

Henri BecquerelAccidentally discovered radioactivityAlpha particles (+2 charge)(Also beta particles, gamma rays)

Ernest Rutherford (1911)Tests Thomsons Plum Pudding Model by shooting alpha particles through a sheet of gold foil

Ernest RutherfordNuclear Model of the Atom

Robert BunsenFound that when heated, different elements produced different colors in a flame

Niels Bohr (1912)Electrons orbit the nucleus somewhat like planets orbit the sunPlanetary Model

Arnold Sommerfeld Expanded the Bohr modelElectrons travel in orbitals, but the orbitals are not the same shape

-- this leads to the electron cloud model of the atom

Electron Cloud Model

Wolfgang Pauli (1924)Predicted that electrons spin while orbiting the nucleus

Paulis Exclusion Principle says no two electrons do the exact same thing at the same time

de Broglie and Schrdinger Propose that electrons move like wave thus the Wave-Mechanical Model

Werner Heisenberg No experiment can measure the position and momentum of a quantum particle simultaneously Heisenbergs Uncertainty Principle

Modern View of the AtomTiny nucleus surrounded by electron cloudNucleus accounts for all of the massArrangement of electrons causes different chemical properties

Electron Cloud ModelNote: Just as no map can equal a territory, no concept of an atom can possibly equal its nature. These models of the atom simply served as a way of thinking about them, though they contained limitations (all models do).