ATOMIC THEORY TIMELINE

DEMOCRITUS

• Democritus was a Greek philosopher (460-370 B.C.)
• He said, "Nothing exists except atoms and empty space;
• everything else is a matter of opinion."
• He was among the first to suggest the existence of atoms.
• He reasoned that atoms were indivisible and indestructible.

DALTON

• John Dalton was an English chemist and school teacher
• Dalton transformed Democrtitus's ideas on atoms into scientific theory
• His results led him to formulate what is called Dalton's Atomic Theory
• 1.) All elements are composed of tiny, individual particles called atoms

DALTON'S ATOMIC THEORY (CONT.)

• 2.) Atoms of the same element are identical. The atoms of any one
• element are different from those of any other element
• 3.) Atoms of different elements can physcially work together or can
• chemically combine in simple, whole-number ratios to form compounds
• Chemical reactions occur when atoms are seperated from each other, joined or rearranged in a different combination

THOMSON

• In 1897, English physicist, J.J. Thomson discovered the electron
• He preformed experiments that involved passing electric current through
• gases at low pressure. The result was a glowing beam, a cathode ray.
• His interpertaion is called the Plum Pudding Model .He discovered that the
• atom is unbreakable & electrons are suspended in a positively charged field.

RUTHERFORD

• In 1911, Rutherford conducted his gold foil expirememt
• He wanted to test the existing plum-pudding model of atomic structure
• A narrow beam of alpha particles was directed at a thin sheet of gold foil
• Most alpha particles went straight through the foil or were slightly deflected
• He discovered that atoms contain a tiny, dense center called a necleus

SCHRÖDINGER

• Schrödinger discovered an equation that predicts the probability
• of finding an electron with a particular amount of energy at a particular location
• in the atom.

QUANTUM MECHANICAL MODEL

• The positivley charged nucleus contains protons and electrons
• The electron cloud is a visual model of the probable locations of
• electrons in an atom. The probability of finding an electron is
• higher in the denser regions of the cloud.