ATOMIC THEORY TIMELINE

DEMOCRITUS

• 460-370 B.C. philosopher and scientist
• First person to believe in small, infinitely divisible particles called atoms

DALTON

• ConclBelieved that each element is composed of tiny, indestructible particles called atoms that are identical in that they have the same mass, volume, and other properties
• Discovered that atoms combine in simple whole-number ratios to form molecules of compounds
• He was also able to determine the relative masses of the atoms using his scale based on H = 1 amu.

THOMSON

• Discovered electrons
• Proposed his theory of electrons with his plum pudding model
• Believed the atom was unbreakable and the atom's structure has electrons suspended in a positively charged electric field

RUTHERFORD

• Predicted that the mass of the atom is due to the mass of the electrons and that there must be a lot of empty space in the atom
• Tested the plum pudding model with his gold foil experiment
• Used gold foil as a target atom and alpha particles as "bullets"
• Rutherford's Results: 98% of particles went straight through, 2% of particles were deflected, and 0.01% of particles didn't go through
• Discovered neutrons as well as the positively charged nucleus and wrongly predicted that electrons were dispersed in the empty space surrounding the nucleus

BOHR

• Created the Bohr model in which electrons travel in orbits around the nucleus
• Each orbit is characterized by a number and a specific amount of energy; the larger the number, the more energy it has and the further it is from the nucleus
• When the electron gains energy and moves up a ring it enters the excited state; when it leaps from a high energy state to one closer to the nucleus it returns to the ground state
• Only works on atoms with one electron (hydrogen)

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