A HISTORY OF ATOMIC THEORY

DEMOCRITUS (460 BC-370 BC)

• One of the first to suggest the existence of atoms
• Had the idea that there must me some tiny amount of mass that can longer longer be divided
• "Nothing exists except atoms and empty space; everything else is opinion"

JOHN DALTON (1766-1844)

• English Chemist
• Studied the ratios in which elements combine in chemical reactions
• Dalton used the observations of his work to come up with his atomic theory.
• Was able to determine the relative mass of elements

DALTON'S ATOMIC THEORY

• Each element is composed of tiny, indestructible particles called atoms
• All atoms of an element are identical
• Atoms combine in simple, whole-number ratios to form molecules of compounds
• Chemical reactions occur when atoms are separated from each other, joined, or rearranged in a different combination. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction

J.J. THOMSON (1856-1940)

• Discovered the electron
• Preformed the cathode-ray tube experiment
• Came up with the plum pudding model of the atom
• This model theorized that atoms are breakable and has electrons suspended in a positively charged electric field

ERNEST RUTHERFORD (1871-1937)

• Wanted to prove Thomson's "empty atom" theory
• Shot alpha particles through gold foil
• Some of the particles were deflected slightly or came back
• Concluded that all the positive charge and almost all the mass are concentrated in a small region
• Rutherford's model is called the nuclear atom, after his dicovery of the nucleus

NIELS BOHR (1885-1962)

• Proposed that an electron is found only in specific circular paths, called orbits, around the nucleus
• In his model of the atom, each orbit has a specific amount of energy
• Found that when an atom gains energy, the electron leaps from a lower orbit to a higher one
• This is called an excited state
• When the electron goes from a higher to a lower orbit, it releases energy as a photon of light- a quantum of energy

ERWIN SCHRÖDINGER (1887-1961)

• Discovered an equation that predicts the probability of finding an electron with a certain amount of energy at a certain location in the atom
• Came up with the Quantum-Mechanical model of the atom, which contained orbitals, regions with a high probability of finding an electron
• This model also contains shells and subshells of the atom, where orbitals can be found