ATOMIC THEORY TIMELINE

DEMOCRITUS

• Greek philosopher
• “Nothing exists except atoms and empty space; everything else is opinion.”
• He was the first to suggest that atoms exist.

DALTON

• English chemist and schoolteacher.
• Proposed the first atomic theory.
• Proposed elements are made up of indestructible, small particles called atoms.
• Said all atoms of the same element are identical.
• Stated atoms combine in simple, whole number ratios to form compounds' molecules.

THOMSON

• Conducted the cathode-ray experiment which discovered electrons.
• Interprets that the atoms is breakable.
• Thinks an atom's structure has electrons contained in a positively charged electric field.
• His plum-pudding model interprets that the total electron's mass makes up the atoms's mass
• Also, he proposes the atoms has a lot of empty space.

RUTHERFORD

• Conducted the gold foil experiment and founded the nucleus of an atom.
• Results were 98% of the particles went straight through, 2% went throught but deflected, and .01% bounced off.
• Proposed the nucleus composes the entire mass of an atom and is positively charged.
• Stated that the nucleus had protons, particles the same amount of charge as an electron but opposite sign.

BOHR

• Created a model where electrons moie in orbital paths around the nucleus.
• Found electrons farther away from the nucleus have more energy.
• Stated specific orbits with specific energies are allowable only.
• Said when atoms gain energy, electrons jump from a lower energy orbit to a further one = excited state.
• Electron jump from a higher energy orbit to a closer one, emitting a quantum of energy = ground state.

BOHR CONTINUED

• Created a model that accurately predicts the spectrum of hydrogen.
• It fails when it applies to elements with more than one electron.

SCHRODINGER

• Founded an equation that predicts the probabilityof finding an electron with a certain amount of energy...
• ...at a certain location in the atom.
• Called the quantum-mechanical model of the atom.