Development of our understandingof the atom

Early ModelsDemocritus was first to suggest atom in 4th century

B.C.Believed matter was made of tiny individual

particles called atomos meaning indivisible. Believed that atoms could not be created,

destroyed, or further divided.

Aristotle was one of the most respected Greek philosophers.Disagreed with Democritus because it was against

his views. Matter is made of earth, fire, air and water.

Did not believe that that atoms moved through empty space.

John DaltonDalton’s model was the “Billiard Ball”

Atomic Theory Matter is composed of atoms Atoms are indivisible Atoms of a given element have same size, mass and chem

props and are different from those of another element Different atoms combine in whole number ratios to form

compounds and are separated, combined and rearranged in chem reactions

Conservation of Mass-is the result of the separation, combination, or rearrangement of atoms-atoms are not created, destroyed, or divided in the process.

Dalton’s Theory had to be revised because he …

Dalton was incorrect about atoms being indivisible.Atoms can be divisible into several subatomic

particles.

Dalton was also incorrect about all atoms of a given element having identical properties. Atoms of the same element can have slightly

different masses.

Early Models

Thomson used the Cathode Ray – determined atoms were made of particlesNegative charges in atom“Plum Pudding” model

Millikan used oil drop experiment Found mass and charge of an electron

Important discoveries

Rutherford experimented with Gold FoilFound nucleus Discovered its positive charge and tiny sizeElectrons moving around the nucleusCame up with “Nuclear Model”

Moving Forward…Bohr said electrons move in orbits

Found in energy levelsExplains bright-line spectrumCalled “Planetary Model” But Bohr knew that it was flawed

What we believe nowOrbital proposed by Schrödinger

Describes most likely where electrons wereQuantum mechanics new theory“Electron Cloud” model