Brief History of Brief History of Atomic TheoryAtomic Theory

11stst atomic models atomic models

In In 400 BC400 BC, the , the model looked like a model looked like a solid indivisible ballsolid indivisible ball

AtomosAtomos Democritus’Democritus’ theory: theory:

Matter could not be Matter could not be divided into smaller and divided into smaller and smaller pieces foreversmaller pieces forever

He named the He named the smallest piece of smallest piece of matter matter “atomos,”“atomos,” an an indivisible particleindivisible particle

This theory was This theory was ignored and forgotten ignored and forgotten for more than for more than 2000 2000 years!years!

Why?Why?

AristotleAristotle & Plato & Plato thought earth, fire, air and water were the elements. Alchemy used this model for about 2000 years.

Dalton’s ModelDalton’s Model

In the In the early 1800searly 1800s, the , the English chemist English chemist

John DaltonJohn Dalton performed experiments performed experiments that confirmed the idea that confirmed the idea of atoms.of atoms.

Dalton’s Atomic TheoryDalton’s Atomic Theory All elements are made of All elements are made of

indivisibleindivisible atoms atoms Atoms of the same element Atoms of the same element

are are identicalidentical, but differ from , but differ from atoms of other elementsatoms of other elements

Different elements’ atoms can Different elements’ atoms can combine to form combine to form compoundscompounds..

ReactionsReactions occur when atoms occur when atoms are are separated, joined, or separated, joined, or rearrangedrearranged..

WHICH OF THESE ARE NO WHICH OF THESE ARE NO LONGER CONSIDERED TO LONGER CONSIDERED TO BE TRUE ? BE TRUE ?

Thomson’s Plum Pudding Thomson’s Plum Pudding ModelModel

In In 18971897, , J.J. J.J. ThomsonThomson discovered that discovered that atoms are made of atoms are made of even even smallersmaller particles.particles.

Thomson ModelThomson ModelThomson used Thomson used

cathode raycathode ray tube tubeThe ray consisted of The ray consisted of

negatively charged negatively charged “electrons”.“electrons”.

Thomson ModelThomson ModelHe proposed a model of the He proposed a model of the

atom called the atom called the ““PlumPlum PuddingPudding” model” model.* .*

Atoms were made from a Atoms were made from a positively positively chargedcharged substancesubstance with negatively with negatively charged electrons charged electrons scatteredscattered about, like raisins in a about, like raisins in a pudding.pudding.

Aka: “raisin pudding model”Aka: “raisin pudding model”

Millikan (1909)Millikan (1909) OIL DROPOIL DROP

experiment experiment

Measures Measures charge on charge on electronelectron

+

-

Rutherford’sRutherford’s gold foilgold foil experimentexperiment (1911) Involved firing a stream of tiny (1911) Involved firing a stream of tiny positively chargedpositively charged particles at a thin sheet of particles at a thin sheet of gold foilgold foil (2000 atoms thick) (2000 atoms thick)

Result: most of space occupied by atom is Result: most of space occupied by atom is emptyempty space! Except for small, space! Except for small, dense, positively charged dense, positively charged NUCLEUSNUCLEUS..

Positive particles named Positive particles named PROTONSPROTONS all protons are in the nucleusall protons are in the nucleus..

Rutherford Nuclear ModelRutherford Nuclear Model all of an atom’s all of an atom’s

positively charged positively charged particles are particles are containedcontained in the in the nucleusnucleus. The . The negatively charged negatively charged particles were particles were scatteredscattered outside the outside the nucleus.nucleus.

Bohr ModelBohr ModelIn 1913, the In 1913, the

Danish scientist Danish scientist Niels BohrNiels Bohr proposed that proposed that each electron is in each electron is in a a specificspecific energyenergy level.level.

Bohr ModelBohr ModelAccording to According to

Bohr’s atomic Bohr’s atomic model, electrons model, electrons move in definite move in definite orbitsorbits around the around the nucleus, much like nucleus, much like planets circle the planets circle the sun. sun.

Wave Model

Louis DeBroglie(1924)

e’s have wavelike properties

Wave ModelErwin Schrodinger(1927)

Quantum mechanics model

e’s location probability

Chadwick (1932) Chadwick (1932) Discovers particle with same mass as Discovers particle with same mass as

protonproton but no charge …the but no charge …the NEUTRONNEUTRON

IndivisibleIndivisible ElectronElectron NucleusNucleus OrbitOrbit Electron Electron CloudCloud

GreekGreek XX

DaltonDalton XX

ThomsonThomson XX

RutherfordRutherford XX XX

BohrBohr XX XX XX

WaveWave XX XX XX