Ch 3 Atomic TheoryCh 3 Atomic TheoryModels of the AtomModels of the Atom

Pre-Dalton Atom ConceptsPre-Dalton Atom Concepts The Greek classical elements The Greek classical elements

are Fire, Earth, Air, and Water. are Fire, Earth, Air, and Water. Chinese Philosophy had 5 Chinese Philosophy had 5

elements that composed elements that composed nature. nature.

Democritus was a Greek Democritus was a Greek philosopher who lived between philosopher who lived between 470-380 B.C. He developed the 470-380 B.C. He developed the concept of the 'atom', Greek concept of the 'atom', Greek for 'indivisible'. Democritus for 'indivisible'. Democritus believed that everything in the believed that everything in the universe was made up of universe was made up of atoms, which were microscopic atoms, which were microscopic and indestructible.and indestructible.

Ch 3 Atomic TheoryCh 3 Atomic Theory

Dalton's model was that the atoms were tiny, Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each indivisible, indestructible particles and that each one had a certain mass, size, and chemical one had a certain mass, size, and chemical behavior behavior

all atoms of an element were identical all atoms of an element were identical compounds consisted of atoms of different compounds consisted of atoms of different

elements combined together elements combined together Compounds have constant composition because Compounds have constant composition because

they contain a fixed ratio of atoms they contain a fixed ratio of atoms chemical reactions involved the rearrangement chemical reactions involved the rearrangement

of combinations of those atoms of combinations of those atoms

John Dalton(1766-1844)

Dalton’s 3 LawsDalton’s 3 Laws

Law of Definite ProportionsLaw of Definite Proportions Atoms combine in simple, whole number ratios to Atoms combine in simple, whole number ratios to

form compoundsform compounds Law of Multiple ProportionsLaw of Multiple Proportions

The mass percentage of elements in a compound is The mass percentage of elements in a compound is fixedfixed

Law of Conservation of MassLaw of Conservation of Mass The mass of the reactants is equal to the mass of the The mass of the reactants is equal to the mass of the

products in any chemical reaction.products in any chemical reaction.

John Dalton(1766-1844)

J. J. Thomson (1897)J. J. Thomson (1897)Thomson’s model had the atom as a sea Thomson’s model had the atom as a sea

of positive charge interspersed with of positive charge interspersed with negative “corpuscles” to neutralize the negative “corpuscles” to neutralize the atom.atom.

It was called the “Plum Pudding” model.It was called the “Plum Pudding” model.His proof was that atoms could be His proof was that atoms could be

deflected in a magnetic field due to their deflected in a magnetic field due to their charge.charge.

Cathode Ray TubeCathode Ray TubeThomson accurately measured the ratio of Thomson accurately measured the ratio of

mass to charge, but not the actual charge mass to charge, but not the actual charge on the electron.on the electron.

Robert Millikan (1902)Robert Millikan (1902)By balancing the electrical force on By balancing the electrical force on

charged oil droplets in an electric field with charged oil droplets in an electric field with the force of gravity, Robert Millikan was the force of gravity, Robert Millikan was able to independently determine the able to independently determine the charge of the electron (-1.6x10-19 C) in charge of the electron (-1.6x10-19 C) in his his Oil Drop ExperimentOil Drop Experiment. .

Millikan Oil Drop ExperimentMillikan Oil Drop Experiment

Ernst Rutherford (1911)Ernst Rutherford (1911)Rutherford bombarded atoms (gold Foil) with Rutherford bombarded atoms (gold Foil) with

Alpha particles (HeAlpha particles (He2+2+).). If the atom was as Rutherford had predicted, If the atom was as Rutherford had predicted,

they should pass through being slightly they should pass through being slightly deflected.deflected.

However, some alpha particles were However, some alpha particles were deflected back indicating a dense, positive deflected back indicating a dense, positive center of the atom.center of the atom.

Gold Foil ExperimentGold Foil Experiment

Rutherford’s Model of the AtomRutherford’s Model of the Atom

James ChadwickJames Chadwick In 1932, Chadwick made a fundamental In 1932, Chadwick made a fundamental

discovery in the domain of nuclear science: he discovery in the domain of nuclear science: he proved the existence of proved the existence of neutronsneutrons - elementary - elementary particles devoid of any electrical charge. In particles devoid of any electrical charge. In contrast with the helium nuclei (alpha rays) contrast with the helium nuclei (alpha rays) which are charged, and therefore repelled by the which are charged, and therefore repelled by the considerable electrical forces present in the considerable electrical forces present in the nuclei of heavy atoms, this new particle need not nuclei of heavy atoms, this new particle need not overcome any electric barrier and is capable of overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the penetrating and splitting the nuclei of even the heaviest elements. Chadwick in this way heaviest elements. Chadwick in this way prepared the way towards the fission of uranium prepared the way towards the fission of uranium 235 and towards the creation of the atomic 235 and towards the creation of the atomic bomb. bomb.