A History of the AtomA History of the Atom

A long time to study a little thingA long time to study a little thing

IntroductionIntroduction

The PhilosophersThe Philosophers

Ancient GreeksAncient GreeksFormed ideas of nature based on their Formed ideas of nature based on their

experiencesexperiencesDid no experiments, just thoughtDid no experiments, just thought

Everything made of :Everything made of :

Earth Air

FireWater

Aristotle Aristotle (384–322 B.C.) (384–322 B.C.)

DemocritusDemocritus(460 – 370 B.C.)(460 – 370 B.C.)

Greek philosopher (not scientist) Idea of the atom (smallest particle)Named the atomDifferent kinds of atoms have different sizes Different kinds of atoms have different sizes

and shapes.and shapes.Apparent changes in matter result from Apparent changes in matter result from

changes in the groupings of atoms and not from changes in the groupings of atoms and not from changes in the atoms themselves.changes in the atoms themselves.

Democritus and AristotleDemocritus and Aristotle

The AlchemistsThe Alchemists

Rearrange the basic elementsRearrange the basic elementsWanted to turn lead in goldWanted to turn lead in gold

ElementsElements

We now know that there are 92 basic We now know that there are 92 basic elementselements

The type of element is determined by the The type of element is determined by the number of protons in the atomnumber of protons in the atom

Modern elemental theoryModern elemental theory

John DaltonJohn Dalton(1766-1844)(1766-1844)

First true atomic theoryAll matter is composed of atoms.All atoms of a given element are

identical and different from those of any other element.

Atoms combine in simple whole number ratios to form compounds.

In a chemical reaction, atoms are separated, combined, or rearranged.

Dalton’s Particle TheoryDalton’s Particle Theory

By the late 1800s…By the late 1800s…Cathode ray tube (aka Crook’s tube) inventedCathode ray tube (aka Crook’s tube) invented

JJ ThomsonJJ Thomson(1897)(1897)

Cathode rays made of Cathode rays made of particles smaller than particles smaller than atomsatoms

First subatomic particlesFirst subatomic particlesDiscovered electronPlum pudding model of

the atom

J. J. ThomsonJ. J. Thomson

Plum pudding modelPlum pudding modelAtom is a sphereAtom is a spherePositive charge is evenly distrubutedPositive charge is evenly distrubutedNegatively charged electrons spread Negatively charged electrons spread

randomly through the sphererandomly through the sphere

Questions raised by ThomsonQuestions raised by Thomson

If electrons are particles smaller than If electrons are particles smaller than atoms, are there other particles?atoms, are there other particles?

What makes up most of the mass of the What makes up most of the mass of the atom?atom?

If electrons are negatively charged, why If electrons are negatively charged, why are atoms neutral?are atoms neutral?

The proton/electron atomThe proton/electron atom

Ernest RutherfordErnest Rutherford(1911)(1911)

Aimed a beam of “alpha particles” at Aimed a beam of “alpha particles” at a thin gold foil a thin gold foil

The beam made a bright spot on a The beam made a bright spot on a fluorescent screenfluorescent screen

Lead block

Uranium

Gold Foil

Florescent Screen

What Rutherford expectedWhat Rutherford expected

What Rutherford expectedWhat Rutherford expected

What Rutherford sawWhat Rutherford saw

What Rutherford sawWhat Rutherford saw

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Rutherford concludedRutherford concluded

There had to be a heavy central core to the atom: the nucleus

Nucleus contains the protons

Rutherford and the Structure of Rutherford and the Structure of AtomsAtoms

James ChadwickJames Chadwick(1932)(1932)

Experimenting with radiation Experimenting with radiation sourcessources

Discovered a new particle with no charge : the neutron

Niels Bohr Niels Bohr (1913)(1913)

Student of Rutherford and ThomsonStudent of Rutherford and ThomsonElectrons travel in orbitals around the Electrons travel in orbitals around the

nucleusnucleusElectron can move from one orbital to Electron can move from one orbital to

another with input or release of energyanother with input or release of energyMore on this later…More on this later…

Bohr’s atomic modelBohr’s atomic model

Structure of the atomStructure of the atom

99.97% of 99.97% of mass in mass in nucleusnucleus

Most of Most of the volume the volume is empty is empty spacespace

Electrons Electrons in cloudin cloud

How big is an atom?How big is an atom?Simulate the size of a hydrogen atom:Simulate the size of a hydrogen atom:

Nucleus : place a baseball on the 50 yard line Nucleus : place a baseball on the 50 yard line of Reliant Stadiumof Reliant Stadium

Electron : put a grain of sand on the back row Electron : put a grain of sand on the back row of the highest sectionof the highest section

Size of AtomsSize of Atoms

Atomic NumberAtomic Number

Atomic MassAtomic Mass

IsotopesIsotopes

Atomic Number, Mass, & Atomic Number, Mass, & IsotopesIsotopes