AtomosAtomos

The History of Atomic TheoryThe History of Atomic Theory

Atomic ModelsAtomic ModelsThis is the Bohr model. In This is the Bohr model. In

this model, the nucleus is this model, the nucleus is orbited by electrons, orbited by electrons, which are in different which are in different energy levels. energy levels.

A model uses ideas to A model uses ideas to explain factsexplain facts

A model can be changed as A model can be changed as new information is new information is collected.collected.

The atomic model The atomic model has changed has changed throughout the throughout the centuries, starting centuries, starting in 400 BC, when it in 400 BC, when it looked like a looked like a billiard ball billiard ball →→

Who are these men?Who are these men?

In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

DemocritusDemocritusDemocritus began the search for a Democritus began the search for a

description of matter more than description of matter more than 24002400 years ago. years ago.

He asked: Could matter be divided He asked: Could matter be divided into smaller and smaller pieces into smaller and smaller pieces forever, or was there a forever, or was there a limitlimit to to the number of times a piece of the number of times a piece of matter could be matter could be divideddivided? ?

400 BC

Atomos Atomos His theory: Matter could not be His theory: Matter could not be divided into smaller and divided into smaller and smaller pieces forever, smaller pieces forever, eventually the smallest eventually the smallest possible piece would be possible piece would be obtained.obtained.

This piece would be This piece would be indivisible.indivisible.

He named the smallest piece He named the smallest piece of matter “atomos,” meaning of matter “atomos,” meaning “not to be cut.”“not to be cut.”

To To Democritus, atoms were Democritus, atoms were smallsmall, hard particles that , hard particles that were all made of the same were all made of the same material but were material but were differentdifferent shapes and sizes.shapes and sizes.

Atoms were Atoms were infiniteinfinite in in number, always moving and number, always moving and capable of joining together.capable of joining together.

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

Why?Why?The eminent The eminent philosophers of philosophers of the time, the time, AristotleAristotle and and Plato, had a Plato, had a more respected, more respected, (and ultimately (and ultimately wrongwrong) theory.) theory.

Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

Dalton’s ModelDalton’s ModelIn the early 1800s, the In the early 1800s, the

English Chemist John English Chemist John Dalton Dalton performed a performed a number of experiments number of experiments that eventually led to the that eventually led to the acceptance of the idea of acceptance of the idea of

atomsatoms..

Dalton’s TheoryDalton’s Theory He deduced that all He deduced that all elementselements are are

composed of atoms. Atoms are composed of atoms. Atoms are indivisible and indestructible indivisible and indestructible particles.particles.

Atoms of the Atoms of the samesame element are element are exactly alike.exactly alike.

Atoms of Atoms of differentdifferent elements are elements are differentdifferent..

CompoundsCompounds are formed by the are formed by the joining of atoms of two or more joining of atoms of two or more elements.elements.

..This theory became This theory became

one of the one of the foundations of foundations of modern chemistry.modern chemistry.

Thomson’s Plum Pudding Thomson’s Plum Pudding ModelModel

In In 18971897, the English , the English scientist J.J. scientist J.J. Thomson provided Thomson provided the first hint that an the first hint that an atom is made of even atom is made of even

smallersmaller particles particles..

He proposed a model of He proposed a model of the atom that is the atom that is sometimes called the sometimes 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 about, like raisins in a pudding.raisins in a pudding.

Thomson studied Thomson studied the the passage passage of an of an electric current electric current through a gas.through a gas.

As the current As the current passed through the passed through the gas, it gave off gas, it gave off rays of rays of negatively negatively charged particlescharged particles..

This surprised This surprised Thomson, because Thomson, because the atoms of the the atoms of the gas were gas were uncharged. Where uncharged. Where had the negative had the negative charges charges comefrom?comefrom?

Where did they come from?

Thomson concluded that the Thomson concluded that the negative charges came from negative charges came from withinwithin the atom. the atom.

A particle smaller than an atom A particle smaller than an atom had to existhad to exist..

The atom was The atom was divisible!divisible!

Thomson called the negatively Thomson called the negatively charged “charged “corpuscles,corpuscles,” today ” today known as known as electronselectrons..

Since the gas was known to be Since the gas was known to be neutral, having no charge, he neutral, having no charge, he reasoned that there must be reasoned that there must be positivelypositively charged particles in the charged particles in the atom.atom.

But he could never find them.But he could never find them.

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

In 1908, the English In 1908, the English physicist Ernest physicist Ernest Rutherford was hard at Rutherford was hard at work on an experiment work on an experiment that seemed to have little that seemed to have little to do with unraveling the to do with unraveling the mysteries of the mysteries of the atomic atomic structure.structure.

Rutherford’s experiment Involved firing Rutherford’s experiment Involved firing a stream of tiny 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)

MostMost of the positively charged of the positively charged “bullets” passed right through “bullets” passed right through the gold atoms in the sheet of the gold atoms in the sheet of gold foilgold foil without changing without changing course at all.course at all.

SomeSome of the positively charged of the positively charged “bullets,” however, did bounce “bullets,” however, did bounce away from the gold sheet as if away from the gold sheet as if they had hit something they had hit something solidsolid. . He knew that positive charges He knew that positive charges repelrepel positive charges. positive charges.

This could only mean that the gold atoms in the This could only mean that the gold atoms in the sheet were mostly sheet were mostly open spaceopen space. Atoms were . Atoms were notnot a pudding filled with a positively charged a pudding filled with a positively charged material.material.

Rutherford concluded that an atom had a Rutherford concluded that an atom had a small, small, dense, positively charged centerdense, positively charged center that that repelledrepelled his positively charged “bullets.”his positively charged “bullets.”

He called the center of the atom the “He called the center of the atom the “nucleusnucleus”” The nucleus is The nucleus is tiny tiny compared to the atom as a compared to the atom as a

whole. whole.

Rutherford reasoned Rutherford reasoned that all of an atom’s that all of an atom’s positively charged positively charged particles were particles were containedcontained in the in the nucleus. The nucleus. The negatively charged negatively charged particles were particles were scatteredscattered outside the outside the nucleus around the nucleus around the atom’s atom’s edgeedge..

Bohr ModelBohr ModelIn 1913, the Danish In 1913, the Danish

scientist Niels Bohr scientist Niels Bohr proposed an proposed an improvement. In his improvement. In his model, he placed each model, he placed each electron in a electron in a specificspecific energy level.energy level.

According to Bohr’s atomic According to Bohr’s atomic model, electrons move in model, electrons move in definite definite orbitsorbits around the around the nucleus, much like nucleus, much like planets circle the sun. planets circle the sun. These orbits, or energy These orbits, or energy levelslevels, are located at , are located at certaincertain distances from the distances from the

nucleusnucleus..

Wave Model

Today’s atomic model is Today’s atomic model is based on the principles of based on the principles of wavewave mechanicsmechanics..

According to the theory of According to the theory of wave mechanics, wave mechanics, electrons electrons do not movedo not move about an atom in a about an atom in a definite path,definite path, like the like the planets around the sun.planets around the sun.

In fact, it is In fact, it is impossibleimpossible to determine the exact to determine the exact location of an electron. The location of an electron. The probableprobable location of location of an electron is based on how much an electron is based on how much energyenergy the the electron has.electron has.

According to the modern atomic model, at atom According to the modern atomic model, at atom has a has a small positively charged nucleussmall positively charged nucleus surrounded by a large region in which there are surrounded by a large region in which there are enough electrons to make an atom neutral.enough electrons to make an atom neutral.

Electron Cloud:Electron Cloud: A space in which electrons are A space in which electrons are

likely to be found.likely to be found. Electrons Electrons whirlwhirl about the about the

nucleus billions of times in one nucleus billions of times in one secondsecond

They are not moving around in They are not moving around in randomrandom patterns. patterns.

Location of electrons depends Location of electrons depends upon how much upon how much energyenergy the the electron has.electron has.

Electron Cloud:Electron Cloud:

Depending on their energy they are locked into a Depending on their energy they are locked into a certain area in the cloud.certain area in the cloud.

Electrons with the Electrons with the lowestlowest energy are found in energy are found in the energy level the energy level closestclosest to the nucleus to the nucleus

Electrons with the Electrons with the highesthighest energy are found energy are found in the in the outermostoutermost energy levels, farther from energy levels, farther from the nucleus.the nucleus.

IndivisibleIndivisible ElectronElectron NucleusNucleus OrbitOrbit Electron Electron CloudCloud

GreekGreek XX

DaltonDalton XX

ThomsonThomson XX

RutherfordRutherford XX XX

BohrBohr XX XX XX

WaveWave XX XX XX