Atomos:Atomos:Not to Be CutNot to Be Cut

The History of Atomic The History of Atomic TheoryTheory

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.

Atomic ModelsAtomic Models This model of the atom This model of the atom

may look familiar to may look familiar to you. This is the Bohr you. This is the Bohr model. In this model, model. In this model, the nucleus is orbited the nucleus is orbited by electrons, which are by electrons, which are in different energy in different energy levels. levels.

A A modelmodel uses familiar uses familiar ideas to explain ideas to explain unfamiliar facts unfamiliar facts observed in nature.observed in nature.

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

Atomic ModelsAtomic Models

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

DemocritusDemocritus

This is the Greek This is the Greek philosopher Democritus philosopher Democritus who began the search who began the search for a description of for a description of matter more than matter more than 24002400 years ago.years ago.

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

400 BC

AtomosAtomos

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

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

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

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 Plato, had a and 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 Model

In the early 1800s, In the early 1800s, the English Chemist the English Chemist John John DaltonDalton performed a performed a number of number of experiments that experiments that eventually led to eventually led to the acceptance of the acceptance of the idea of atoms.the idea of atoms.

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

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

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

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

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

This theory This theory became one became one of the of the foundations foundations of modern of modern chemistry.chemistry.

Thomson’s Plum Pudding Thomson’s Plum Pudding ModelModel

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

Thomson ModelThomson Model

He proposed a He proposed a model of the atom model of the atom that is sometimes that is sometimes called the “called the “PlumPlum PuddingPudding” model. ” model.

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

Thomson ModelThomson Model

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

Thomson ModelThomson Model

This surprised This surprised Thomson, Thomson, because the because the atoms of the gas atoms of the gas were uncharged.were uncharged.

Where had the Where had the negative charges negative charges come from?come from?

Where did they come from?Where did

they come from?

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

A particle smaller than an atom had to exist.

The atom was divisible!

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

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

But he could never find them.

Development of the Atomic

Theory Group Work

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

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

Rutherford’s experiment Involved Rutherford’s experiment Involved firing a stream of tiny firing a stream of tiny positively positively chargedcharged particles at a thin sheet particles at a thin sheet of of gold foilgold foil (2000 atoms thick). (2000 atoms thick).

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

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

RutherfordRutherford This could only mean that the gold atoms This could only mean that the gold atoms

in the sheet were mostly in the sheet were mostly open spaceopen space. . Atoms were Atoms were notnot a pudding filled with a a pudding filled with a positively charged material.positively charged material.

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

He called the center of the atom the He called the center of the atom the ““nucleusnucleus.”.”

The nucleus is The nucleus is tinytiny compared to the atom compared to the atom as a whole.as a whole.

RutherfordRutherford

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

http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

Bohr ModelBohr Model

In 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 model, he placed each electron in a each electron in a specificspecific energy energy level.level.

Bohr ModelBohr Model

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

The Wave ModelThe Wave Model

The Wave ModelThe Wave Model

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

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

The Wave ModelThe Wave Model In fact, it is In fact, it is impossibleimpossible to determine the to determine the

exact location of an electron. The exact location of an electron. The probableprobable location of an electron is based location of an electron is based on how much on how much energyenergy the electron has. the electron has.

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

Electron CloudElectron Cloud A space in which A space in which

electrons are likely electrons are likely to be found.to be found.

Electrons Electrons whirlwhirl about the nucleus about the nucleus billions of times in billions of times in one second.one second.

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

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

Electron CloudElectron Cloud

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

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

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

IndivisiblIndivisiblee

ElectronElectron NucleusNucleus OrbitOrbit Electron Electron CloudCloud

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DaltonDalton XX

ThomsonThomson XX

RutherforRutherfordd

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BohrBohr XX XX XX

WaveWave XX XX XX