Atomos: Not to Be CutAtomos: Not to Be Cut

The History of Atomic Theory DateThe History of Atomic Theory Date

Objective: Identify Scientists and their Objective: Identify Scientists and their contributions to the Atomic Theorycontributions to the Atomic Theory

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

may look familiar to you. may look familiar to you. This 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 familiar A model uses familiar ideas to explain ideas to explain unfamiliar facts observed unfamiliar facts observed in nature.in nature.

A model can be changed A model can be changed as new information is as 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 (4in 400 (4thth century) BC, century) BC, when it looked when it looked like like a billiard ball a 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.

Scientist Date Contribution Model

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DemocritusDemocritus This is the Greek This is the Greek

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

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

4 BC

AtomosAtomos

His theory: Matter could not His theory: Matter could not be divided into smaller and be 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; Atoms not Atoms not divisible.divisible.

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.”

AtomosAtomos

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

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

Democritus’ theory was Democritus’ theory was ignored and forgotten ignored and forgotten for more than for more than 2000 2000 years!years!

Aristotle 3BCAristotle 3BC

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

Dalton’s ModelDalton’s Model

In the early 1800s, In the early 1800s, the English the English Chemist John Chemist John Dalton Dalton 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 of are composed of atoms. atoms.

Atoms are indivisible and Atoms are indivisible and indestructible particles.indestructible particles.

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

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

CompoundsCompounds are formed by are formed by the joining of atoms of two the joining of atoms of two or more elements.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, like about, like raisins in a pudding.raisins in a pudding.

Thomson ModelThomson ModelThomson 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 particles.charged 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?

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

A particle smaller than an atom had to exist. Atoms made of smaller particles.

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.

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 little seemed to have little to do with unraveling to do with unraveling the mysteries of the the mysteries of the atomic structure.atomic 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 of the positively charged “bullets” passed charged “bullets” passed right through the gold right through the gold atoms in the sheet of atoms in the sheet of gold gold foilfoil without changing without changing course at all.course at all.

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

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

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

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.

RutherfordRutherford

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.nucleus. The 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 model, he placed each each electron in a electron in a specificspecific energy energy levellevel..

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 certainlocated at certain distances from the distances from the nucleus.nucleus.

Current Theory- 1920+Current Theory- 1920+

Today’s atomic Today’s atomic model is based on model is based on the principles of the 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 about an atom in a an atom in a definite definite path,path, like the planets like the planets around the sun.around the sun.

Electron CloudElectron Cloud

Electron CloudElectron Cloud A space in which A space in which

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

Electrons Electrons whirlwhirl about about the nucleus billions of the nucleus billions of times in one secondtimes in 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 Cloud:Electron Cloud: In fact, it is In fact, it is impossibleimpossible to to

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

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

Electron Cloud:Electron Cloud:

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

IndivisibleIndivisible ElectronElectron NucleusNucleus OrbitOrbit Electron Electron CloudCloud

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DaltonDalton XX

ThomsonThomson XX

RutherfordRutherford XX XX

BohrBohr XX XX XX

WaveWave XX XX XX