The History The History of the Atomof the Atom

IntroductionIntroductionOur understanding of the physical world Our understanding of the physical world

has grown at an incredible rate in the last has grown at an incredible rate in the last 200 years.200 years.

The key to the advances made in The key to the advances made in chemistry has been our growing chemistry has been our growing knowledge about atoms. knowledge about atoms.

In this unit we will look at some of the early In this unit we will look at some of the early historical discoveries that helped us build historical discoveries that helped us build a useful “working model” of the atom.a useful “working model” of the atom.

Atomic ModelsAtomic ModelsAtomic models use Atomic models use

familiar ideas to familiar ideas to explain unfamiliar explain unfamiliar facts observed in facts observed in nature.nature.

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

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.

DemocritusDemocritus Greek philosopherGreek philosopher 400 BC400 BC Began the search for Began the search for

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

Thought matter could Thought matter could not be divided not be divided indefinitely.indefinitely.

DemocritusDemocritus Matter cannot be divided Matter cannot be divided

into smaller and smaller into 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.”

““Atomos”Atomos” To Democritus, atoms To Democritus, atoms

were small, hard were small, hard particles that were all particles that were all made of the same made of the same material but were material but were different shapes and different shapes and sizes.sizes.

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

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

Why?Why?Aristotle and Aristotle and

Plato had a Plato had a more more respected, respected, (and (and ultimately ultimately wrongwrong)) theory. theory.

• They favored the earth, fire, air and water approach to the nature of matter.

DaltonDalton In the early 1800s, In the early 1800s,

the English the English Chemist John Chemist John Dalton performed a Dalton performed a number of number of experimentsexperiments that 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 All matter is made of All matter is made of

atoms.atoms. Atoms of an element are Atoms of an element are

identical.identical. Each element has different Each element has different

atoms.atoms. Atoms of different elements Atoms of different elements

combine to form combine to form compounds.compounds.

Atoms are rearranged in Atoms are rearranged in reactionsreactions

Atoms cannot be split.Atoms cannot be split.Dalton’s “Billiard Ball” Model: atoms are solid and indivisible.

J.J. ThomsonJ.J. ThomsonIn 1897, the In 1897, 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 “Plum Pudding” ModelThomson “Plum Pudding” Model

Atoms were made Atoms were made from a positively from a positively charged substance charged substance with negatively with negatively charged charged electronselectrons scattered around. scattered around.

(like raisins in a pudding).(like raisins in a pudding).

Thomson’s ExperimentThomson’s Experiment

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

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

This surprised This surprised Thompson. Where had Thompson. Where had the negative charges the negative charges come from?come from?

Thomson’s “Plum Pudding” ModelThomson’s “Plum Pudding” Model

Negative Negative charges must charges must come from come from insideinside the atom.the atom.

A particle A particle smaller than an smaller than an atom had to atom had to exist!exist!

Where did they come from?

The atom was divisible!

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

•Since the gas was known to be neutral, he reasoned there must be positively charged particles in the atom – but he never found 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 firing a experiment firing a stream of tiny stream of tiny positively charged positively charged particles at a thin particles at a thin sheet of gold foil. sheet of gold foil.

Gold Foil ExperimentGold Foil Experiment

Most of the positively Most of the positively charged “bullets” passed charged “bullets” passed straight through the atoms straight through the atoms in the gold foil.in the gold foil.

Some of the “bullets” Some of the “bullets” bounced away from the bounced away from the gold sheet as if they had hit gold sheet as if they had hit something solid. something solid.

He knew that positive He knew that positive charges charges repelrepel positive positive charges.charges.

Gold Foil ExperimentGold Foil Experiment

Rutherford’s ConclusionsRutherford’s Conclusions The atom is mostly The atom is mostly

empty space.empty space. He called the small, He called the small,

dense, positively dense, positively charged center of the charged center of the atom a “nucleus”.atom a “nucleus”.

Negatively charged Negatively charged particles were scattered particles were scattered outside the nucleus outside the nucleus around the atom’s around the atom’s edge.edge.

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

scientist Niels Bohr scientist Niels Bohr proposed an proposed an improvement to the improvement to the atomic model.atomic model.

He placed each He placed each electron in a electron in a specificspecific energy level.energy level.

Bohr ModelBohr ModelElectrons move in Electrons move in

definite definite orbitsorbits around around the nucleus, much like the nucleus, much like planets circle the sun. planets circle the sun.

These orbits, or These orbits, or energyenergy levelslevels, are located at , are located at certaincertain distances from distances from the nucleus.the nucleus.

Quantum Mechanical ModelQuantum Mechanical ModelToday’s atomic Today’s atomic

model is based on model is based on wavewave mechanicsmechanics..

Electrons Electrons do not do not movemove about an about an atom in a definite atom in a definite path, like the path, like the planets around planets around the sun.the sun.

Electron CloudElectron CloudA 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

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

Electron CloudElectron CloudElectrons with the Electrons with the

lowestlowest energy are energy are found found closestclosest to to the nucleusthe nucleus

Electrons with the Electrons with the highesthighest energy energy are found are found farthestfarthest from the nucleus.from the nucleus.

Depending on their energy, they are

locked into a certain area in the cloud.

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