Chapter 3: The Atom “The Building Blocks of Matter”

Chapter 3: The AtomChapter 3: The Atom

““The Building Blocks of The Building Blocks of Matter”Matter”

Early Definitions of the AtomEarly Definitions of the Atom►The Greek philosopher The Greek philosopher DemocritusDemocritus (460 (460

B.C.B.C. – 370 – 370 B.CB.C.) was among the first to .) was among the first to suggest the existence of suggest the existence of atomsatoms (from (from the Greek word “atomos”)the Greek word “atomos”) He believed that atoms were He believed that atoms were indivisibleindivisible and and

indestructibleindestructible His ideas did agree with later scientific His ideas did agree with later scientific

theory, but did not explain chemical theory, but did not explain chemical behavior, and was behavior, and was not based on the not based on the scientific methodscientific method – but just – but just philosophyphilosophy

Aristotle Aristotle

► Aristotle believed in Aristotle believed in 4 elements: 4 elements:

► Earth, Air, Fire, and Earth, Air, Fire, and Water.Water.

European AlchemistsEuropean Alchemists

► 1100 A.D.1100 A.D.

► Their work Their work evolvedevolved into what is now into what is now modern modern chemistrychemistry..

► Trying to change Trying to change ordinary materials ordinary materials into gold.into gold.

The 1700’sThe 1700’s

►Little work was done on atomic theory Little work was done on atomic theory until the 1700’suntil the 1700’s

►Aristotle’s and Aristotle’s and Democritus’ theories stoodDemocritus’ theories stood►However some Laws were attained: However some Laws were attained:

Law of Conservation of Law of Conservation of MassMass Law of Law of DefiniteDefinite Proportions Proportions Law of Law of MultipleMultiple Proportions Proportions

Law of Conservation of MassLaw of Conservation of Mass

► 1789: by French Chemist 1789: by French Chemist Antoine LavoisierAntoine Lavoisier► Mass is neither Mass is neither createdcreated nor destroyed during nor destroyed during

ordinary chemical ordinary chemical reactionsreactions or physical or physical changeschanges

► Simply, the mass of what you end up with Simply, the mass of what you end up with (products), must equal the mass of what you (products), must equal the mass of what you started with (reactants)started with (reactants)

Law of Definite ProportionsLaw of Definite Proportions

►1800: 1800: FrenchFrench chemist chemist Joseph ProustJoseph Proust ►Chemical compounds contain the Chemical compounds contain the

samesame elements in exactly the same elements in exactly the same proportionsproportions by mass, regardless of t he by mass, regardless of t he size of the samplesize of the sample

►Ex.) Salt, NaCl: always consists of Ex.) Salt, NaCl: always consists of 39.34% Sodium39.34% Sodium 60.66% Chlorine60.66% Chlorine

Law of Multiple Law of Multiple ProproportionsProproportions

► John DaltonJohn Dalton first expressed this first expressed this observation in 1803 and it is observation in 1803 and it is sometimes called sometimes called Dalton's LawDalton's Law

►When elements When elements combinecombine they do so in they do so in a a ratioratio of small whole of small whole numbersnumbers. . Ex.) carbon and oxygen react to form CO Ex.) carbon and oxygen react to form CO

or COor CO22, but not CO, but not CO1.81.8

Video About Dalton

Dalton’s Atomic TheoryDalton’s Atomic Theory (experiment based!) (experiment based!)

3) Atoms cannot be subdivided, created, or destroyed.

4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds

1) All matter is composed of extremely small particles called atoms.

2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

John Dalton(1766 – 1844)

Dalton Continued…Dalton Continued…

5) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

Experiments determining the structure Experiments determining the structure of the atomof the atom

► AtomAtom: smallest : smallest particle of an particle of an element that retains element that retains the chemical the chemical properties of that properties of that element.element.

► 1897: 1897: J. J. Thomson-J. J. Thomson- used used

Cathode ray tubesCathode ray tubes

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive end.positive end.

Thomson’s ExperimentThomson’s Experiment

Voltage source

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Voltage source

Thomson’s ExperimentThomson’s Experiment

►By adding an electric field…… By adding an electric field……

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field, he found that By adding an electric field, he found that the moving pieces were negatively chargedthe moving pieces were negatively charged

+

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1904: Thompson’s Model1904: Thompson’s Model► These were concluded These were concluded

to be electrons.to be electrons.► Stability suggested Stability suggested

that were there is a that were there is a negative, there should negative, there should be a positive. He be a positive. He couldn’t find the couldn’t find the positive (for a while). positive (for a while).

► Described the atom as Described the atom as plum pudding.plum pudding.

► A cloud of positive A cloud of positive stuff, with the stuff, with the electronselectrons able to be removed. able to be removed.

1909: Millikan’s Experiment1909: Millikan’s Experiment

Oil

Atomizer

Oil spray

Microscope

-

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Millikan’s ExperimentMillikan’s Experiment

X-rays

X-rays charge the oil drops.

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Millikan’sMillikan’s Experiment Experiment

Some drops would fall slower than others…

From the mass of the drop and the charges on the two plates, he calculated the mass of and charge magnitude on an electron 1 electron = 9.11 x 10 –31 kg

1911: Earnest Rutherford’s 1911: Earnest Rutherford’s ExperimentExperiment

► Used uranium which as it decays Used uranium which as it decays produces produces alphaalpha particles. particles.

► Aimed alpha particles at Aimed alpha particles at gold foilgold foil by by drilling hole in a lead block.drilling hole in a lead block.

► Since the mass is evenly distributed in Since the mass is evenly distributed in gold atoms, and if Thompson was correct gold atoms, and if Thompson was correct in his structural picture of the atom, in his structural picture of the atom, Rutherford believed the alpha particles Rutherford believed the alpha particles should go straight through the foil.should go straight through the foil.

► Used gold foil because it could be made Used gold foil because it could be made very thin (only a few atoms thick).very thin (only a few atoms thick).

Lead block

Uranium

Gold Foil

Florescent Screen

What he expected

Because

Because, he thought the mass was evenly distributed in the atom.

What he got

Video About Rutherford

How he explained it

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► Atom is mostly Atom is mostly emptyempty► Small Small densedense, , positivepositive piece piece

at center.at center.► Alpha particlesAlpha particles

(have a +2 charge)(have a +2 charge) are deflected by it are deflected by it

if they get close if they get close enough. enough.

+

So Rutherford discovered the NUCLEUS!!

Modern ViewModern View

► The atom is mostly The atom is mostly empty space.empty space.

► Two Two regionsregions► Nucleus-Nucleus- protons protons

and and neutronsneutrons..► ElectronElectron cloud- cloud-

region where you region where you might find an might find an electron.electron.

► More on this later…More on this later…

STOP

© 2009, Prentice-Hall, Inc.

Discovery of the NucleusDiscovery of the Nucleus

Ernest Rutherford Ernest Rutherford shot shot particles at particles at a thin sheet of a thin sheet of gold foil and gold foil and observed the observed the pattern of scatter pattern of scatter of the particles.of the particles.

© 2009, Prentice-Hall, Inc.

The Nuclear AtomThe Nuclear Atom

Since some Since some particles were particles were deflected at large deflected at large angles, angles, Thompson’s model Thompson’s model could not be could not be correct.correct.

© 2009, Prentice-Hall, Inc.

The Nuclear AtomThe Nuclear Atom►Rutherford postulated a very small, Rutherford postulated a very small,

dense nucleus with the electrons dense nucleus with the electrons around the outside of the atom.around the outside of the atom.

►Most of the volume of the atom is Most of the volume of the atom is empty space.empty space.

© 2009, Prentice-Hall, Inc.

Other Subatomic ParticlesOther Subatomic Particles

►ProtonsProtons were discovered by Rutherford were discovered by Rutherford in 1919.in 1919.

►NeutronsNeutrons were discovered by were discovered by James James ChadwickChadwick in 1932. in 1932.

© 2009, Prentice-Hall, Inc.

Subatomic ParticlesSubatomic Particles

► ProtonsProtons and and electronselectrons are the only particles are the only particles that have a charge.that have a charge.

► Protons and neutrons have essentially the Protons and neutrons have essentially the same mass.same mass.

► The mass of an The mass of an electronelectron is so small we ignore is so small we ignore it.it.

© 2009, Prentice-Hall, Inc.

Symbols of ElementsSymbols of Elements

ElementsElements are symbolized by one or two are symbolized by one or two letters.letters.

© 2009, Prentice-Hall, Inc.

Atomic NumberAtomic Number

All All atomsatoms of the same element have the of the same element have the same number of same number of protonsprotons: :

The atomic number (Z)The atomic number (Z)

© 2009, Prentice-Hall, Inc.

Atomic MassAtomic Mass

The mass of an atom in The mass of an atom in atomic mass atomic mass unitsunits (amu) is the total number of (amu) is the total number of protonsprotons and and neutronsneutrons in the atom. in the atom.

© 2009, Prentice-Hall, Inc.

IsotopesIsotopes

► Isotopes are atoms of the same Isotopes are atoms of the same elementelement with with different different massesmasses..

► Isotopes have different numbers of Isotopes have different numbers of neutronsneutrons..

116C

126C

136C

146C

Review Video

AssignmentAssignment

►Project 1: Atomic Structure TimelineProject 1: Atomic Structure Timeline

Atomic Structure TimelineAtomic Structure TimelineRequirements:Requirements:► MustMust include a title. include a title.► MustMust include a spot for each of the 11 include a spot for each of the 11

items.items.► MustMust include include at leastat least the the WhoWho, , WhenWhen, ,

and and WhatWhat..► MustMust have a picture (color is preferred) have a picture (color is preferred)

for each item.for each item.Options:Options:1.1. Comic book formatComic book format2.2. Standard timelineStandard timeline

11 Important 11 Important Figures/DiscoveriesFigures/Discoveries

► Democritus Democritus ► AristotleAristotle► Antoine LavoisierAntoine Lavoisier – – LawLaw ► Joseph Proust – Law Joseph Proust – Law ► John Dalton – LawJohn Dalton – Law► John Dalton – TheoryJohn Dalton – Theory► J.J. ThomsonJ.J. Thomson► Robert Millikan Robert Millikan ► Ernest Rutherford – Gold FoilErnest Rutherford – Gold Foil► Rutherford – Discovery of protonRutherford – Discovery of proton► Chadwick – Discovery of neutronChadwick – Discovery of neutron