Chapter 3

Chapter 3Chapter 3

Atoms and their structureAtoms and their structure

History of the atomHistory of the atom Not the history of atom, but the idea of Not the history of atom, but the idea of

the atomthe atom Original idea Ancient Greece (400 B.C..)Original idea Ancient Greece (400 B.C..) Democritus Greek philosopherDemocritus Greek philosopher

History of AtomHistory of Atom Looked at beachLooked at beach Made of sandMade of sand Cut sand - smaller sandCut sand - smaller sand

Smallest possible Smallest possible piece?piece?AtomosAtomos - not to be cut - not to be cut

Another GreekAnother Greek AristotleAristotle - Famous philosopher - Famous philosopher All substances are made of 4 elementsAll substances are made of 4 elements Fire - HotFire - Hot Air - lightAir - light Earth - cool, heavyEarth - cool, heavy Water - wetWater - wet Blend these in different proportions to Blend these in different proportions to

get all substances get all substances

Hyle

Who Was Right?Who Was Right? Greek society was slave basedGreek society was slave based Beneath Famous to work with handsBeneath Famous to work with hands did not experimentdid not experiment Greeks settled disagreements by Greeks settled disagreements by

argumentargument Aristotle was more famousAristotle was more famous He wonHe won His ideas carried through middle His ideas carried through middle

ages.ages.

Who’s Next?Who’s Next? Late 1700’s - Late 1700’s - John DaltonJohn Dalton- England- England Teacher- summarized results of his Teacher- summarized results of his

experiments and those of other’sexperiments and those of other’s In In Dalton’s Atomic TheoryDalton’s Atomic Theory Combined ideas of elements with that of Combined ideas of elements with that of

atomsatoms

Dalton’s Atomic TheoryDalton’s Atomic Theory All All mattermatter is made of tiny is made of tiny indivisibleindivisible

particles called atoms.particles called atoms. Atoms of the same element are Atoms of the same element are

identical, those of different atoms are identical, those of different atoms are different.different.

Atoms of different elements combine Atoms of different elements combine in whole number ratios to form in whole number ratios to form compoundscompounds

Chemical reactions involve the Chemical reactions involve the rearrangement of atoms. No new rearrangement of atoms. No new atoms are created or destroyed.atoms are created or destroyed.

Law of Definite Proportions (#3)Law of Definite Proportions (#3) Each compound has a specific ratio of Each compound has a specific ratio of

elementselements It is a ratio by mass It is a ratio by mass Water is always 8 grams of oxygen for Water is always 8 grams of oxygen for

each gram of hydrogen each gram of hydrogen

Law of Multiple ProportionsLaw of Multiple Proportions if two elements form more that one if two elements form more that one

compound, the ratio of the second compound, the ratio of the second element that combines with 1 gram of element that combines with 1 gram of the first element in each is a simple the first element in each is a simple whole number.whole number.

What?What? Water is 8 grams of oxygen per 1 Water is 8 grams of oxygen per 1

gram of hydrogen.gram of hydrogen. Hydrogen Peroxide is 16 grams of Hydrogen Peroxide is 16 grams of

oxygen per 1 gram of hydrogen.oxygen per 1 gram of hydrogen. 16 to 8 is a 2 to 1 ratio16 to 8 is a 2 to 1 ratio True because you have to add a True because you have to add a

whole atom, you can’t add a piece of whole atom, you can’t add a piece of an atom.an atom.

Parts of AtomsParts of Atoms J. J. ThomsonJ. J. Thomson - English physicist. 1897 - English physicist. 1897 Made a piece of equipment called a Made a piece of equipment called a

cathode ray tube.cathode ray tube. It is a vacuum tube - all the air has been It is a vacuum tube - all the air has been

pumped out.pumped out.

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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 endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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 endpositive end

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 endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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 endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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 endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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 endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

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 By adding an electric field

+

-

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 By adding an electric field

+

-

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 the By adding an electric field he found that the moving pieces were negative moving pieces were negative

+

-

Thomson’s ModelThomson’s Model Found the Found the

electronelectron Couldn’t find Couldn’t find

positive (for a positive (for a while) while)

Said the atom was Said the atom was like like plum puddingplum pudding

A bunch of A bunch of positive stuff, positive stuff, with the with the electrons able to electrons able to be removedbe removed

Rutherford’s experimentRutherford’s experiment Ernest RutherfordErnest Rutherford-English physicist. -English physicist.

(1910)(1910) Believed in the plum pudding model of Believed in the plum pudding model of

the atom.the atom. Wanted to see how big they are Wanted to see how big they are Used radioactivityUsed radioactivity Alpha particles - positively charged Alpha particles - positively charged

pieces given off by uranium pieces given off by uranium Shot them at Shot them at gold foilgold foil which can be which can be

made a few atoms thick made a few atoms thick

Rutherford’s experimentRutherford’s experiment When the alpha particles hit a florescent When the alpha particles hit a florescent

screen, it glows.screen, it glows. Here’s what it looked like (pg 101)Here’s what it looked like (pg 101)

Lead block

Uranium

Gold Foil

Florescent Screen

He ExpectedHe Expected The alpha particles to pass through The alpha particles to pass through

without changing direction very muchwithout changing direction very much BecauseBecause The positive charges were spread out The positive charges were spread out

evenly. Alone they were not enough to evenly. Alone they were not enough to stop the alpha particlesstop the alpha particles

What he expected

Because

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

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

What he got

How he explained it

+

Atom is mostly emptyAtom is mostly empty Small dense,Small dense,

positive piecepositive piece at centerat center

Alpha particles Alpha particles are deflected byare deflected by

it if they get close it if they get close enough enough

+

Density and the AtomDensity and the Atom Since most of the particles went Since most of the particles went

through, it was mostly empty.through, it was mostly empty. Because the pieces turned so much, Because the pieces turned so much,

the positive pieces were heavy.the positive pieces were heavy. Small volume, big mass, big densitySmall volume, big mass, big density This small dense positive area is the This small dense positive area is the

nucleusnucleus

Structure of the AtomStructure of the Atom There are two regionsThere are two regions The nucleusThe nucleus With protons and neutrons With protons and neutrons Positive chargePositive charge Almost all the massAlmost all the mass Electron cloud- Most of the volume of an Electron cloud- Most of the volume of an

atomatom The region where the electron can be foundThe region where the electron can be found

Other piecesOther pieces ProtonProton - positively charged - positively charged

pieces 1840 times heavier than pieces 1840 times heavier than the electronthe electron

NeutronNeutron - no charge but the - no charge but the same mass as a proton.same mass as a proton.

Where are the pieces?Where are the pieces?

Subatomic particlesSubatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.673 x 10-24

1.675 x 10-24

Modern ViewModern View The atom is The atom is

mostly empty mostly empty spacespace

Two regionsTwo regions NucleusNucleus- -

protons and protons and neutronsneutrons

Electron cloudElectron cloud- - region where region where you might find you might find an electronan electron

Size of an atomSize of an atom Atoms are small.Atoms are small. Measured in picometers, 10Measured in picometers, 10-12-12 meters meters Hydrogen atom, 32 pm radiusHydrogen atom, 32 pm radius Nucleus tiny compared to atomNucleus tiny compared to atom IF the atom was the size of a stadium, the IF the atom was the size of a stadium, the

nucleus would be the size of a marble.nucleus would be the size of a marble. Radius of the nucleus near 10Radius of the nucleus near 10-15-15m.m. Density near 10Density near 101414 g/cm g/cm

Counting the PiecesCounting the Pieces Atomic Number Atomic Number = number of protons= number of protons # of protons determines kind of atom# of protons determines kind of atom the same as the number of electrons in the same as the number of electrons in

the neutral atomthe neutral atom Mass Number = Mass Number = the number of protons the number of protons

+ neutrons+ neutrons that’s all the parts with massthat’s all the parts with mass

SymbolsSymbols Contain the symbol of the element, the Contain the symbol of the element, the

mass number and the atomic numbermass number and the atomic number

SymbolsSymbols Contain the symbol of the element, the Contain the symbol of the element, the

mass number and the atomic numbermass number and the atomic number

X Massnumber

Atomicnumber

SymbolsSymbols Find the Find the

– number of protonsnumber of protons– number of neutronsnumber of neutrons– number of electronsnumber of electrons– Atomic numberAtomic number– Mass NumberMass Number

F19 9

SymbolsSymbols Find the Find the

–number of protonsnumber of protons–number of neutronsnumber of neutrons–number of electronsnumber of electrons–Atomic numberAtomic number–Mass NumberMass Number

Br80 35

SymbolsSymbols if an element has an atomic if an element has an atomic

number of 34 and a mass number number of 34 and a mass number of 78 what is the of 78 what is the –number of protonsnumber of protons–number of neutronsnumber of neutrons–number of electronsnumber of electrons–Complete symbolComplete symbol

SymbolsSymbols if an element has 91 protons and if an element has 91 protons and

140 neutrons what is the 140 neutrons what is the –Atomic numberAtomic number–Mass numberMass number–number of electronsnumber of electrons–Complete symbolComplete symbol

SymbolsSymbols if an element has 78 electrons and if an element has 78 electrons and

117 neutrons what is the 117 neutrons what is the –Atomic numberAtomic number–Mass numberMass number–number of protonsnumber of protons–Complete symbolComplete symbol

IsotopesIsotopes Dalton was wrong.Dalton was wrong. Atoms of the same element can have Atoms of the same element can have

different numbers of neutronsdifferent numbers of neutrons Have different mass numbersHave different mass numbers called called isotopesisotopes The The Mass NumberMass Number is the mass of one is the mass of one

atom of one isotopeatom of one isotope

Naming IsotopesNaming Isotopes Put the mass number after the name of Put the mass number after the name of

the element, and write the complete the element, and write the complete symbol for itsymbol for it

carbon- 12carbon- 12 carbon -14carbon -14 uranium-235uranium-235

Atomic MassAtomic Mass How heavy is an atom of oxygen?How heavy is an atom of oxygen? There are different kinds of oxygen atoms.There are different kinds of oxygen atoms. More concerned with More concerned with average average atomic mass.atomic mass. Based on abundance of each element in Based on abundance of each element in

nature.nature. Don’t use grams because the numbers Don’t use grams because the numbers

would be too smallwould be too small

Measuring Atomic MassMeasuring Atomic Mass Unit is the Unit is the Atomic Mass Unit Atomic Mass Unit (amu)(amu) One twelfth the mass of a carbon-12 One twelfth the mass of a carbon-12

atom. atom. Each isotope has its own atomic mass Each isotope has its own atomic mass

we need the average from percent we need the average from percent abundance.abundance.

Calculating averagesCalculating averages You have five rocks, four with a mass of 50 You have five rocks, four with a mass of 50

g, and one with a mass of 60 g. What is the g, and one with a mass of 60 g. What is the average mass of the rocks?average mass of the rocks?

Total mass = 4 x 50 + 1 x 60 = 260 gTotal mass = 4 x 50 + 1 x 60 = 260 g Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 5 5 Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 55 5 5

Calculating averagesCalculating averages Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 5 5 5 5 Average mass = .8 x 50 + .2 x 60Average mass = .8 x 50 + .2 x 60 80% of the rocks were 50 grams80% of the rocks were 50 grams 20% of the rocks were 60 grams20% of the rocks were 60 grams Average = % as decimal x mass + Average = % as decimal x mass +

% as decimal x mass + % as decimal x mass + % as decimal x mass + % as decimal x mass +

Atomic MassAtomic Mass Calculate the atomic mass of copper if Calculate the atomic mass of copper if

copper has two isotopes. 69.1% has a mass copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of of 62.93 amu and the rest has a mass of 64.93 amu.64.93 amu.

Atomic MassAtomic Mass Magnesium has three isotopes. 78.99% Magnesium has three isotopes. 78.99%

magnesium 24 with a mass of 23.9850 magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the with a mass of 25.9826 amu. What is the atomic mass of magnesium?atomic mass of magnesium?

If not told otherwise, the mass of the If not told otherwise, the mass of the isotope is the mass number in amu isotope is the mass number in amu

Atomic MassAtomic Mass Is not a whole number because it is an Is not a whole number because it is an

averageaverage of several different isotopes. of several different isotopes. Is the decimal numbers on the periodic Is the decimal numbers on the periodic

table.table. Is always larger than the Atomic Is always larger than the Atomic

NumberNumber