Atoms and their structure History of the atom n Not the history of atom, but the idea of the atom n...

Atoms and their structureAtoms and their structure

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

atomatom Original idea Ancient Greece (400 B.C..)Original idea Ancient Greece (400 B.C..) Democritus and Leucippus Greek Democritus and Leucippus Greek

philosophers philosophers

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?Atomos - not to be cutAtomos - not to be cut

Another GreekAnother Greek Aristotle - Famous philosopherAristotle - 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

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 argumentGreeks settled disagreements by argument Aristotle was more famousAristotle was more famous He wonHe won His ideas carried through middle ages.His ideas carried through middle ages. Alchemists change lead to goldAlchemists change lead to gold

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

experiments and those of other’sexperiments and those of other’s In Dalton’s Atomic TheoryIn Dalton’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 identical, Atoms of the same element are identical,

those of different atoms are different.those of different atoms are different. Atoms of different elements combine in Atoms of different elements combine in

whole number ratios to form compoundswhole number ratios to form compounds Chemical reactions involve the Chemical reactions involve the

rearrangement of atoms. No new atoms rearrangement of atoms. No new atoms are created or destroyed.are created or destroyed.

Dalton’s Theory and Law of Dalton’s Theory and Law of Conservation of massConservation of mass

+

1 mas unit 4 mass units 5 mass units

5 mass units1 mass unit 4 mass units

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 16 grams of oxygen to 2 Water is always 16 grams of oxygen to 2

grams of hydrogengrams of hydrogen HH22O = 2 g H to 16 g O (always)O = 2 g H to 16 g O (always) HH22OO22 = 2 g H to 32 g O = 2 g H to 32 g O CO = 12 g C to 16 g OCO = 12 g C to 16 g O COCO2 2 = 12 g C to 32 g O= 12 g C to 32 g O

Law of Multiple ProportionsLaw of Multiple Proportions If two or more different compounds are If two or more different compounds are

composed of the same two elements, composed of the same two elements, the masses of the second element the masses of the second element combined with a certain mass of the combined with a certain mass of the first element can be expressed as ratios first element can be expressed as ratios of small whole numbers.of small whole numbers.

Example: HExample: H22O & HO & H22OO22

Water has 16 g O, peroxide 32 g OWater has 16 g O, peroxide 32 g O32g/16g = 1:2 ratio32g/16g = 1:2 ratio

What?What? Another example:Another example:

CO is Carbon monoxideCO is Carbon monoxide

COCO22 is Carbon dioxide is Carbon dioxide

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

cathode ray tube (CRT)cathode ray tube (CRT) It is a vacuum tube - all the air has been It is a vacuum tube - all the air has been

pumped out.pumped out. Thompson pumped a pure gas at low Thompson pumped a pure gas at low

pressure into the CRTpressure into the CRT A current flows easily through a gas at A current flows easily through a gas at

low pressure. Why?low pressure. Why?

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

Pure gas at low pressurecathodeanode


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


Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative (cathode) appear to move from the negative (cathode) to the positive end (anode)to the positive end (anode)


Voltage source

+-

Passing an electric current makes a beam of Passing an electric current makes a beam of light appear to move from the cathode to light appear to move from the cathode to the anodethe anode

Stream of light changed colors if he Stream of light changed colors if he changed gases. Why?changed gases. Why?


Voltage source

+-

Voltage source


By adding an electric field By adding an electric field

Voltage source


By adding an electric field By adding an electric field

+

-

Voltage source


By adding an electric field he found that the beam By adding an electric field he found that the beam of light took a curved path towards the positive of light took a curved path towards the positive plate.plate.

Hypothesis: Something in light has a neg chargeHypothesis: Something in light has a neg charge

+

-

cathodeAnode

Gas at low pressure

FactsFacts Different gases glow with different colors as Different gases glow with different colors as

current passes through the tubecurrent passes through the tube The part of glass tube directly opposite the The part of glass tube directly opposite the

cathode glowscathode glows An object placed between the cathode and An object placed between the cathode and

the opposite end of tube casts a shadow on the opposite end of tube casts a shadow on the glassthe glass

A paddlewheel between the electrodes rolls A paddlewheel between the electrodes rolls along on its rails from the cathode towards along on its rails from the cathode towards the anodethe anode

Thomson measured the charge/mass ratio of the negatively Thomson measured the charge/mass ratio of the negatively charged particle and found it to be very high:charged particle and found it to be very high:

charge charge massmass

1.1. If neg charged particle was in light it must be very smallIf neg charged particle was in light it must be very small2.2. Same charge to mass ratio no matter what gas was usedSame charge to mass ratio no matter what gas was used3.3. Used different types of metals for electrodes and still got Used different types of metals for electrodes and still got

the same ratiothe same ratio

How did he know the particles How did he know the particles charge was negative?charge was negative?

Thomson’s Final HypothesisThomson’s Final Hypothesis Mysterious particle is a fundamental Mysterious particle is a fundamental

part of all matterpart of all matter Smaller than the atomSmaller than the atom

Named: Named: electronelectron

What about a positive charge?What about a positive charge? Thomson wanted to find a positive particleThomson wanted to find a positive particle He repeated his experiment with a different tubeHe repeated his experiment with a different tube When the negative particles go in one direction When the negative particles go in one direction

the positive must travel the opposite waythe positive must travel the opposite way He put holes in the cathode (neg electrode) and He put holes in the cathode (neg electrode) and

the +charged particles passed through the the +charged particles passed through the holes and let off scintillations on a screen holes and let off scintillations on a screen behind the cathodebehind the cathode

He discovered the proton but not the He discovered the proton but not the location,charge, or masslocation,charge, or mass

Thomsom’s ModelThomsom’s Model Found the electronFound the electron Couldn’t find positiveCouldn’t find positive

(for a while) (for a while) Said the atom was Said the atom was

like plum puddinglike plum pudding A bunch of positive A bunch of positive

stuff, with the stuff, with the electrons able to be electrons able to be removed removed

Mass evenly spread Mass evenly spread outout

Robert MillikanRobert Millikan Found the charge and mass on the Found the charge and mass on the

electronelectron

Handout: Oil Drop ExperimentHandout: Oil Drop Experiment

Read- answer questionsRead- answer questions

Other piecesOther pieces Proton - positively charged pieces 1840 Proton - positively charged pieces 1840

times heavier than the electrontimes heavier than the electron Neutron - no charge but the same mass Neutron - no charge but the same mass

as a proton.as a proton. Where are the pieces?Where are the pieces?

Rutherford’s experimentRutherford’s experiment Ernest Rutherford English physicist. (1910)Ernest Rutherford English physicist. (1910) Believed in the plum pudding model of the Believed in the plum pudding model of the

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

given off by uranium given off by uranium Shot them at gold foil which can be made a Shot them at gold foil which can be made a

few atoms thick 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 72)Here’s what it looked like (pg 72)

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

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

Modern ViewModern View The atom is mostly The atom is mostly

empty spaceempty space Two regionsTwo regions Nucleus- protons Nucleus- protons

and neutronsand neutrons Electron cloud- Electron cloud-

region where you region where you might find an might find an electronelectron

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

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

1.67 x 10-24

1.67 x 10-24

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 The region where the electron can be

foundfound

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 All the things with massAll the things with mass

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 different mass numbersdifferent mass numbers called called isotopesisotopes

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 neutronsnumber of neutrons


–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


–Mass numberMass number



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


–Mass numberMass number



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

the elementthe element 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 atom. One twelfth the mass of a carbon-12 atom. 12.01 amu – most abundant isotope is 12.01 amu – most abundant isotope is

carbon-12carbon-12 Other isotopes must be greater than this Other isotopes must be greater than this

because the average is over 12because the average is over 12 Each isotope has its own atomic mass we Each isotope has its own atomic mass we

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

Lithium = 6.941 amuLithium = 6.941 amu– Most abundant is lithium – 7Most abundant is lithium – 7– Others are probably lower since average is Others are probably lower since average is

less than 7less than 7

– A relative measure is usually compared to A relative measure is usually compared to a known standarda known standard

– First – define the standardFirst – define the standard– *Standard for periodic table – Carbon –12 *Standard for periodic table – Carbon –12

= exactly 12.000 amu= exactly 12.000 amu

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

average. average. are the decimal numbers on the periodic are the decimal numbers on the periodic

table.table.

Atomic Weight/Relative Atomic Atomic Weight/Relative Atomic MassMass

12.01 amu – most abundant isotope is12.01 amu – most abundant isotope is

carbon – 12.carbon – 12.

Other isotopes of carbonOther isotopes of carbon

must be greater than thismust be greater than this

since average is over 12since average is over 12

6.941 amu6.941 amu most abundant is most abundant is 77LiLi

others are probably lowerothers are probably lower

since avg is less than 7since avg is less than 7

What is a “relative mass”?What is a “relative mass”? A relative measure is usually compared A relative measure is usually compared

to a known standard.to a known standard. 11stst – you must define the standard – you must define the standard

Standard on periodic table:Standard on periodic table:– Carbon – 12 = 12.000 amu exactlyCarbon – 12 = 12.000 amu exactly– 1 amu = 1/12 carbon-12 atom1 amu = 1/12 carbon-12 atom– 1 amu = 1.66 x 101 amu = 1.66 x 10-24-24g (mass of a g (mass of a

proton)proton)

Calculate the number of atoms in Calculate the number of atoms in 1.00794 g of H.1.00794 g of H.

Calculate the number of atoms in Calculate the number of atoms in 12.011 gram of Carbon12.011 gram of Carbon

Avogadro’s Number/The MoleAvogadro’s Number/The Mole 1 mole = 6.022 x 101 mole = 6.022 x 102323 = Avogadro’s Number = Avogadro’s Number

1 mole of H atoms = 6.022 x 101 mole of H atoms = 6.022 x 102323 atoms atoms 1 mole of C atoms = 6.022 x 101 mole of C atoms = 6.022 x 102323 atoms atoms

From periodic table:From periodic table: 1 mole of C = 12.011g1 mole of C = 12.011g 1 mole of H = 1.00794 g1 mole of H = 1.00794 g How much tells you how manyHow much tells you how many

Calculations with the moleCalculations with the mole Calculate the number of atoms of Ag in Calculate the number of atoms of Ag in

1.25 g of Ag.1.25 g of Ag.

Calculate the mass of 1,000,000 Au Calculate the mass of 1,000,000 Au atomsatoms

How about molecules?How about molecules? Remember there are some elements that Remember there are some elements that

exist as molecules aloneexist as molecules alone– Examples: The diatomic “7”Examples: The diatomic “7”

– Oxygen gasOxygen gas OO2 2 Hydrogen gas HHydrogen gas H22

– Nitrogen gasNitrogen gas NN2 2 Iodine Iodine I I22

– Flourine gasFlourine gas FF22

– Chlorine gasChlorine gas ClCl22– Bromine gasBromine gas BrBr22

ExampleExample In a sample of the air in this room there In a sample of the air in this room there

are approximately 2.33 X 10are approximately 2.33 X 102323 molecules of oxygen gas (Omolecules of oxygen gas (O22). How ). How

many moles of oxygen gas are in this many moles of oxygen gas are in this sample?sample?

More Mole FunMore Mole Fun Calculate the number of atoms in a Calculate the number of atoms in a

cube of Au that is 3 cm by 2 cm by 2cm. cube of Au that is 3 cm by 2 cm by 2cm. The density of gold is 19.3 g/cm The density of gold is 19.3 g/cm33

1 amu = 1.66 x 101 amu = 1.66 x 10-24-24 g g

How many atoms are there in 1.00794g How many atoms are there in 1.00794g of hydrogen?of hydrogen?

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