History of the History of the Atomic ModelAtomic Model

Chapter 4Chapter 4

Sir William Crookes (1879)Sir William Crookes (1879)

Invented the cathode ray tube and investigated Invented the cathode ray tube and investigated electrical charges in gases.electrical charges in gases.

Postulated a negatively charged particle that Postulated a negatively charged particle that was eventually named the electron.was eventually named the electron.

Schematic of a Cathode Ray TubeSchematic of a Cathode Ray Tube

Henri Becquerel (1896)Henri Becquerel (1896)

Noticed that certain substances Noticed that certain substances emitted radiation without any emitted radiation without any external energy source.external energy source.

Two types of radiation called Two types of radiation called alpha (alpha () and beta () and beta () due to ) due to their penetrating power.their penetrating power.

Third was later added on – Third was later added on – gamma (gamma ())

John Joseph (JJ) Thomson John Joseph (JJ) Thomson (1903)(1903)

Using the cathode ray tube Using the cathode ray tube invented by Crookes, invented by Crookes, discovered the electron, e.discovered the electron, e.

Also calculated the charge to Also calculated the charge to mass ratio of an electron.mass ratio of an electron.

QQee/m/mee = -1.7588x10 = -1.7588x101111 C/kg C/kg

Proposed the “plum pudding” Proposed the “plum pudding” model.model.

Postulated positive particles.Postulated positive particles.

0-1

9.11x10-31 kg

Robert Millikan (1909)Robert Millikan (1909)

Working with Harvey Fletcher, Working with Harvey Fletcher,

Determined the charge on a single Determined the charge on a single electron, Q = -1.602x10electron, Q = -1.602x10-19-19 C C

Since Thomson found Q/m, they Since Thomson found Q/m, they were able to calculate an were able to calculate an electron’s masselectron’s mass

Which is 1,836 times lighter than a Which is 1,836 times lighter than a hydrogen atom.hydrogen atom.

What is an Electron’s Mass?

Millikan’s Oil Drop ExperimentMillikan’s Oil Drop Experiment

Ernest Rutherford (1909)Ernest Rutherford (1909)

Discovered the Discovered the positively charged positively charged dense central portion dense central portion of the atom using his of the atom using his “gold foil experiment”“gold foil experiment” NucleusNucleusAtoms are mostly Atoms are mostly empty space!empty space!Discovered the Discovered the proton ( p) in 1918proton ( p) in 19181

1

Rutherford’s experimentRutherford’s experiment

Results of foil experiment if Plum Pudding Results of foil experiment if Plum Pudding model had been correctmodel had been correct

Failure!Failure!

Actual ResultsActual Results

An atom viewed in cross sectionAn atom viewed in cross section

The nucleus is 100 The nucleus is 100 thousand times thousand times smaller than the smaller than the atom!atom!

If the nucleus were If the nucleus were the size of a marble, the size of a marble, the atom would be as the atom would be as big as a football big as a football stadiumstadium!!

Timeline So FarTimeline So Far

1803 – Dalton’s Atomic Theory1803 – Dalton’s Atomic Theory1879 – Crookes postulates a negatively charged 1879 – Crookes postulates a negatively charged particle particle electron. electron.1896 – Bequerel discovers radioactive materials.1896 – Bequerel discovers radioactive materials.1903 – Thompson defines the charge to mass 1903 – Thompson defines the charge to mass ratio of the electron (thereby “discovering” it). ratio of the electron (thereby “discovering” it). 1909 – Millikan discovers the charge (and 1909 – Millikan discovers the charge (and therefore mass) of the electrontherefore mass) of the electron1909 & 1918 – Rutherford discovers that the atom 1909 & 1918 – Rutherford discovers that the atom is mostly empty space and eventually the proton.is mostly empty space and eventually the proton.

Problem to SolveProblem to Solve

If electrons are negatively charged and If electrons are negatively charged and protons are positively charged, what keeps protons are positively charged, what keeps them separated?them separated?

Why don’t they simply smash together like Why don’t they simply smash together like magnets?magnets?

Hang on to your hats, it’s about to get Hang on to your hats, it’s about to get weird!weird!

Updated Atomic Model (post 1909)Updated Atomic Model (post 1909)

Discarded Plum PuddingDiscarded Plum Pudding

Electrons must orbit around central Electrons must orbit around central “nucleus” “nucleus” Planetary Model Planetary Model

Nearly all of the mass is located in the Nearly all of the mass is located in the dense, central, positively charged nucleus.dense, central, positively charged nucleus.

Still, why don’t the electrons fall into the Still, why don’t the electrons fall into the nucleus like opposite magnetic poles?nucleus like opposite magnetic poles?

Enter the DaneEnter the Dane

Niels Bohr (1885 – 1962)Niels Bohr (1885 – 1962) Danish physicistDanish physicist Went to Cambridge with the Went to Cambridge with the

complete works of Dickens complete works of Dickens

(to learn the language, duh!).(to learn the language, duh!). Worked on the electron/nucleus Worked on the electron/nucleus

problemproblem Often thought of as the Father of Often thought of as the Father of

Quantum MechanicsQuantum Mechanics

Bohr’s Solution (1913)Bohr’s Solution (1913)

Electrons are located Electrons are located in specific energy in specific energy levelslevels

Electrons move in a Electrons move in a definite orbit around definite orbit around the nucleusthe nucleus

Areas between orbits Areas between orbits are not allowed!are not allowed!

More Problems to SolveMore Problems to Solve

Why are electrons limited to specific Why are electrons limited to specific orbits?orbits? Quantum mechanics has all the answers… Quantum mechanics has all the answers…

uh…sort of…maybe…probably…kind of…uh…sort of…maybe…probably…kind of…

With exception of hydrogen, atoms weigh With exception of hydrogen, atoms weigh more than sum of protons and electrons more than sum of protons and electrons –– there must be another particlethere must be another particle……

Walther Bothe & Herbert BeckerWalther Bothe & Herbert Becker (1928) (1928)

Aimed alpha radiation at light Aimed alpha radiation at light elements like boron.elements like boron.

Found it gave off an extremely Found it gave off an extremely penetrating radiation.penetrating radiation.

Thought it produced high energy Thought it produced high energy gamma rays.gamma rays.

Bothe

Radiation ComparisonRadiation Comparison

Alpha Radiation – positively charged, Alpha Radiation – positively charged, easily stopped (paper, skin, etc)easily stopped (paper, skin, etc)

Beta Radiation – negatively charged, Beta Radiation – negatively charged, stopped by a sheet of aluminumstopped by a sheet of aluminum

Gamma radiation – high energy light (no Gamma radiation – high energy light (no charge), penetrates a lot of materialcharge), penetrates a lot of material

New Radiation - ??? – penetrates even New Radiation - ??? – penetrates even moremore

Irene Joliot-CurieIrene Joliot-Curie (1897-1956) (1897-1956)

Daughter of Marie & Pierre CurieDaughter of Marie & Pierre Curie

Continued work of Bothe and Becker (1932)Continued work of Bothe and Becker (1932) Aimed Bothe’s new beam at paraffinAimed Bothe’s new beam at paraffin Ejected high energy protonsEjected high energy protons Misinterpreted results – cost her a Nobel PrizeMisinterpreted results – cost her a Nobel Prize

Also found a way to transmute elements! (1934)Also found a way to transmute elements! (1934) Inexpensive way to create radioisotopes for medicineInexpensive way to create radioisotopes for medicine

Just like her mother, died from radiation exposureJust like her mother, died from radiation exposure

Did not believe the work of Did not believe the work of Joliot-CurieJoliot-Curie She said beam was light wavesShe said beam was light waves He thought not enough massHe thought not enough mass

Discovered neutron in 1932.Discovered neutron in 1932.

No charge.No charge.

Mass slightly more than a Mass slightly more than a proton.proton.Jimmy Neutron

James Chadwick (1891-1974)James Chadwick (1891-1974)

Subatomic ParticlesSubatomic Particles

NameName AbbrevAbbrev LocationLocation Mass (kg)Mass (kg) Charge (C)Charge (C) Effective Effective ChargeCharge

ElectronElectron ee-- oror e e OrbitOrbit 9.019x109.019x10-31-31 -1.602x10-1.602x10-19-19 -1-1

ProtonProton pp++ oror p p NucleusNucleus 1.673x101.673x10-27-27 +1.602x10+1.602x10-19-19 +1+1

NeutronNeutron n n oror n n NucleusNucleus 1.675x101.675x10-27-27 00 00

0-1

1 1

10

Comparison (cont’d)Comparison (cont’d)

Protons: Heavy, positive charge Protons: Heavy, positive charge repelled by the nucleus.repelled by the nucleus.Electrons: Nearly massless, negative Electrons: Nearly massless, negative charge charge repelled by other electrons repelled by other electrons surrounding an atom.surrounding an atom.Neutrons: Heavy, no charge Neutrons: Heavy, no charge no no interactions with nucleus or electrons.interactions with nucleus or electrons. Can pass through a lot of material.Can pass through a lot of material. If fast enough, can break nucleus apart!If fast enough, can break nucleus apart!

Atomic RequirementsAtomic Requirements

All atoms of a given element must have the All atoms of a given element must have the same number of protons!same number of protons! 1 Proton ~1 Dalton (1 Da = 1.6605x101 Proton ~1 Dalton (1 Da = 1.6605x10-27 -27 kg)kg)

In order to be neutral, atoms must have same # In order to be neutral, atoms must have same # of electrons as protons (charges must cancel out of electrons as protons (charges must cancel out to zero)to zero) Electrons do not add weight (compared to pElectrons do not add weight (compared to p++ & n) & n)

Total weight comes from additional neutrons.Total weight comes from additional neutrons. 1 Neutron ~ 1 Da1 Neutron ~ 1 Da

Solving the Mass ProblemSolving the Mass Problem

Total mass is sum of protons, neutrons, Total mass is sum of protons, neutrons, and electrons.and electrons.

1 Atom of Helium has 2 protons, 2 1 Atom of Helium has 2 protons, 2 electrons, and 2 neutrons = 4 Daelectrons, and 2 neutrons = 4 Da

But, it turns out that some atoms of But, it turns out that some atoms of elements weigh more (or less) than the elements weigh more (or less) than the others…others…

Elements and NeutronsElements and Neutrons

Atoms can have more (or less) neutronsAtoms can have more (or less) neutrons

Isotopes are atoms with the same # of Isotopes are atoms with the same # of protons, but different # of neutronsprotons, but different # of neutrons

Since protons determine chemical identity, Since protons determine chemical identity, neutrons just add mass.neutrons just add mass.

Isotopes occur in different ratios in nature.Isotopes occur in different ratios in nature.

Two isotopes of Sodium.Two isotopes of Sodium.

Specifying IsotopesSpecifying Isotopes

Two ways to write an isotope:Two ways to write an isotope: C or simply C C or simply C Carbon-12Carbon-12

These isotopic symbols tell you how many These isotopic symbols tell you how many of each particle is in the isotope.of each particle is in the isotope.

The Mass number (A) is the sum of pThe Mass number (A) is the sum of p++ & n & n

The Z number is the number of pThe Z number is the number of p++

The Charge, Q, is # pThe Charge, Q, is # p++ - # e - # e--

12 6

12

Uranium-235Uranium-235

Uranium has Uranium has 9292 protons protons

Needs Needs 9292 electrons to cancel them out electrons to cancel them out

Its mass number is Its mass number is 235235 ( (9292 p p++ + + 143143 n) n)

U235 92

92 -92 = 0, so no charge!

Nitrogen-14 ionNitrogen-14 ion

N14 7

-3

Mass Number

(A)# p+ + # n

Z Number# p+

(nuclear charge)

Charge# p+ - # e-

Atomic Symbol

Nitrogen has 7 protons and 7 neutrons. It tends to gain 3 extra electrons when it forms an ion.

Let’s Practice!

Mass of AtomsMass of Atoms

Mass of an atom is sum of its pMass of an atom is sum of its p++, n, & e, n, & e--..

Subatomic particles are really (really) light.Subatomic particles are really (really) light.

Use the Dalton (Da) to measure the mass Use the Dalton (Da) to measure the mass – also called the – also called the unified mass unitunified mass unit (u) or (u) or atomic mass unitatomic mass unit (amu) (amu)

1 Da = 1/121 Da = 1/12thth mass of a Carbon-12 atom mass of a Carbon-12 atom

1 Da = 1.6605x101 Da = 1.6605x10-27-27 kg kg

Particle WeightsParticle Weights

Electrons – lightest of subatomic particlesElectrons – lightest of subatomic particles 0.0005 Da (usually ignored)0.0005 Da (usually ignored)

ProtonsProtons 1.0073 Da1.0073 Da

Neutrons – heaviest of subatomic particlesNeutrons – heaviest of subatomic particles 1.0087 Da1.0087 Da

pp++ & n are usually rounded to 1 Da, while & n are usually rounded to 1 Da, while ee-- are assumed to be 0 Da are assumed to be 0 Da

Atomic MassAtomic Mass

Mass of a specific isotope.Mass of a specific isotope. Carbon-12 = 12.00 DaCarbon-12 = 12.00 Da Carbon-13 = 13.00 DaCarbon-13 = 13.00 Da Oxygen-16 = 15.9949 DaOxygen-16 = 15.9949 Da

Must be measured experimentally due to Must be measured experimentally due to Mass DeficitMass Deficit

6 p+ + 6 n

6 p+ + 7 n

8 p+ + 8 n???

Mass DeficitMass Deficit

When subatomic particles combine, their When subatomic particles combine, their masses change (don’t ask why…)masses change (don’t ask why…)Sometimes, the combination weighs less, Sometimes, the combination weighs less, other times, it weighs more (depends on other times, it weighs more (depends on numbers of each combining).numbers of each combining).Gain/lose mass according to Einstein’s Gain/lose mass according to Einstein’s famous equation: E = mcfamous equation: E = mc22

Atomic & Hydrogen (Nuclear) bombs Atomic & Hydrogen (Nuclear) bombs release this energy!release this energy!

Carbon-12Carbon-12

Was chosen as the Was chosen as the “standard” atom.“standard” atom.

All atomic masses are based All atomic masses are based on this specific isotope.on this specific isotope.

Mass of C-12 defined as Mass of C-12 defined as exactlyexactly 12.00 Da. 12.00 Da.

Masses of other atoms are Masses of other atoms are relative to C-12.relative to C-12.

Atomic Masses of IsotopesAtomic Masses of Isotopes

Uranium IsotopesUranium Isotopes Oxygen Isotopes Oxygen Isotopes

IsotopeIsotope Mass (Da)Mass (Da)

1414 14.00859614.008596

1515 15.00306615.003066

1616 15.99491515.994915

1717 16.99913216.999132

1818 17.99916117.999161

1919 19.00358019.003580

2020 20.00407720.004077

IsotopeIsotope Mass (Da)Mass (Da)

232232 232.037162232.037162

233233 233.039635233.039635

234234 234.040952234.040952

235235 235.043930235.043930

236236 236.045568236.045568

237237 237.048730237.048730

238238 238.050788238.050788

Abundance of IsotopesAbundance of Isotopes

Isotopes naturally occur in different Isotopes naturally occur in different proportions.proportions.

Oxygen isotopes:Oxygen isotopes:

IsotopeIsotope Mass (Da)Mass (Da) AbundanceAbundance

O-16O-16 15.99491515.994915 99.757%99.757%

O-17O-17 16.99913216.999132 0.038%0.038%

O-18O-18 17.99916117.999161 0.205%0.205%

Atomic WeightAtomic Weight

Different from Atomic MassDifferent from Atomic Mass

Also called “Relative Atomic Mass”Also called “Relative Atomic Mass”

Average weight of all naturally occurring Average weight of all naturally occurring isotopes.isotopes.

Provides more accurate weight of a typical Provides more accurate weight of a typical samplesample

A weighted averageA weighted average

Calculating Atomic WeightCalculating Atomic Weight

Atomic Mass * Abundance = mass fractionAtomic Mass * Abundance = mass fraction 100100

Carbon has 2 stable isotopes:Carbon has 2 stable isotopes: Carbon-12 = 12.00Da – is 98.93%Carbon-12 = 12.00Da – is 98.93% Carbon-13 = 13.00Da – is 1.07%Carbon-13 = 13.00Da – is 1.07%

98.93/100*12.00 Da = 11.872 Da98.93/100*12.00 Da = 11.872 Da 1.07/100*13.00 Da = 0.139 Da1.07/100*13.00 Da = 0.139 Da

12.011 Da12.011 Da

Oxygen’s Atomic WtOxygen’s Atomic Wt

15.9949Da * 99.757/100 = 15.9560Da15.9949Da * 99.757/100 = 15.9560Da

16.9991Da * 0.038 / 100 = 0.0065Da16.9991Da * 0.038 / 100 = 0.0065Da

17.9992Da * 0.205 / 100 = 0.0369Da17.9992Da * 0.205 / 100 = 0.0369Da

Total Total = 15.9994Da = 15.9994Da

That’s the # on the Periodic Table!

IsotopeIsotope Mass (Da)Mass (Da) AbundanceAbundance

1616 15.99491515.994915 99.757%99.757%

1717 16.99913216.999132 0.038%0.038%

1818 17.99916117.999161 0.205%0.205%

SummarySummary

3 subatomic particles3 subatomic particles electron (-1, nearly massless)electron (-1, nearly massless) proton (+1, ~1 Da)proton (+1, ~1 Da) neutron (0, ~1 Da)neutron (0, ~1 Da)

Isotopes – same protons, different # neutronsIsotopes – same protons, different # neutrons Written two waysWritten two ways

Atomic Mass – mass of 1 atomAtomic Mass – mass of 1 atom

Atomic Weight – average weight of all isotopesAtomic Weight – average weight of all isotopes