Chapter 3: Atomic Chapter 3: Atomic StructureStructure

ScientistsScientists

Democritus- Matter composed of Democritus- Matter composed of atoms (indivisible) (~450B.C.)atoms (indivisible) (~450B.C.)

Lavoisier – conservation of massLavoisier – conservation of massProust – law of constant Proust – law of constant

compositioncompositionDalton – modern atomic theory Dalton – modern atomic theory

(KNOW the 4 postulates)(KNOW the 4 postulates)

Law of Constant Law of Constant CompositionComposition

Faraday – Atoms contain Faraday – Atoms contain charged particlescharged particles

Thomson – atoms are Thomson – atoms are divisible, he discovered divisible, he discovered electronselectrons

Millikan – found the charge Millikan – found the charge and mass of electronsand mass of electrons

ScientistsScientists

Becquerel –discovered Becquerel –discovered radioactivityradioactivity

Marie Curie – isolated Marie Curie – isolated radioactive elements.radioactive elements.

Rutherford – demonstrated Rutherford – demonstrated existence of neutrons and the existence of neutrons and the nucleusnucleus

RutherfordRutherfordModern atomModern atomNucleus - central charge Nucleus - central charge

concentrated into a very concentrated into a very small volume in comparison small volume in comparison to the rest of the atom to the rest of the atom

tiny tiny electrons circling around circling around the nucleus like planets the nucleus like planets around the sun.around the sun.

Modern Atomic TheoryModern Atomic Theory

Atoms are composed of three Atoms are composed of three fundamental particlesfundamental particles

Protons pProtons p++ Neutrons n Neutrons no o Electrons Electrons ee--

The nucleus is made of protons and The nucleus is made of protons and neutrons, (positively charged)neutrons, (positively charged)

Electrons orbit the nucleus in and Electrons orbit the nucleus in and electron cloud (negatively charged)electron cloud (negatively charged)

An atom is neutral, the # p+ = # e-An atom is neutral, the # p+ = # e-

ChartChart

ParticleParticle LocatioLocationn

ChargChargee

Mass (g)Mass (g) Mass Mass AMUAMU

ProtonProton Inside Inside NucleusNucleus

++ 1.673 x 1.673 x 1010-24-24

~ 1~ 1

NeutronNeutron Inside Inside NucleusNucleus

00 1.675 x 1.675 x 1010-24-24

~1~1

ElectronElectron Outside Outside nucleusnucleus

-- 9.109 x 9.109 x 1010-28-28

~0~0

Atomic NumberAtomic Number

Atoms identity comes from Atoms identity comes from the number of protons in the the number of protons in the nucleusnucleus

In a chemical reaction, atoms In a chemical reaction, atoms gain/lose electrons and become gain/lose electrons and become an an ionion. .

Ion is a charged particle. This can Ion is a charged particle. This can be + or – depending on whether an be + or – depending on whether an electron is gained or lost.electron is gained or lost.

Calculating charges and Calculating charges and writing ions.writing ions.

If an electron is gained, the If an electron is gained, the charge becomes negative.charge becomes negative.

If an electron is lost, the charge If an electron is lost, the charge becomes positive.becomes positive.

Ex. Magnesium Ex. Magnesium Number of protons = 12Number of protons = 12 Number of electrons = 10Number of electrons = 10 Charge of ion = 2+ or MgCharge of ion = 2+ or Mg+2+2

IsotopesIsotopes

IsotopesIsotopes - - atoms of the same atoms of the same element (same #p+) but with element (same #p+) but with different number of neutrons.different number of neutrons.

All elements have isotopes. All elements have isotopes. Isotopes of elements are Isotopes of elements are

almost indistinguishable (they almost indistinguishable (they exhibit the same properties)exhibit the same properties)

The mass number is used to The mass number is used to identify isotopes. identify isotopes.

Mass number = the sum of Mass number = the sum of the pthe p++ and n and n00

Mass number → 37Mass number → 37 ClClAtomic number →17Atomic number →17

More ExamplesMore Examples

Other examplesOther examples Cl-35Cl-35 C-12 C-14C-12 C-14 3535 1212 1414 ClCl C C C C 17 17 6 6 6 6

Even MORE examplesEven MORE examples

IonsIons 5656 16 16 27 27 FeFe+2+2 O O2-2- Al Al+3+3

2626 8 8 13 13

Mass of atoms-Mass of atoms-

Masses measured in amu’sMasses measured in amu’sAMU = atomic mass unit = AMU = atomic mass unit =

1/12 the weight of a carbon-1/12 the weight of a carbon-12 atom12 atom

Atomic Mass = atomic weight Atomic Mass = atomic weight = average atomic mass= average atomic mass

Calculation of average atomic Calculation of average atomic massmass

= Average mass of an element’s = Average mass of an element’s atomsatoms

Lithium – 6 7.42% = 6 x 0.0742 = Lithium – 6 7.42% = 6 x 0.0742 = 0.44520.4452

Lithium – 7 92.58% =7 x .9258 = + Lithium – 7 92.58% =7 x .9258 = + 6.48066.4806

6.9258 6.9258 amuamu

You try it!You try it!

Neon – 20 Neon – 20 90.92% = 90.92% = Neon – 21 Neon – 21 0.26% = 0.26% = Neon – 22Neon – 22 8.82% = 8.82% = 20.179 20.179

amuamu

Changes in the nucleusChanges in the nucleus

Nuclear Reactions – Nuclear Reactions – Change the composition of Change the composition of the nucleus. the nucleus.

Atoms undergo nuclear decay Atoms undergo nuclear decay and produce new elements!and produce new elements!

Why are some atoms Why are some atoms radioactive?radioactive?

Changes in the NucleusChanges in the Nucleus

What governs nuclear What governs nuclear stability?stability?

part of reason is the # of ppart of reason is the # of p++ and # nand # noo

strong nuclear forcestrong nuclear force– force which holds the force which holds the nucleus togethernucleus together

Pattern of stable nucleiPattern of stable nuclei

““belt of stability” – as atomic belt of stability” – as atomic number increases, you need number increases, you need more neutrons to keep the more neutrons to keep the atom stableatom stable

All atoms with an atomic All atoms with an atomic number greater than 83 are number greater than 83 are radioactive radioactive

Radioactive DecayRadioactive Decay Radioactive Decay – emission of radiationRadioactive Decay – emission of radiation 3 types:3 types: Alpha: Alpha: High-energy alpha particlesHigh-energy alpha particles 2p2p++ and 2 n and 2 n00. . 44 22 Weak, stopped by paper or Weak, stopped by paper or

clothingclothing Mass number 4Mass number 4

Radioactive DecayRadioactive Decay

Beta:Beta: High speed electronsHigh speed electrons 00 00 ee -1-1 -1-1 Mass number = 0Mass number = 0 Can pass through clothing, some Can pass through clothing, some

damage to skindamage to skin

Radioactive DecayRadioactive Decay

Gamma:Gamma: Most dangerousMost dangerous Consists of radiation wavesConsists of radiation waves Only stopped by heavy dense Only stopped by heavy dense

material like lead/concretematerial like lead/concrete 00 00

Writing nuclear equations:Writing nuclear equations:

Atomic massAtomic mass Chemical symbolChemical symbol Atomic Number Atomic Number

Alpha particlesAlpha particles

When a nucleus emits an alpha When a nucleus emits an alpha particle, the mass decreases by particle, the mass decreases by 4 amu’s and the number 4 amu’s and the number decreases by 2 amu’s. decreases by 2 amu’s.

226226 222222 44 Ra → RnRa → Rn ++ 8888 8686 22

Beta particlesBeta particles

When a nucleus emits a beta When a nucleus emits a beta particle, the mass of the atom is particle, the mass of the atom is practically unchanged, but the practically unchanged, but the atomic number increases by one atomic number increases by one unit. unit.

131131 131131 00 II → Xe + → Xe + 5353 5454 -1-1

Gamma RaysGamma Rays

When a nucleus emits a When a nucleus emits a gamma ray, both the atomic gamma ray, both the atomic number and atomic mass number and atomic mass remain the same. remain the same.

113113 113113 00 In → In → In + In + 4949 4949 00

Application of Nuclear Application of Nuclear ChemistryChemistry

Use of half life + Radioactive DatingUse of half life + Radioactive Dating Nuclear Bombardment – ReactionsNuclear Bombardment – Reactions Create radioactive isotopes used in Create radioactive isotopes used in

medicinemedicine Power GenerationPower Generation Fission – Limerick Generating PlantFission – Limerick Generating PlantFusion – “research”Fusion – “research”

Radioisotope – an isotope Radioisotope – an isotope that is radioactive. that is radioactive.

Half-life – The amount of time Half-life – The amount of time it takes for ½ of a sample of a it takes for ½ of a sample of a radioactive isotope to decay. radioactive isotope to decay. (1/2 of the radioactive atoms)(1/2 of the radioactive atoms)

Ex. 90Sr = 28.8 yrs Ex. 90Sr = 28.8 yrs

Radiocarbon DatingRadiocarbon Dating

uses carbon-14uses carbon-14 carbon-14 is radioactivecarbon-14 is radioactive half-life is 5370 yrshalf-life is 5370 yrs Produced naturally from reaction Produced naturally from reaction

between N-14 and cosmic raysbetween N-14 and cosmic rays Rate of production carbon-14 = Rate of production carbon-14 =

rate of decay of carbon-14rate of decay of carbon-14