Unit 3

History of the Atom400 B.C. – Democritus & LeucippusBeach sand smaller piece of sand atomos (indivisible)

Everything is composed of imperishable, indivisible elements called atomos.

Aristotle’s viewsAll substances are made of 4 elementsFire, Air, Earth, Water

Blend these elements in different proportions to get all substances

So who was right?Greeks settled argument by…Debating!Aristotle was a better speaker so he won.

His views carried on through the Middle Ages.

1808 - DaltonEnglish schoolteacherRecognized that elements were made of atoms

Combined LOTS of researchDalton’s Atomic Theory

Dalton’s Atomic Theory1. All matter is made of tiny,

indivisible particles called atoms.

2. Atoms of a given element are identical in their physical and chemical properties.

3. Atoms of different elements differ in their physical & chemical properties

Dalton’s Atomic Theory4. Atoms of different elements combine in simple whole # ratios to form compounds

5. In a chemical reaction, atoms are combined, separated, & rearranged, but never created, destroyed, or changed.

From Dalton’s Theory…Law of Definite Proportions (#4)

2 samples of a compound have the same proportions by mass

500 kg NaCl = 60.66% Cl & 39.34% Na

2 mg NaCl = Law of Conservation of Mass (#5)

Mass of reactants = mass of products

60.66% Cl & 39.34% Na

From Dalton’s Theory…Law of Multiple Proportions

If 2 or more compounds are composed of the same elements, the ratio of the mass of the elements is always a small, whole #

NO 1.14 g O: 1 g NNO2 2.28 g O: 1 g N

O [NO]: O [NO2] = 1.14 : 2.28 = 1 : 2

Another way to look at it…Water (H2O) has 8 g of oxygen per g of hydrogen.

Hydrogen peroxide (H2O2) is 16 g of oxygen per g of hydrogen.

16 to 8 is a 2 to 1 ratio of oxygen.Always whole #s because you have to add a whole atom --- you can’t add a piece of an atom.

Dalton’s Atomic ModelThe Solid Sphere Model

The research continues…1897 – JJ Thomson – cathode ray tube

MoviePumped air out of glass tube and applied voltage to metal electrodes at either end of the tubeAnode = positive chargeCathode = negative charge

Voltage source

+-

Metal Disks

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

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s ExperimentThomson’s Experiment

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

Also placed a paddle wheel in the center – and it turned! So they must have mass!

+

- By adding an electric field By adding an electric field

Thomson’s ConclusionsThe cathode ray consists of particles that have mass & a negative chargeParticles called ELECTRONS

Plum pudding modelNegative electrons in a ball of positive charge

The research continues…We know that an electron has a negative charge and atoms are neutral

Mass of an electron MUCH less than an atom

Something must be missing…

Rutherford - 1909Beam of small, positively charged particles called ALPHA particles

Aimed at thin gold foil (a few atoms thick)

Measured the angles of deflection

Movie

Lead block

Uranium

Gold Foil

Flourescent Screen

What Rutherford ExpectedThe Plum Pudding ModelAlpha particles would pass through without changing direction very much.

Because…The positive charges were spread out evenly & would not stop the positive alpha particles.

What he expected

Because

Because, he thought the positive charges were evenly distributed in the atom

Because, he thought the positive charges were evenly distributed in the atom

What he got

Rutherford’s Explanation

+

Atom is mostly empty.Small dense,

positive piece at center.

Alpha particles

are deflected by it if they get hit the

dense positive center.

+

Conclusions from RutherfordThe small dense, positive place at the center is called the NUCLEUSRadius of nucleus is less than 1/10000 the radius of the atom

PROTON – positive charged particle in nucleusCharge is exactly equal but opposite to an electron

BUT still not enough mass

Still More Research…NEUTRONS

Found in nucleus with protonsDo not have a chargeSame mass as protons

The Nuclear Model - RutherfordElectrons revolve around nucleus in elliptical orbits

Also called planetary model

The Bohr ModelElectrons are found in certain levels or shells around the nucleus

More research…1924 – de Broglie - Electrons behave like waves around the nucleus

Heisenberg’s Uncertainty PrincipleA ceiling fan

Planck & Einstein – quantum theoryElectrons found in clouds instead of strict orbitals

Historical models of the atom (solid sphere, plum pudding, nuclear model, & planetary model all predicted exact locations of particles. The modern quantum theories combine all of this research with more recent findings that suggest a certain level of unpredictability.

Subatomic particles

ElectronProton

Neutron

Name

Symbol

Charge

Relative mass

Actual mass (g)

e-

p+

n0

-1+10

1/1840

1

1

9.11 x 10-

28

1.67 x 10-

241.67 x 10-

24

Most of the mass is in the nucleus!

The AtomAll atoms have protons and electrons

Most atoms have neutronsElements differ from each other in the number of protons in an atom

Protons & neutrons made of quarks

Atomic NumberThe number of protons in an atom

Same in all atoms of an elementExample – Hydrogen=1Atomic number also reveals the number of electrons in a neutral atom

Mass NumberNumber of particles in the nucleus

= # of Protons + # of neutronsExample – Neon has a mass # of 20Can vary among atoms of an element

Different elements can have the same mass number

Not specifically on periodic table

Using Atomic SymbolsEach element has a name & a symbol

Examples – Sulfur = SSodium = Na

The subscript to the right tells you how many atoms are presentS8 = 8 sulfur atoms

Using Atomic SymbolsContain the symbol of the element, the mass number, and the atomic number.

X Massnumber

Atomicnumber

Symbols/NotationFind the …

Atomic numberMass Numbernumber of protonsnumber of neutronsnumber of electronsName

Na2411

Symbols/NotationSymbols/Notation Find the …

–Atomic number

–Mass Number

–number of protons

–number of neutrons

–number of electrons

–Name

Br80 35

IsotopesAll atoms of an element have the same number of protons but not necessarily the same # of neutrons

ISOTOPE – Atoms of the same element with different numbers of neutrons

Naming IsotopesName – mass numberExamples

Helium-3, Helium-4Symbols/Notation

3

2He He

4

2

Name That ElementName That Element if an element has an atomic

number of 34 and a mass number of 78 what is the

–number of protons

–number of neutrons

–number of electrons

–Complete symbol– Name

Name That ElementName That Element if an element has 91 protons and

140 neutrons what is the

–Atomic number

–Mass number

–number of electrons

–Complete symbol– Name

Name That ElementName That Element if an element has 78 electrons and

117 neutrons what is the

–Atomic number

–Mass number

–number of protons

–Complete symbol

–Name

Atomic MassThere are different isotopes of each element that all have different masses

Therefore we look at AVERAGE atomic mass for an element

Based on abundance of each isotope in nature

Atomic Mass ExampleYou have five rocks -- four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?

(50 + 50 + 50 + 50 + 60) / 5260/5 = 52 g

Atomic MassAtoms are so small that even picograms aren’t useful 1 g = 10-12 pg

So… we use a different unit

Atomic Mass Units (amu)AKA a Dalton (Da)Defined as 1/12 the mass of a carbon-12 atom.

Very close to mass numberExample – Oxygen (O)

Mass number = 16(Average) atomic mass = 15.999 amu

Calculating averagesAverage atomic mass = (% as decimal x mass) + (% as decimal x mass)

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

Atomic Mass of CompoundsAdd the masses of the parts!EXAMPLE

H2O = 2 hydrogen atoms + 1 oxygen

H2O = 2 (1.0079) + 15.999H2O = 18.015 amu in 1 molecule

Another number problem…Most samples of elements have LOTS of atoms

We want things to be simple! Scientists created a new unit

The MOLE# of atoms in exactly 12 g of Carbon-12

Avogadro’s number = the number of particles in one mole of an element

Avogadro’s number = 6.022 x 1023 particles (atoms or molecules)

Avogadro as a conversion factor0.30 mol F = _________ atoms F

0.30 mol F

mol F

atoms F6.022 x 1023

1

Molar MassMass in grams of 1 mole of an elementUnits are g/molMolar mass of 1 mole = atomic massCopper (Cu) atomic mass = 63.55 amuTherefore molar mass = 63.55 g/mol

Molar mass as a conversion factor3.50 mol Cu = _________ g Cu

3.50 mol Cu

mol Cu

g Cu63.55

1

Molar mass as a conversion factor2 step conversions – 3g Cu = _________ atoms

Calculating compounds – molar mass of H2O?

2 step conversions with compounds – 4g H2O = _____ atoms

Atomic Nuclei ReviewNucleus made of protons & neutrons

Isotope – nucleus with same # of protons & different # neutrons

Why don’t nuclei repel each other?1935 Yukawa – attractive force between neutrons & protons stronger than proton repulsion

Only works when particles very close together

Not as strong in large nucleus

Spontaneous Nuclear ChangeAKA transmutationAll to increase stability1. Fission – large nucleus splits2. Fusion – small nuclei combine3. Radioactive decay – release of

particles

Radioactive Decay1. Convert neutrons to protons

Emission of high energy negative particles called BETA PARTICLES

If a neutron loses its negative charge, it becomes a proton

Increases atomic number, no change to mass number

Radioactive Decay1. Convert neutrons to protonsExample

Th Pa234 234

90 91+ β

0

-1

Radioactive Decay2. Convert protons to neutronsNucleus captures an electron

If a proton combines with an electron, it forms a neutron

Atomic number decreases by 1, no change in mass number

Pa238

91 U238

92

Radioactive Decay3. Losing ALPHA PARTICLESAn alpha particle is a positive particle identical to a helium-4 nucleus ( He)

Example

4

2

U Th + He 238

92

234

90

4

2

Radioactive DecayAlso capable of producing a high energy photon called GAMMA RAY EMISSION

Does not change nucleus

Types of RadiationALPHA BETA GAMMA

Symbol

How change the nucleus

Penetration

Low Medium High

Shielding provided by

Skin Paper, clothing

Lead

Danger Low Medium High

Nuclear reactorsUranium atoms split (fission) and releases neutrons

Neutrons trigger more uranium to split nuclear chain reaction

Boron absorbs neutrons so is used to control the speed of the chain reaction

Fission products like iodine & cesium also produce alpha, beta, & gamma particles to stabilize

Nuclear reactorsHeat must be removed to allow decay to continue (decay = stabilization) so cooling systems are vital

Fukushima, JapanPower failed from earthquakeDiesel generators failed from tsunamiRelease of radioactive isotopes of cesium & iodide

Half LifeRate of decay of a radioactive sample

Constant – not influenced by temperature, pressure, etc.

Often used to determine age – RADIOACTIVE DATING

Half LifeMost Carbon on Earth is C-12All animals have C-14 from plantsAll living plants & animals have fixed ratio of C-14 to C-12

Once dead, C-14 levels decreaseMeasure ratio & compare to known

Half-Life ExampleC-14 half life = 5715 yearsYou find an artifact with a C-14:C-12 ratio that is 1/8 the modern ratio. How old is the artifact?

Half-Life Example1. Determine the number of

half-lives that the artifact has undergone.

2. Multiply the # of half-lives by the length of the half-life

Half-Life ExampleRatio is one eighth the modern ratio1 ½ ½ ¼ ¼ 1/8 3 half lives

5715 years x 3 half lives=17,145 years old

More Half-Life ExamplesThe half-life of Ra-226 is 1599 years. How many years are needed for the decay of 15/16 a given amount?

Assume the half-life of a substance is 3.824 days. How much time will it take for ¼ of a sample to remain?

More Half-Life ExamplesAssume the half-life of a substance is 3.0 minutes. How long will it take for 16 mg to become 1.0 mg?