Chapter 4

Atomic Structure

History of the Atom• 1. Democritus vs. Aristotle pg. 102-103• 2. John Dalton and conservation of mass

pg. 104-105• 3. Cathode ray tube and Sir William Crookes

pg. 107-108• 4. Mass and charge of electron (J.J.

Thompson) and oil drop experiment pg. 108-109

• 5. Plum pudding model vs. Rutherford’s experiment pg. 110-112

Democritus• Greek philosopher who asked questions about

matter. – Can you divide matter infinitely?

• Democritus says no!– Tiny particles called atoms, indivisible!

• Matter is composed of atoms, which move through empty space.• Atoms are solid homogeneous, indestructible,

and indivisible.• Atoms have different sizes and shapes. These

properties, and movement determine properties of matter

John Dalton

• Matter is composed of small particles called atoms that are indivisible and indestructible.

• Atoms of a given element are identical in size, mass, and chemical properties, and are different from those of another element.

• Different atoms combine in simple whole number ratios to form compounds.

• In a chemical reaction, atoms are separated, combined or rearranged.

Dalton’s Inaccuracies• Atoms are not indivisible!

– Subatomic particles – electrons, protons, and neutrons

• Atoms of the same element can have slightly different masses! - isotopes

The Atom

• EXTREMELY small particle of an element that retains the properties of that element is an atom.

• If the atom is the size of an orange, an orange would be the size of the EARTH

Subatomic Particles - Electron

• Cathode Ray TubeThin beam of electronstravels from cathode to anode!Cathode rays are a stream of charged particles. Particles carry anegative charge…now called electrons!

Anode

Cathode

Vacuum

J.J. Thompson

• Determined that the mass of the charged particle (electron) was much less than that of the hydrogen atom. – Dalton was WRONG about the atom being the

smallest particle!

Millikan Oil-Drop Experiment• Determined the charge of an electron.Charge up the oil particleswith electrons.Change the electric fieldchanges the rate of oildroplets!

Charge of electron 1.602 x 10-19 coulombs Mass of electron = 9.1 x 10-28 grams

Plum Pudding Model• Matter isn’t all negatively charged, so how do

we have negatively charged subatomic particles without positively charged ones??

• J.J. Thompson thought an atom was a positively charged sphere with electrons hanging out within.

Rutherford and the Nucleus• Experiment proved that plum pudding model was

incorrect! • Atom is mostly emptyspace through which e- can move. Almost all ofthe positive charge and atomic mass resides in the center – NUCLEUS!

Nucleus is positively charged to deflect alpha particles and to balance electron charge.

Subatomic Particles

• Electron – VERY tiny, negatively charged

• Proton – located in the nucleus, charge opposite of an electron (positive!)

• Neutron – located in the nucleus, same mass as a proton, neutral!

Warm – Up!

• What experiment determined the mass and charge of an electron?

• Dalton concluded that the atom was the smallest particle of matter. Was he correct?

• What did the gold foil experiment prove?

Atomic Theory Today• Quantum Mechanical Model• All atoms are made up of electrons, protons, and

neutrons. Electrons are located outside of the nucleus, protons and neutrons are located inside the nucleus.

• Electrons exist in a cloud surrounding the nucleus. Attracted to the nucleus so they hang around!

• Nucleus accounts for 99.97% of the atomic mass, and occupies a VERY small volume.

• A neutral atom has the same number of electrons and protons!

CurrentAtomic Model

Neutral atom:# Protons = # Electrons

Simulation!!

Make sure you can answer…

• What are John Dalton’s 4 theorems• How does John Dalton’s theory relate to

conservation of mass?• How was the electron discovered?• Who discovered the mass of an e-? HOW?• What was Rutherford’s contribution?• Describe the structure of the atom.

Properties of Atoms

Atomic ## of Protons = # of Electrons (in neutral atom)

Atomic massa weighted average

Practice

What is the isotopic symbol for each?

Isotopes and Ions• Isotope –

• Things to remember – – The # of protons of an element NEVER changes,

and is ALWAYS the same as the Atomic #. – If the # neutrons is different = ISOTOPE– If the # electrons is different = ION

• + = cation Less electrons• - = anion More electrons

Atoms with the same number of protons but different number of neutrons.

Isotopic symbol :32

70Ge

Mass of Atoms

• Mass of electron = 1/1840th of a proton• Mass of proton ≈ mass of neutron• 1 atomic mass unit (amu) ≈ mass of proton

Carbon 12 atom = 12 amu

Why aren’t the masses of elements in whole numbers?

Atomic Mass = Average of Isotopes

• Weighted average mass – mass of each isotope contributes to total mass according to how much of that isotope exists.

K

Three isotopes = 39K 40K 41K

Potassium

19 19 19

93.26% 0.01% 6.73%Percent Composition:

Calculate the Atomic Mass of K

1. Use % composition and convert to relative abundance (divide by 100)

93.26% composition = 0.9326 relative abundance

2. Amu = ((Mass of Isotope1)x(Relative Abundance1)) + ((Mass of Isotope2)x(Relative Abundance2))…

((0.9326)x(39)) + ((0.0001)x(40)) + ((0.0673)x(41)) =39.1347 amu

Warm Up!Element Atomic # Mass #Calcium 20 46Oxygen 8 17Mercury 80 204

What is the number of protons, electrons, and neutrons for each?

What is the isotope symbol (shorthand notation) for each?

Agenda• Question for today: What does radioactive

mean and what makes certain atoms radioactive?

• Isotope calcs• Radioactive particles• Decay practice

Amu = (R.A.)x(Mass) + ((R.A.)x(Mass))…• What element is this? Isotope Mass of Isotope Percent abundance 6X 6.015 amu 7.59%

7X 7.016 amu 92.41%Find the atomic massWhat element is this? (Use the Periodic Table)

• Boron has two isotopes: Boron-10 (% abundance – 19.8%, mass = 10.013 amu) and Boron-11 (% abundance – 80.2%, mass – 11.009 amu). Calculate the atomic mass of Boron.

• Bromine has two isotopes with the first having a mass of 78.918336 amu and occupying 50.69% and the second isotope having a mass of 80.916289 amu and occupying 49.31%. What is the average atomic mass of bromine?

• Verify the atomic mass of Magnesium:24Mg = 23.985042 amu and percent abundance of

78.99% , 25Mg = 24.985837 amu and percent abundance of 10.00%, 26Mg = 25.982593 amu and percent abundance of 11.01%.

One more…

• Copper has two naturally occurring isotopes, Cu-63 and Cu-65. The atomic mass of Cu is 63.55 amu. Calculate the percent abundances of the two isotopes.

Radioactivity – emit radiation

• Nuclear reactions – change an element into a new element!! Lots of energy involved!– Unlike a chemical reaction because we are doing

more than rearranging – we CHANGE the identity.– Change in the atom’s nucleus.

• UNSTABLE nuclei are unhappy and lose energy by emitting radiation – radioactive decay.

• They form STABLE atoms of a different element.

Radioisotopes

• Isotopes of atoms with unstable nuclei.

• Undergo radioactive decay to attain stability. Emit 3 types of radiation

– alpha, – beta, – gamma,

What are the charges on radioactive particles?

Types of Radiation

• Alpha radiation – (remember the gold foil experiment?!?!) made up of POSITIVE “alpha particles”.

• 2 protons and two neutrons (no electrons!)

4He2+ or 2

Alpha decay

U238

92 Th234

90He4

2+

Ra226

88 Rn222

86He4

2+

Cm247

96 Pu243

94He4

2+

Types of Radiation

• Beta radiation – negatively charged beta particles

• Unstable neutron turns into a proton and ejects 1 electron

e- or

Types of Radiation• Gamma radiation – emits gamma rays, high

energy photon that has no mass nor charge.

• Gamma rays almost always accompany alpha and beta radiation and account for the energy lost in the nucleus.

Usually omitted from nuclear equations.

U238

92 Th234

90He4

2+ + 2

Penetrating Power of Radiation

Penetrating Power

Alpha particles most mass and charge. 4He2+

Beta particles less mass (only the mass of an electron) and a neg charge.

Gamma rays have no mass and no charge.

2

Isotopic mass

Least

Most

In the Nucleus• Radioactive decay – transmutation

– Atomic # is altered = identity of element changed

Nucleons

Strong nuclear force between all nucleons. Repulsive force between 2 protons (electrostatic). Neutron attraction have to overcome the repulsive forces – as atomic # increases we need more neutrons to stabilize the nucleus!!!

+ +

+

Low atomic #’s have a 1:1 neutron to proton ratio

4He

High atomic #’s are stabilized by a 1.5:1 ratio 200Hg

If atom is not in band (belt) of stability it undergoes radioactive decay to get there!

2

80

Decay Practice

4He2238Pu94

+ 234U92decay

Thorium-229 is used to increase the lifetime of fluorescent bulbs. What type of decay occurs when thorium-229 decays to form radium-225?Write out the nuclear equation.

Write a balanced nuclear equation for the decay shown on the right. Identify A and B

AB

Bismuth -212

4He2

229Th90

+ 225Ra88

4He2

212Bi83

+ 208Tl81

208Tl81

+ 208Pb82

A

B

Warm – Up!!• What is the band of stability and how does it relate

to the proton to neutron ratio?• How does the neutron to proton ratio change

when polonium-210 decays into lead-206? What type of decay does polonium-210 undergo?

(Low atomic # elements are happy with a 1:1 ratio of neutrons to protons. Heavier elements need a 1.5:1 ratio and all elements above 82 are radioactive.)

Half Life

• Time required for one half of the nuclei to decay into its products.

• Strontium-90 half life is 29 years.If you had 10 g now, in 29 years you would have

5g.

Half Life CalculationsN = N0 (½)n

N – remaining amount of elementN0 – initial amount of element

n – number of half lives that have passed

Kr-85 has a half life of 11 years. Kr is used in indicator lights of appliances. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left?

N = ?N0 = 2 mg

n = 33 years/11 years (years that have passed/half life)

Kr-85 has a half life of 11 years. Kr is used in indicator lights of appliances. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left?

N = 2.0 mg (½)(33/11)

N = 2.0 mg (½)3

N = 2.0 mg (⅛) N = 0.25 mg left after 33 years

Half Life Practice

• The half life of Ra-222 is 3.8 days. How much is left of a 10 mg sample after 15.2 days?

N = N0 (½)n

N = 10mg (½)(15.2/3.8)

N = 10mg (½)4

N = 10mg (1/16)N = 0.625mg

Half Life PracticeBandages can be sterilized by exposure to gamma radiation from cobalt-60, which has a half life of 5.27 years. How much of a 10 mg sample of cobalt-60 is left over after 10.54 years? After four half lives?

N = N0 (½)n

N = 10 mg (½)10.54/5.27

N = 10 mg (½)4

Half – Life Calculations

• Do the problem intuitively…

Think about how many half lives have passed and just do the division

Two half lives (10 mg/2)/2 = 2.5 mg

Four half lives 10 mg/2/2/2/2 = 0.625 mg

Warm - Up!• Americium-241 has a half life of 430 years. How

much of a 15 mg sample is left after 2150 years?

• A radioisotope has a half life of 197 years. How much remains of a 2.0 g sample after 10 years?

• Strontium has a half life of 29 years. How long will it take for a 56 g sample to decay to 1.75 g?

Nuclear Reactions• Induced Transmutation – FORCE an element

to change its identity by bombarding it with radioactive particles!

• Particle accelerators move particles at extremely high speeds to overcome repulsive forces.

4He2

14N7

+ 17O +8

1H1

Nuclear Fission

• Splitting of a nucleus into fragments - LARGE release of energy!

• Large atoms want to be smaller for stability (Atomic # larger than 60 = large atoms)

neutron

U-235 U-236

Kr - 92

Ba - 141

Neutrons Perpetuate Fission

Critical Mass

• Fissionable material must have sufficient mass to split– Not massive enough – subcritical – no chain reaction– Extremely massive – violent nuclear reaction

Violent NuclearReactions

Subcritical masses that gettogether to form supercriticalmass.

Equal to 20,000 tons of TNT

Nuclear Reactors

Reactor core controlled by Cd or B to absorb neutrons

Assignment : Look up Chernobyl Nuclear Power Plant!

Nuclear Fusion

• Bind low atomic mass (less than 60 because 60 is ideal) to form more stable atom.

• Combination of nuclei called fusion. • How the sun works…

• Need very high energy to initiate and sustain.

41H1 2+ energy + 4He2

Nuclear waste

• What happens to all the radioactive waste accumulated at a power plant?

• Treated with advanced technologies so the material doesn’t deteriorate

• Sealed and buried underground

• Describe the process that occurs during a nuclear chain reaction and explain how to monitor a chain reaction in a nuclear reactor.

True/False• Great amounts of energy can be liberated from small

amounts of matter in a nuclear reaction.• The amount of U-235 in a nuclear reactor should

always be kept subcritical. • Nuclear power plants do not contribute to air

pollution.• Nuclear power use is dangerous because plants are

commonly are out of control.

Warm UpIsotope Percent Abundance Mass (amu)

X-50 4.35 49.946

X-52 83.79 51.941

X-53 9.50 52.941

X-54 2.36 53.939

Find the atomic mass and identify the element.

How many electrons, and protons does this element have?

How many electrons protons and neutrons does 52X2+ have? Is it in cationicor anionic form?

Warm – Up!

• What is an isotope?

• What is a radioactive isotope?

• What does it mean for something to be radioactive?

• What are the three types of radioactive particles?

• What are the charges on those particles?

• Complete the following nuclear reaction (Remember that = e-)

Warm Up!

Cs137

55 Ba137

56 +

Warm – Up!!

• What happens to the atomic mass number and the atomic number of a radioisotope when it undergoes alpha emission?

• High speed electrons emitted by an unstable nucleus are ________ particles.

• What isotope of what element is produced if krypton-81 undergoes beta decay?

Write out the nuclear reaction.

Agenda:Nuclear FissionNuclear Power Plants

Homework:# 34 – 79 odds pg 894 due ThursdayTest Thursday

Today’s Question

What are the main components of a nuclear power plant?

How does a power plant control the chain reaction?

Nuclear Fission

• The splitting of an atom into parts

Uranium-235 when hit with a neutron at high velocity breaks apart into 2 new elements as well as additional neutrons.

Those neutrons go on to strike other U-235 atoms and cause a chain reaction

Fission reactions produce a ton of energy. If a chain reaction is uncontrolled the consequences can be extreme

Nuclear Reactors

• Nuclear fission produces the energy generated in a nuclear power plant

• Uses fuel rods made out of enriched uranium• Control rods made out of other metals are

used to control the chain reaction by absorbing extra neutrons

• Heat generated from fission boils water and the steam spins a turbine which produces electricity

Nuclear waste

• What happens to all the radioactive waste accumulated at a power plant?

• Treated with advanced technologies so the material doesn’t deteriorate

• Sealed and buried underground

• What happens when we run out of U-235?• Breeder Reactors-cost inefficient-technically challenging

Nuclear Fusion

• Process by which 2 or more atomic nuclei join together to form a single heavier nucleus

• Generally produces large quantities of energy• Stars• Hydrogen bomb• Possible answer to energy source with the

least amount of waste products

Nuclear Weapons

• 2 types Fission based and Fission/Fusion Based

• Limited development by a few developed countries globally

Fission Weapons

• Chemical explosives start a chain reaction

Fission/Fusion Weapons

• Fission reaction acts as a trigger for the fusion reaction