CHAPTER 4/25 Atomic Structure

Nuclear Chemistry

A Long, Long Time Ago…• Greek Philosophers 4 elements are Earth, Water, Fire, and Air

• Aristotle- first recorded atomic thoughtsMatter is continuous (no atoms)

• Democritus (400 B.C.) - first recorded atomic theoryAtoms are smallest part of matter, each type of matter has different atoms

A Long Time Ago…

• Antoine Lavoisier (1782)Used experiments in closed containers to develop the Law of Conservation of Matter

• Joseph Proust (1799)Analyzed water to develop the Law of Definite

Proportions

• John Dalton (1803)Compiled past research to develop the first useful atomic theory

John Dalton

Dalton’s theory had 4 major tenets

1. All matter is composed of atoms

2. Atoms are indivisible

3. Atoms of 1 element are alike, but different from those of other elements

4. Atoms combine in small, whole number ratios to form compounds

A Little While Ago…

J.J. Thomson (1897) discovers the electron using a cathode ray tube

Proposes the “Plum Pudding” Model of the atom

Cathode Ray Tube Experimenthttp://www.aip.org/history/electron/jjappara.htm

The Nucleus Exists!

Ernest Rutherford (1911)

uses the gold foil experiment to “discover” the nucleus

The Gold Foil Experiment

Results of Gold Foil Experiment…Over 98% of the particles went straight through

About 2% of the particles went through but were deflected by large angles

About 0.01% of the particles bounced off the gold foil

About that 0.01% of particles…..

…" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." E. Rutheford

Rutherford’s Conclusion

Rutherford's Nuclear Model

1. The atom contains a tiny dense center called the nucleus the volume is about 1/10 trillionth the volume of the atom

2. The nucleus is essentially the entire mass of the atom

3. The nucleus is positively chargedthe amount of positive charge of the nucleus balances the negative charge of the electrons

4. The electrons move around in the empty space of the atom surrounding the nucleus

Coworker James Chadwick later adds neutrons

Properties of Subatomic Particles

Particle Symbol LocationRelativeElectricalCharge

Relative Mass

ActualMass (g)

Electron e- Space surrounding the nucleus

1- 1_ 1840

9.11 x 10-28

Protonp+

Nucleus1+ 1 1.637 x 10-24

Neutronn0

Nucleus0 1 1.675 x 10-24

Understanding Periodic Blocks• Atomic Number

• # of protons• # of electrons

• Element Symbol

• Atomic MassThe weighted average

atomic mass of isotopes of an atom

Mass Number #protons + # neutrons

Can 20.0983 Neutrons Exist???

No, 39.0983 is an average mass of all natural K atoms

• All K atoms MUST have 19 protons• Some have 20 neutrons, some 19, some 21…• Average is 20.0983 neutrons• Individual isotopes are identified by the number of neutrons

Identifying Protons, Neutrons, and Electrons

• Chlorine-35 (element-mass number)• # of protons = 17• # electrons = # protons• # neutrons = mass number – protons

• 35-17 = 18

• Why no electrons in atomic mass?• Electron has mass 1/1837 of Proton and Neutron

IsotopesNuclear Notation or Isotope Notation

# protons = ______

# neutrons = ______

# electrons = ______

Problems that can arise..

Charges and Ions – Only changes the number of electrons

Finding the mass number – use symbol, p+ and no or periodic table in that order!

Atomic Mass Units• Carbon-12

• 6 protons• 6 neutrons

atomic mass unit is equal to one-twelfth of the mass of the nucleus of a carbon-12 atom

Atomic Mass Problems

Boron has two naturally occurring isotopes: boron-10 (19.8%, 10.013 amu) and boron 11 (80.2%, 11.009 amu). What is the atomic mass of boron?

Find the Abundance x Mass for each isotope and then add products together.

0.198 x 10.013 = 1.98

0.802 x 11.009 = 8.83

1.000 10.81 amu

Chemical vs. Nuclear Change• Chemical Change- produces new kinds of matter with new

properties

• Involves breaking and forming BONDS• Accomplished by rearrangement of

ELECTRONS• Nuclear Change- produces a new nucleus that contains less

energy

• Involves emission or capture of nuclear particles

• Accomplished by changing PROTONS and NEUTRONS in nucleus

HighHighMedMedLowLowActual Actual HazardHazard

Low-MedLow-MedMedMedHighHighBiological Biological HazardHazard

LowLowMedMedHighHighEnergyEnergy

Hard (2 cm Hard (2 cm lead)lead)

Medium Medium (aluminum (aluminum

foil)foil)

Easy Easy

(skin/(skin/clothes)clothes)

ShieldingShielding

0000γγ00

-1-1ee4422HeHeSymbolSymbol

GAMMA (GAMMA (γγ))BETA (BETA (ββ))ALPHA (ALPHA (αα))

Other Radiation

• Ionizing Radiation- has significant energy to change atoms and molecules into ions

• Types: alpha, beta, gamma, x-rays

• Effects to living organisms: changes in, DNA (cell death/cancer)

• Nonionizing radiation- does not have significant energy to ionize atoms or molecules (types: microwaves, visible light, radiowaves)

Nuclear Stability(Electrostatic Force vs. Nuclear Force)

Electrostatic force arises from the interaction between two protons (repulsive force)

Nuclear force arises between protons and neutrons due to their close proximity to one another

Neutron to Proton Ration and the Band of Stability

As the atomic numberincreases, more and more neutrons are needed to create a strong nuclearforce to oppose and increasing electrostatic force

1:1

1.5:1

Nuclear Reactions

Nuclear Decay

Radioactive Decay Half-lives

• Half-life is the time taken for half of the atoms of a radio-active substance to decay.

Number of half-lives TE/t1/2

5 variables to consider1.Initial quantity2.Final quantity3.# of half-lives4.Value of a half life5.Time Elapsed

Sample Problem

A sample of radioactive iridium has a half life of 12 years. In 60 years, how much iridium would remain from a 50g sample?

32

1

50

Final

nFinal

2

1

50

Initial=

Final=

Time Elapsed=

# of half lives=

Value of 1 half life=

50g

12 years

60 years

?

n

Initial

Final

2

1

60/12 = 5

5

2

1

50

Finalgx

x

6.132/50

5032

Half-life

• Half-lives can range from a millionth of a second to millions of years

Radioactive Dating• Uses carbon-14 to tell age of fossils

• C-14 is present constantly in atmosphere• 15.3 decays/min in living organism• decays/min decreases by ½ every 5370 years an organism is

dead.• Only useful to 60,000 yrs ago

Transmutation• All nuclear reactions are:

transmutation reactionsoSome transmutation reactions are inducedoAll transuranium elements

(atomic #93 and greater) have

been produced through induced

transmutation.

Conversion of an element into anatom of another

element

Writing Induced Transmutation Reactions

Write the balanced nuclear equation for the induced transmutation of aluminum-27 into sodium-24 by neutron bombardment. An alpha particle is released in the reaction.

HeNanAl 42

2411

10

2713

nCmHePu 10

24296

42

23994

Write the balanced nuclear equation for the alpha particle bombardment of Pu-239. One of the reaction products is a neutron.

Nuclear Reactions for Energy

• Fission- nucleus broken into 2 smaller nuclei

• Fusion- smaller nuclei join to form a larger, more stable nuclei

Reactants and Products

Nuclear Power- generated by a controlled fission chain reaction

Control rods- absorb neutrons to slow the chain reaction

• Made of cadmium• Inserted or withdrawn

to keep temp of reaction steady

Moderators- slow neutrons down so they DO hit uranium fuel rods

• Made of water, beryllium, or graphite

• Intended to allow neutrons to be absorbed by uranium

Cooling and Shielding

• Water- acts as a coolant and transfers heat between reactor and turbines that produce electricity

• Steel & concrete- surround core and protect personnel by absorbing radiation