Chapter 23Nuclear Chemistry

Objectives:1)Determine the half-life of a radioactive isotope from a decay graph.2)Recall the different radiation types and their penetration abilities.3)Carry out balancing of nuclear reactions.4)Interpret the N/Z ratio importance and how an atom might decay to reach a stable configuration from the band of stability chart.5)Determine the age of a sample given the half-life and the activity % of an isotope.

Atomic CompositionAtomic Composition

• Protons– + electrical charge

– mass = 1.672623 x 10-24 g

– relative mass = 1.007 atomic mass units (amu)

• Electrons– negative electrical charge

– relative mass = 0.0005 amu

• Neutrons– no electrical charge

– mass = 1.009 amu

IsotopesIsotopes

• Atoms of the same element (same Z) but different mass number (A).

• Boron-10 (10B) has 5 p and 5 n: 105B

• Boron-11 (11B) has 5 p and 6 n: 115B

10B

11B

RadioactivityRadioactivity

• One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of ______________(1876-1934).

• She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

• Ernest Rutherford found Ra forms Rn gas when emitting an alpha particle.

• 1902—Rutherford and Soddy proposed radioactivity is the result of the natural change of the isotope of one element into an isotope of a different element.

Types of radiationTypes of radiation

Penetrating abilityPenetrating ability

Nuclear ReactionsNuclear Reactions

• Alpha emission

Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.

Nucleons are rearranged but conserved

Nuclear ReactionsNuclear Reactions• Beta emission

Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

How does this happen?

Radioactive Decay SeriesRadioactive Decay SeriesA radioactive isotope (parent) is found to decay to form a product that is also radiactive (daughter).

Other types of nuclear reactionsOther types of nuclear reactions

Positron (0+1): a positive electron

An electron and proton combine to form a neutron.0

-1e + 11p --> 1

0n

207 207

K-capture: the capture of an electron from the first (inermost) or K shell

Origin of the ElementsOrigin of the Elements• The Big Bang Theory • In the first moments therewere only 2 elements:_______________ and __________________.

H is 88.6% of all atomsHe is 11.3% of all atomsH + He = 99.9% of all atom & 99% of mass of the universe.

This tells us about the origin of the elements, and so does the existence of isotopes.

Stability of NucleiStability of Nuclei

• Very few isotopes are stable.

• Up to calcium (Z=20), stable isotopes often have equal numbers of _____________ and _______________.

• As the mass increases, elements have larger number of ___________.

• Beyond bismuth (Z=83 with 126 neutrons) all isotopes are unstable and __________________.

IsotopesIsotopes

• Hydrogen: – 1

1H, protium

– 21H, deuterium

– 31H, tritium

(radioactive)

• Helium, 42He

• Lithium, 63Li and 7

3Li

• Boron, 105B and 11

5B

• Iron

– 5426Fe, 5.82% abundant

– 5626Fe, 91.66% abundant

– 5726Fe, 2.19% abundant

– 5826Fe, 0.33% abundant

IsotopesIsotopes

• Except for 11H the mass number is always at least 2 x

atomic number.

• Repulsive forces between protons must be moderated

by ____________.• Heaviest naturally occurring non-radioactive isotope is

209Bi with _________ protons and _________neutrons• There are 83 x 126 = 10,458 possible isotopes. Why

so few actually exist?

IsotopesIsotopes

• Up to Z = 20 (Ca), n = p (except for 73Li, 11

5B, 199F)

• Beyond Ca, n > p (A > 2 Z)

• Above Bi all isotopes are radioactive. Fission leads to

smaller particles, the heavier the nucleus the greater the

rate.

• Above Ca: elements of EVEN Z have more isotopes

and most stable isotope has EVEN N.

Stability of NucleiStability of Nuclei

• Out of > 300 stable isotopes:

EvenEven OddOdd

OddOdd

EvenEven

ZZNN

157157 5252

5050 55

31311515PP

191999FF

2211H, H, 66

33Li, Li, 101055B, B, 1414

77N, N, 1801807373TaTa

Stability of NucleiStability of Nuclei

EvenEven OddOdd

OddOdd

EvenEven

ZZ NN

157157 5252

5050 55

• Suggests some PAIRING of NUCLEONS• There are “nuclear magic numbers”

2 He 28 Ni8 O 50 Sn20 Ca 82 Pb

24395Am --> 4

2 + 23993Np

emission reduces Z

emission increases Z

6027Co --> 0

-1 + 6028Ni

Isotopes with low n/p ratio, below band of stability decay, decay by _____________________or ______________________

Band of Stability and Radioctive Band of Stability and Radioctive DecayDecay

Binding EnergyBinding Energy

Eb is the energy required to separate the nucleus of an atom

into _____________ and _____________.

For deuterium, 21H ---> 1

1p + 10n

Eb per nucleon = Eb/2 nucleons

Mass of 21H = 2.01410 g/mol

Mass of proton = 1.007825 g/mol

Mass of neutron = 1.008665 g/mol

∆m = 0.00239 g/mol

From Einstein’s equation: Eb = (∆m)c2 = 2.15 x 108 kJ/mol

Eb per nucleon = Eb/2 nucleons = 1.08 x 108 kJ/mol

Binding EnergyBinding Energy

Half-LifeHalf-Life

• HALF-LIFE is the time it takes for _______________________________________.• The rate of a nuclear transformation depends only on the “reactant” concentration.• Concept of HALF-LIFE is especially useful for 1st order reactions.

Half-LifeHalf-Life

Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?

Do: mass * (t1/2)^number of half-lives

KineticsKinetics

Activity (A) = Disintegrations/time = (k)(N)

where N is the number of atoms

Decay is first order, and so

ln (A/Ao) = -kt

The half-life of

radioactive decay is

t1/2 = 0.693/k

Copper acetate containing Copper acetate containing 6464Cu is used to study brain tumors. Cu is used to study brain tumors. This isotope has a half-life of 12.7 h. If you begin with 25.0 mg This isotope has a half-life of 12.7 h. If you begin with 25.0 mg

of of 6464Cu , what mass in micrograms remains after 64 h?Cu , what mass in micrograms remains after 64 h?

Radiocarbon datingRadiocarbon dating• Radioactive C-14 is formed in the upper atmosphere by

nuclear reactions initiated by neutrons in cosmic radiation14N + 1

on ---> 14C + 1H• The C-14 is oxidized to CO2, which circulates through the

biosphere.• When a plant dies, the C-14 is not replenished.• But the C-14 continues to decay with t1/2 = 5730 years.• Activity of a sample can be used to date the sample.

A piece of charred bone found in the ruins of a Native A piece of charred bone found in the ruins of a Native American village has a American village has a 1414C:C:1212C ratio that is 72% of the radio C ratio that is 72% of the radio

found in living organisms. Calculate the age of the bone found in living organisms. Calculate the age of the bone fragment.fragment.

Artificial Nuclear ReactionsArtificial Nuclear Reactions• New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B.• Reactions using neutrons are called n, reactions because a ray is usually emitted.• Radioisotopes used in medicine are often made by n, reactions.• Example of a n, reaction is production of radioactive 31P for use in studies of P uptake in the body.

3115P + 1

0n ---> 3215P +

Balance the equations below (both mass and Balance the equations below (both mass and charge to be conserved in a nuclear charge to be conserved in a nuclear

reaction): reaction):

+ +

+ +

+ + + 2

Transuranium ElementsTransuranium Elements

Elements beyond 92Elements beyond 92(transuranium)(transuranium) made starting made startingwith an with an n,n, reaction reaction

2382389292U + U + 11

00n ---> n ---> 2392399292U + U +

2392399292U U ---> ---> 239239

9393Np + Np + 00-1-1

2392399393Np Np ---> ---> 239239

9494Np + Np + 00-1-1

106106SgSg

Nuclear FissionNuclear FissionFission chain has three general steps:1. Initiation. Reaction of a single

atom starts the chain (e.g., 235U + neutron)

2. Propagation. 236U fission releases neutrons that initiate other fissions

3. Termination.

109109MtMt

Nuclear Fission and PowerNuclear Fission and Power

• Currently about 103 nuclear

power plants in the U.S. and

about 435 worldwide.

• 17% of the world’s energy

comes from nuclear.

Radiation UnitsRadiation UnitsCurie: 1 Ci = ___________ distintegrations/s

SI unit is the becquerel: 1 Bq = __________

Rad: measures amount of energy absorbed

1 rad = __________J absorbed/kg tissue

Rem: based on rad and type of radiation. Quantifies

biological tissue damage

Usually use “millirem”

Effects of RadiationEffects of Radiation

Nuclear Medicine: ImagingNuclear Medicine: Imaging

Technetium-99m is used in more than 85% of the diagnostic scans done in

hospitals each year. Synthesized on-site from ___________.99

42Mo ---> 99m43Tc + 0

-199m

43Tc decays to 9943Tc giving off ray.

Tc-99m contributes in sites of high activity.

Nuclear Medicine: ImagingNuclear Medicine: Imaging

Imaging of a heart using Tc-99m before and after exercise.

BNCT: Boron Neutron Caption BNCT: Boron Neutron Caption TherapyTherapy• 10B isotope (not 11B) has the ability to

capture slow neutrons• In BNCT, tumor cells preferentially

take up a boron compound, and subsequent irradiation by slow neutrons kills the cells via the energetic 10B --> 7Li neutron capture reaction (that produces a photon and an alpha particle)

• 10B + 1n ---> 7Li + 4He + photon

Food IrradiationFood Irradiation

• Food can be irradiated with rays from 60Co or 137Cs.• Irradiated milk has a shelf life of 3 mo. without

refrigeration.• USDA has approved irradiation of meats and eggs.

End of ChapterEnd of Chapter

• Go over all the contents of your textbook.

• Practice with examples and with problems at the end of the chapter.

• Practice with OWL tutor.• Practice with the quiz on CD of

Chemistry Now.• Work on your OWL assignment for

Chapter 23.