last updated: 10/23/2015CHE 140 Packet 4 - 1 Packet 4 Nuclear Chemistry

last updated: 04/21/23 CHE 140 Packet 4 - 1

Packet 4

Nuclear Chemistry

Concept Area I: Terminology

CHE 140 Packet 4 - 2

Nuclear Chemistry Handout

Concept Area II: Different Types of Radiation

a. You should be able to identify the different main types of radioactivity: α, β, β+, and γ.

b. You should be able to identify the relative ease of penetration of the main forms of radioactivity.

c. You should be able to identify the danger to living tissue for the main forms of radioactivity when outside or inside the tissue.



First, Radiation is a Natural Phenomenon; however, humans can also create it.


How much radiation are we exposed to on

average?Radiation exposure

depends on where we live, what we do, and what medical procedures we have done.

Let’s look at two other books’ tables on annual exposure…

+135

similar table Timberlake page 141


Sources of radiation we’re exposed to:

+292

+67


Sources of radiation

we’re exposed to continued:


Risk of Radon Exposure


So, what are the main kinds of radiation?• alpha, α or :

• beta, β or :

• positron, β+ or :

• gamma, γ or :

He42

e01

e01

γ00


Positively charged alpha particles Positively charged alpha particles and positrons are attracted to the and positrons are attracted to the negative pole of a magnet.negative pole of a magnet.

Negatively charged beta particles Negatively charged beta particles are attracted to the positive pole are attracted to the positive pole of a magnet.of a magnet.

Uncharged gamma rays Uncharged gamma rays are neither attracted nor are neither attracted nor repelled by a magnet.repelled by a magnet.

Positively charged positrons Positively charged positrons would be attracted to the would be attracted to the negative pole of a magnet.negative pole of a magnet.

CHE 140 Packet 4 - 11Timberlake page 130 & 131

(g/particle)

Actual Mass

6.65×10–24

9.11×10–28

9.11×10–28

0

1.67×10–24

1.67×10–24

Penetrating power of radioactive particles:


The penetrating power of radioactive particles can be measured in terms of what depth of water it takes to stop 50% of the radiation.

First, why water?

Water is the main component of living tissue!

Now, the penetrations:

Alpha radiation has the highest mass and charge and thus the lowest penetration – about 0.03 mm.

Beta radiation penetrates about 2 mm.

Gamma rays have the lowest mass and thus the highest penetration at about 10 cm.

.right image Timberlake page 131

What can shield us?


Alpha radiation is the _________, so it can be stopped by the __________ barrier.

Beta and positron radiation are the ______________ ones. They can be stopped by a block of wood, thin sheets of metal.

Gamma radiation is the __________. It’s actually a form of ________. It needs several centimeters of lead to be stopped.


So which form of radiation is the most dangerous…

• outside the human body• What type of radiation is worst to sit beside?

• Why?

• inside the human body• What type of radiation is worst to ingest?

• Why?


Note about cancer radiation treatment: The radiation used in cancer treatment is given in units of mCi (millicuries) which are not easily converted to rems. However, thyroid cancer is given the highest dose amount - as high as 75 mCi – if over 50 mCi are given the person must stay in the hospital to limit others exposure to radiation. Most other cancers seem to use less than 10 mCi per dose and usually there are few if no precautions for exposing others to the radiation used for treatment.


So, how is radiation measured?

• curies, Ci – indicate that 3.7×1010 disintegrations per second have occurred

• becquerel, Bq – indicate that one disintegration per second has occurred (SI unit)

• rad – stands for radiation absorbed dose, and indicates how much radiation one gram of material has absorbed

• rem – stands for radiation equivalent in humans, and shows the true amount of radiation absorbed since alpha, beta and gamma radiation are absorbed differently


How do we detect radiation?Or, how a Geiger counter works:

The radiation ionizes the gas inside the detector. The electrons released from the ionization are detected by the voltmeter. We hear the detections as clicks. A Geiger counter only tells us radiation of some form is present. It can not differentiate between the different types of radiation.


Concept Area III: Nuclear Reactions (Alchemy!)

a. You should understand what determines the stability of the nucleus, binding energy, mass defect, energy released in nuclear reactions.

b. You should be able to explain what happens to the nucleus with the loss of subatomic particles.

c. You should be able to complete nuclear reactions.d. You should be able to explain the difference

between nuclear fission and fusion.e. You should be able to identify particles used in

artificial transmutation and explain what each does.f. You should understand what a nuclear chain

reaction is.


Comparison of Chemical and Nuclear ReactionsChemical Reactions

• One substance is converted into another, but atoms never change identity (transmutate).

• Only valence electrons are involved as bonds break and form.

• Reactions are accompanied by relatively small changes in energy and no measurable changes in mass.

• Reaction rates are influenced by temperature, concentration, catalysts, and the compound in which an element occurs.

Nuclear Reactions• Atoms of one element typically

are transmutated into atoms of another element.

• Protons, neutrons, and other particles are involved; valence electrons rarely take part.

• Reactions are accompanied by relatively large changes in energy and measurable changes in mass.

• Reaction rates are affected by number of nuclei, but not by temperature, catalysts, or normally the compound in which an element occurs.


Nuc

lear

Dec

ay S

erie

s


Spontaneous Fission…• We know according to the Rutherford’s nuclear

atom view, all the protons and neutrons are at the center of the atom.

• We also know that like charges (like protons) repel each other.• So why do only some atoms (usually the big ones)

spontaneous “fall apart” into other atoms? • Why don’t all atoms (like the small ones)

fall apart too?


Why does the nucleus of any atom stay together and not fly apart?


Okay, so if nucleons like each other, why is atomic size limited?

• The attractive strong nuclear force is only good for adjacent (touching) nucleons.

• The repulsive electrostatic force works over great distances!

• So, the strong nuclear force can only compensate for (or overcome) the electric force for smaller atoms.


Stability of Atomic Nuclei

In this chart, each circle is a known isotope. The blue and green circles are stable or non-radioactive isotopes; the red circles are radioactive isotopes – they spontaneous “fall apart”.

Notice very few of the known isotopes are stable!


Nuclear Binding Energy• An atomic nucleus can contain as many as 83 protons and still be

stable – beyond 83, all atoms thus far discovered are unstable.• For stability, the nuclear binding force must be greater than the

electrostatic force. This can be determined by the nuclear binding energy, Eb.

• The nuclear binding energy, is defined as the amount of energy needed to separate the nucleus of an atom into its protons and neutrons. • If Eb is positive, energy is needed to separate.• If Eb is negative, energy is released when separated – thus

spontaneous – thus is happens readily, and we get radioactive atoms!• We also need to consider mass defect. The mass of a nucleus is

always less than the sum of the masses of its protons and neutrons because the “missing mass” has been converted into energy that holds the nucleons together!


Note that the nucleons for 56Fe have the least mass because 56Fe has the most binding energy keeping it together; thus it is the most stable atom. The more energy used to hold the nucleons together, the greater the mass defect in the nucleons.

Since there are isotopes to either side of our most stable nucleus, this implies that there are two types of nuclear reactions: fusion and fission.


This chart shows the binding energy versus atomic mass.

In fusion, nucleon mass is usually lost as isotopes “fuse” together because more energy is usually needed to keep the nucleons together in the larger isotopes.


This chart shows the binding energy versus atomic mass as well – a few isotopes are labeled.

In fission, nucleon mass is changed as isotopes split apart. They gain mass if less binding energy is needed and lose mass if more binding energy is needed.

Scientists predict that there will be islands of stability in the larger atomic numbers – we just haven’t found where these islands are yet.

Balancing Nuclear Reactions

In a balanced nuclear equation, the

sum of the mass numbers of the reactant and the products are equal.

sum of the atomic numbers of the reactant and the products are equal.

and the atomic symbol must match the atomic number!

For example:


HeCmCf 42

24796

25198


Types of Natural Nuclear Reactions• alpha emission or alpha decay

• First recognized in 1903 by Rutherford and Soddy.

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

• Nucleons are rearranged but conserved.

• Called alpha emission/decay because the atomic isotope transmutates to another atomic isotope by emitting an alpha particle.


Alpha Decay

Timberlake page 132


Complete the following alpha decay nuclear equations:

1.

2.

____HeU 42

23692

PbHe____ 21482

42

Timberlake page 133


Types of Natural Nuclear Reactions• beta emission or beta decay

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

• How does this happen?

• Called beta decay because the atomic isotope transmutates to another atomic isotope by emitting an beta particle (an electron from the nucleus!).


Beta Decay

updated version on Timberlake page 134


Complete the following beta decay nuclear equations:

1.

2.

____I 01-

13153 e

He____ 32

01- e

Timberlake page 133


Other Types of Nuclear ReactionsPositron Emission: a positive electron

K-capture/e– capture : the capture of an electron from the first shell of electrons (used to be called the K shell)

207 207

This occurs because an electron and proton combine to form a neutron. -1e +

1p → 0n

0 1 1


Gamma emitter: only gamma radiation is emitted. Rare to occur without alpha or beta radiation emitted also.

The “m” after the 99 in the reactants means that the Tc is in an excited metastable state.

Other Types of Nuclear Reactions

γTcTc 9943

99m43

Thyroid imaging using 99mTc

← asymmetric scan

indicates disease

← normal

other PET images on page 147 Timberlake


Summary of Nuclear

Reactions:

right image Timberlake page 137, left image from previous edition


Complete the following nuclear reactions:1.

2.

3.

4.

____ Ar 01-

3718 e

B____ 115

01

____ClS 3517

3516

Si____ 3014

01


Artificial Nuclear Reactions• Rutherford identified the first artificial

nuclear reaction with alpha particles (remember the gold foil?). In 1919, he noticed the following reaction:

• Later Cockcroft and Walton used artificially accelerated protons to cause nuclear reactions:

HONHe 11

178

147

42

He2pLi 42

11

73


• In 1934, Fermi was able to use low energy neutrons to transmutate elements:

• Many of the elements greater than 92 began with the above reaction to form various isotopes that then naturally decayed to the new elements:

γOnP 3215

10

3115

e

e01

23994

23993

01

23993

23992

23992

10

23892

Pu Np

NpU

UnU


Nuclear Fission – splitting nuclei

• So far in our nuclear reactions, one element has transmutated into another element.

• However, sometimes we can split one atom into two new atoms – nuclear fission!

• In samples of uranium that had been bombarded with neutrons, barium was found. At first they weren’t sure why…


Nuclear Fission!

bottom image similar to Timberlake page 149


Nuclear fission chain reactions have three general steps:

Initiation: the reaction of a single atom is need to start the chain. Fission of 235U is initiated by the absorption of a neutron.

Propagation: This part of the process repeats itself over and over, each step producing more product. The fission of 236U releases neutrons that initiate the fission of other uranium atoms.

Termination: Eventually the chain will end. This could come about if the reactant is used up, or if the neutrons that continue the chain escape from the sample without being captured by 235U.

Timberlake page 150


Nuclear Fusion – fusing nuclei together • In fission we nuclei into multiple atoms.• In fusion we several atoms into one!• Our sun does fusion all the time.• The only way humans have been

able to accomplish is in high yield nuclear weapons where a fission reaction then triggers a fusion reaction.

• However, there is hope that this type of reaction could be used one day to make “clean” energy.

nHeHH 10

42

21

31

splitput together/fuse

Timberlake page 151 has reaction image


Summary• So what, besides atomic isotopes, can be

formed or released in nuclear reactions?

• What types of particles can we use to cause nuclear reactions?


Concept Area IV: Nuclear Calculations

a. You should be able to tell the amount of radioactive substance left using half-life information.


Radioactive Decay Rates• Radioactive elements are constantly decaying

into other elements.

• How long does it take?• Hmmm….. This sounds like, __________.• To find out we can calculate the half-life!

• A half-life is defined as the amount of time it takes half of the radioactive material to decay.


What is the half-life for 226Ra?


XeβI 13154

01

13153

• If we start with 20 g of 131I, how many grams would we have after 3 half-lives?

• How long is one half-life?

Timberlake page 142


14C DatingScientists use the amount of 14C

remaining in an organic sample to determine how long ago the sample died.

Once something dies, the amount of 14C should stay constant in the remains since the plant or animal is no longer interacting physically (by breathing or eating) with the environment.

See Timberlake page 156 Chemistry Link to the Environment for more


• Carbon-14 is formed in the upper atmosphere by nuclear reaction initiated by neutrons in cosmic radiation:

• Once carbon-14 is formed, it is oxidized to 14CO2. Thus, carbon-14 can now enter the natural carbon cycle on the planet.

How does 14C Form?


Does the amount of 14C remain constant? No!

Analyzing the amount of C-14 in each ring of trees, we can estimate the amount of carbon-14 around that year.

So, when we use carbon-14 dating, it is just an estimate of how old the item is!


How does 14C decay?

• The rate of decay of carbon-14 is constant.• The half-life is 5730 years.


Concept Area V: Applications

a. You should be familiar with how nuclear chemistry can be used.


Atomic energy is used all around us!


Generating Electrical Power

similar image Timberlake page 151


X-rays & CT-Scans!


Positron emission tomography (PET) from 15O is used here to image

brain activity.

normalAlzheimer’s

updated chart on Timberlake page 146; similar pictures on page 147


Radioactive isotopes are used for a wide range of medical applications!


Tracers• We can use radioactive isotopes of elements as tracers.• Thus we can follow where the compound goes – for

instance, how effectively is the fertilizer taken up by the plant?


Food Preservation

See Timberlake page 156 Radiation and Food for more


Alas, Nuclear Weapons


The End of Packet 4

Any Questions?

and the end of CHE 140!

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last updated: 10/23/2015CHE 140 Packet 4 - 1 Packet 4 Nuclear Chemistry