Nuclear Chemistry

Types of RadiationThere are four main types of ionizing radiation:

1.alpha rays: Helium nuclei - 2 protons + 2 neutrons2.positron rays: Positrons - the antimatter counterparts to electrons; same mass

but charge is +13.beta rays: Electrons4.gamma rays: High energy photons - ν ≥ 1020 Hz - higher energy than x-

rays

radiation symbol chargemass

(per particle)penetrating

power

alpha +2 6.65 × 10-24 g lowest

positron +1 9.11 × 10-28 g medium

beta -1 9.11 × 10-28 g medium

gamma 0 0 highest

42 He4

2or

e01

or01

00

e0-1

or0-1

Nuclear Reactionso Chemical reactions never alter the identity of the atoms involvedo In nuclear reactions, the total numbers of nucleons – protons, neutrons and electrons –

remains constant, but chemical identity can change when the particles interchangeo Nuclear reactions permit the transmutation of elements

Balancing Nuclear Reactions

(1) The sum of the mass numbers of reacting nuclei must equal the sum of the mass numbers of the nuclei produced (conservation of mass).

(2) To maintain charge balance, the sum of the atomic numbers of the products must equal the sum of the reactants

Example of Balancing a Nuclear Reaction

Mass number (protons+neutrons) 226 4 + 222Atomic number (protons only) 88 2 + 86

particleradium 226 radon 222

226 4 22288 2 86Ra He Rn

7 Classes of Nuclear Reaction: 1 - 2

1. Alpha emissionIn this spontaneous reaction an alpha particle is emitted.

2. Beta emission In this spontaneous reaction an beta particle (electron) is emitted.

This implies that the nucleons are transformed during the process, where one of the neutrons is converted into a proton and a high-energy electron is ejected:

Note that beta emission increases the atomic number by one.

234 4 23092 2 90U He Th

235 0 23592 1 93U Np

1 0 10 1 1n p

7 Classes of Nuclear Reaction: 3 - 43. Positron emission

This is the opposite of a beta emission where a new particle, called the positron is emitted.

Essentially, a proton decomposes to a neutron and a positron which is emitted by the nucleus. This transformation, at the nucleon level, implies the reaction occurs within the nucleus:

Positrons and electrons are antiparticles, meaning that If the two were to meet, they annihilate each other releasing two high-energy gamma ray photons.This reaction decreases the atomic number by 1.

4. Electron (K) captureAn electron from the atom is incorporated into the nucleus. The electron is from the 1s level also known by the older name of K-level. This spontaneous process is the opposite of beta emission, where a proton takes on an electron to form a neutron

This reaction gives same net product as positron emission, decreasing the atomic number by 1.

207 0 20784 1 83Po Bi

1 0 11 1 0p n

7 0 74 1 3Be e Li

7 Classes of Nuclear Reaction: 55. Nuclear fission

Nuclear fission is always an induced process, where a large nucleus splits into several smaller nuclei. The actual fission process below is the decay of uranium-236. 236U is made by neutron bombardment of 235U. :

Fission is accompanied by the release of a lot of energy, and this is the source of the power both of the atomic bomb and nuclear power

235 1 236 141 92 192 0 92 56 36 0U n U Ba Kr 3 n

7 Classes of Nuclear Reaction: 6 - 76. Nuclear fusion

Fusion is the combining of two light elements into a heavier nucleus. A lot of energy is released. (hydrogen bomb - where the hydrogen refers to the heavier hydrogen isotopes deuterium and tritium).

7. Nuclear transmutation The combination of heavier nuclei to produce artificial elements. All the elements beyond uranium are artificial, in the sense that none have stable isotopes.

This classification includes all the remaining artificial, i.e. induced, nuclear reactions

10 252 257 15 98 103 0B Cf Lr 5 n

2 3 4 11 1 2 0H + H He + n

Practice Question 1

o Complete or balance the following nuclear reaction equations by identifying the missing components. Classify each reaction according to one of the 7 classes of reaction introduced.

a)

b)

c)

d)

e)

f)

g)

239 23992 93U Np 0

1e emission

1 31 1 H He Nuclear fusion

35 32 417 15 2 Cl P

38 3819 18K Ar Positron emission

Nuclear fission

Electron capture

Nuclear transmutation

235 95 192 39 0U Y + +3 n

44 4422 21Ti+ Sc

246 254 196 102 0Cm No 4 n

Induced α emission, nuclear transmutation

21H

10n

01e

13753I

0-1e

126C

Example Problem

Gamma rays (and neutrinos)o Gamma rays accompany most nuclear reactions.

o When a nucleus emits an alpha particle, the resulting nucleus is usually in an unstable, higher energy form - Excited nuclear state

o It quickly decays to the nuclear ground state by releasing the excess energy as a gamma ray.

o The most dangerous part of a nuclear process, as gamma rays have high penetrating power.

o The energy of the gamma rays varies greatly, but is always characteristic of the type of nuclear transformation.

o Gamma spectroscopy analyzes the wavelengths of the emitted gamma rays, which can be used to identify the source of the nuclear reaction.

o Hence it is possible to detect the kind of radioactive process occurring when radioactivity is detected

Why are some nuclei stable, others not?o Black dots – stable nuclides

o Red dots - unstable nuclideso Pink region - all combinations of N (number of

neutrons) and Z (number of protons) that cannot exist

o Zone of black dots - belt of stability- neutrons are needed to stabilize protons (+ve repulsion)

o At low Z, N ≈ Z; at high Z, N > Zo Unstable nuclides surround the belt of stabilityo Z > N - Nuclides with more protons allowed

for stability, attain it, by changing the ratio of N/Z by: Heavy nuclei - alpha (α) emission Light nuclei- positron emission or

electron capture o N > Z -Nuclides with more neutrons than

protons convert neutrons into protons This is achieved by beta () emission

1N

Z

Binding energy

42He

Regionof

greateststability

5626 the most stable nuclide in the UniverseFe

Fusion zone

Susceptible to fission reactions

Binding energy is the energy change that occurs if a nucleus were formed directly from its component protons and neutrons. The source of the energy is a loss of mass

2E m c

All nuclei strive to maximize stability via nuclear reactions

Calculating binding energyo We use the tabulated masses of free protons and neutrons, and compare them to that of any

given nuclideo The difference in mass is converted to energy using the Einstein equation from special

relativity The mass of one mole of “free” protons is 1.0072765 g/mol The mass of one mole of “free” neutrons is 1.0086649 g/mol The mass of one mole of “free” electrons is 0.0005486 g/mol

gmolm 8 1.0072765 8 1.0086649 15.994916 8 0.0005468 0.136986

g2 8 2 10mol kJm

s molgkg

0.136986 1kJE m c (2.9979246 10 ) 1.2312 10

1000 1000J

10 kJ8mol kJ

molbinding

E 1.2312 10 E 7.70 10 of nucleons

16 16

proton neutron isotope electron

Example: Calculate the binding energy of the 16O isotope with mass of 15.994916 g/mol, and the binding energy per nucleon. Note that the isotope mass includes the electrons

Example – Binding Energy

Example 1

Given the mass of 55Mn (z = 25) is 54.9380 amu. Calculate the binding energy for a mole of nucleons

8.49*1011 J/mol

Range of binding energieso In the sample calculation on the previous slide, we calculated the binding energy of the 16O

nuclide and also the binding energy per nucleono A nucleon is either a proton or a neutrono It is found that the binding energies of all the known nuclides falls in the range from:

~ 0 – 9.0 108 kJ/mol of nucleon = 0 – 900 GJ/mol of nucleon

o The most stable nuclide is 56Feo Sometime values are given in MeV, which is a measurement per individual nucleono Chemists most often work out energies in units of kJ for mole quantities of matter, so that

we will always work out total and per-nucleon binding energies in kJ/molo However, the MeV unit is widely used in the nuclear industry. The conversion factor is:

1 eV = 96.485342 kJ/mol 1 MeV = 9.6485342 107 kJ/mol ≈ 108 kJ/mol = 100 GJ/mol

o The range of binding energies per nucleon in MeV is therefore:

~ 0 – 9.0 MeV

Nuclear decay is measured as the number of nuclei that disintegrate in a given period of time. This is termed the activity (A) of a sample and is directly proportional to the number of radioactive atoms (N) in the sample:

where k is the rate constant (decay constant).

By integrating this equation, we get a first-order rate equation:

where A0 and N0 are the initial activity and number of radioactive atoms while A and N are the activity and number of radioactive atoms after time ‘t’.

At t1/2, N = ½ N0 and A = ½ Ao. A relationship between k and t1/2 can be found as:

Rates of Nuclear Decay

NA kN

t

o

Aln k t

A

o

Nln k t

N

o1/ 2

o

A / 2 1ln k t ln 0.693

A 2

1/ 2

0.693t

k

Examples – Radioactive Decay

60Co is a -emitter used to treat cancer. Calculate the fraction of 60Co left after 20 years of preparation.

Example 1

7.79 %

Example 2

A 2.00 mg sample of pure 32P was prepared was found to contain 0.40 mg of 32P after 33.3 days. Determine t1/2.

14.3 days

Examples – Dating

Example 3 - Uranium Dating.

A sample of ore contains 5.20 mg of 238U and 1.85 mg 206Pb. Calculate the age of the rock. (t1/2 = 4.51*109 years)

t = 2.26*109 years

Example 4 - Radio Carbon Dating

A charcoal sample taken from Stonehenge has an activity of 9.65 events per minute per gram of carbon. Determine the age of the sample given that ambient activity of carbon is 15.3 min-1g-1 and t1/2 = 5730 years.

t = 3830 years

Nuclear Decay – The Uranium Series

All isotopes of uranium are unstable, but 238U decays extremely slowly.

This is the 238U decay series, showing the steps in the decay of this nucleus to eventually produce stable 206Pb

In this region there are several pathways, but all lead to the same species, 206Pb

Nucleosynthesis by transmutationo All the elements beyond 92U are synthetic. They are made either as a by product of operating

a nuclear reaction (neutron source) or by deliberate transmutationo Latest to be recognized: It was made by a nucleosynthesis reaction:

o Roentgenium is the most recent of the transuranium elements which start with 93Np, complete the actinides and most of the 6d series of the PT

o Early efforts at transmutation date back all the way to Rutherford. For example, he performed the reaction:

o Alpha-particles: high repulsive forces experienced when the 2+ alpha nucleus comes near a strongly positive charged nucleus

o Neutron: uncharged particle reaches target much easier, and efforts at transmutation switched to neutron bombardment

o The discovery of fission was a result of such studies: neutron bombardment of

272111Rg

209 64 272 183 28 111 0Bi Ni Rg n

4 14 17 12 7 8 1He N O H

23592U

o Successful neutron bombardment: an example is the “manufacture” of plutonium, which is a two-stage process:

o This is the process that is used in “breeder” reactors to convert unusable 238U into fissionable 239Pu

o This method works well up to element 101, Mendelevium, but beyond this larger “bullets” must be used, as for example in:

o Such reactions suffer from very strong nuclei-nuclei repulsion, and require great effort and very specialized apparatus

o Many transuranium elements from the Actinides have been made in sufficient quantity to have some of their basic chemical properties investigated

o The “super-heavy” elements of the 6d block are usually only made a few atoms, or “events” at a time and nothing is known of their chemistry. Roentgenium is “heavy” gold!

1 238 239 239 00 92 92 93 1n U U Np

239 0 23993 -1 94Np β + Pu

10 252 257 15 98 103 0B Cf Lr 5 n

Nucleosynthesis by transmutation

Energy ranges of the main types of nuclear radiation and detection of radiation

particles - energy range 3.5 - 10 MeV particles (and positrons) - energy range 0.18 - 3.6 MeV radiation - energy range 0.008 - 7.11 MeV o Radiation may be detected by a Geiger counter.

o This device is suitable for any type of radiation capable of ionizing argon gas, causing a current to flow and a needle or loudspeaker to sound.

o This includes those we have talked about, , , and , as well as high-energy X-rays. As a group, these are called "ionizing radiation".

Radiation Safetyo The unit of radioactivity:

Curie Ci; 1 Ci = 3.70 1010 events per second (Becquerel, Bq)

The Curie measures the rate of nuclear decay - similar to half-life. For a given isotope, with a known half-life, the Curie indicates much of that isotope is present

o The unit of radiation intensity:rad = radiation absorbed dose = amount of radiation that results in absorption of 1 10-5 J g-1 of absorbing material.

It varies with the type of radiation and its source. All sources of nuclear radiation are distinguishable by type and energy.

o The Röntgen equivalent for man: Unit of exposure – related to riskrem ; 1 = 1 rad 1 RBE

This measures the relative effect of different types of radiation (, , or X) on humans, which is expressed as the RBE, relative biological effectiveness. Multiplying the RBE by rad results in a unit that tells just how much a certain radioactive source, with a certain intensity, will harm you.

Nuclear Power -The fission processo Typical reaction is 235U fission is illustrated by the equation:

E = –2.1 1010 kJ / mol

o This is not the only possible reaction: a variety of daughter isotopes are produced (As, Br, Sr, Zn, and Zr), some of which are stable, but most of which are radioactive themselves (e.g. as -, + or emitters).

o These reaction can release 1, 2 or 3 neutrons, and on average 235U fission releases 2 neutrons for every one captured.

o To be self-sustaining, a nuclear reactor needs to control the fast neutrons produced in fission.

o The right number of fast neutrons must be slowed down to a speed where they can be captured by 235U nuclei.

o This is accomplished by using a moderator. Properties of a moderator include:a) must not absorb many neutrons, since these are required to sustain a nuclear chain

reactionb) must be light in mass so that the neutrons are not slowed too muchc) must not react with neutrons to form radioactive speciesd) must have a high probability of collision with a neutron

1 235 141 92 10 92 56 36 0n U Ba Kr 3 n

Nuclear chain reactionso The splitting of 235U requires first that it is hit by a neutron to make 236U, which then

spontaneously breaks into many pieces, including two lighter units and several neutronso The substances which have been used as moderators are water, D2O (heavy water) and

graphite. Water and D2O are favoured because they are liquids, and can double as heat transfer agents by circulation through the reactor and a heat exchanger.

Energy considerations in nuclear power

o Nuclear reactors have all sorts of technical difficulties associated with handling radioactive fuels and by-products. Why bother?

o A small amount of nuclear fuel can release a large amount of energy. The origin of this energy is the nuclear binding energy.

o The nucleons in the daughter isotopes have much less total binding energy than the parent. This excess binding energy is released and converted into heat.

o For this fission reaction the total mass change is 0.19 u per 235U nucleus. o This converts to 7.3 1010 J or 73 GJ per gram of 235U. o In consumer terms, where 1 kW hr of power is equivalent to 3.6 MJ, each gram of 235U can

supply 20,000 kW hr of heat energy. (average household uses ~10 GJ per Month)

o This is the maximum possible there are significant losses in electric power generation.

Environmental factors for nuclear power

o The major difficulty with non-nuclear power generation is simply the huge scale of these operations.

o A 2000 MW coal-fired power station releases 42,000 tons of CO2, 600 tons of SO2 (and related acid gases) and 10 tons of fly ash per day.

o Scrubbers can reduce the acid gases and fly ash considerably, but nothing can be done about the CO2 greenhouse gas

o Power in Alberta is generated near Edmonton from large coal thermal generating stationso It must be transmitted great distances, with considerable losses during transmissiono Nuclear reactors are usually located close to residential areas: thus they overcome the large

losses from long-distance transmission

The CANDU reactor for electric power generation

o Schematic diagram of a CANDU nuclear reactor as used in a typical electric generating station

o This is a unique Canadian design that has been used in several countries including Canadao There are none in Western Canada, but several in Ontario, Quebec and New Brunswicko The name stands for CANada Deuterium Uranium. It is unique in its ability to burn normal

uranium dioxide instead of needing enriched fuels

Application of Nuclear Reaction in Medicine

Imaging methods

Chemical Identification

Treatment

The pattern of radiation emitted from a nuclear reaction in the body can be reconstructed to make images of parts of the body. An agent that undergoes a nuclear reaction must be administered (invasive techniques)

An agent that undergoes a nuclear reaction can be attached to a drug molecule that binds to a specific area in the body of interest. Ex) cancer cells, blood clots, diseased cells, receptors specific to an organ.( in the brain). Imaging methods are used to observe the phenomenon of interest.

In radiation therapy radiation produced from a nuclear reaction can be used a destroy diseased tissue. Sometime a drug is administered to make the tissue more susceptible to radiation damage.

A whole-body scan (photographic process) undertaken with a phosphate complex labeled with 99mTc that accumulates in bones. 99mTc emits gamma radiation with is detected.

The dopamine accumulates in the putamen of a normal human brain (right). A Parkinson’s disease patient shows much lower accumulation in the putamen (left)

The emitted positron immediately encounters an electron, which undergoes an annihilation event releasing two photons. The detector senses the photons and where they come from which is used to reconstruct an image.

18 0 189 1 8F O

ImagingA common positron emitter is 18F,

PET – Positron Emission Tomography

It is incorporated into [18F]-dopamine.