Nuclear Chemistry

Reactions and properties of nucleus

Mp

nMass number

(number of nucleons)

nuclide : nuclear speciesIsotopes : nuclides of same chemical species

Nuclear equation

Atomic number(number of protons)

ni

piM ni

piM

Pi = Pj ni = nj

Nuclear Chemistry

Selected elementary particles

0

-1e- 1

0n

1

1p+

0

1e+

0

-1e- 1

0n

1

1p+ +

(atomic) mass unit : 1/12 the mass of a single atom of 12C same as “dalton” 1u(amu) = 1 dalton = 1.66 x 10-27 Kg

Nuclear Chemistry

Einstein mass-energy relationship

0

-1e-1

0n

1

1p+ +

balanced nuclear equation, but m = -8.4 x 10-4 u

Change in mass corresponds to change in energy, and vice versa.

i.e. E = m C2 = - 1.25x 10-13J = 0.782 MeV 1 eV = 1.60 x 10-19J

Energy obtained when e was accelerated by 1V potential difference.

cf. In chemical reaction, m ~ 0

Ex. CH4 의 연소열 H= -890KJ m = -9.9 x 10-9g

In nuclear reactionm = -0.0046 gU

238

92 Th234

90 He4

2+

Nuclear Chemistry

Nuclear binding energy

mass of nucleus < mass of nucleons

nuclear binding energy

i.e. E = m C2 : nuclear binding energy

stability of nucleus

Nuclear decay process : spontaneous

i.e. G < 0

m = mass defect

1. Beta decay

C14

6 N14

7 e-0

-1+ +

electron with energy

Atomic number : +1, mass number : unchanged

# of nucleons

~ m < 0

Neutrino

Nuclear Chemistry

2. Positron emission

C11

6 B11

5 e+0

1+ +

Atomic number : -1, mass number : unchanged

3. electron capture

U231

92 Pa231

91e-0

-1+

+

When positron emission cannot occur

4. Alpha decay Atomic number : -2, mass number : -4

U238

92 Th234

90 He4

2+

5. others Neutron emission, proton emission, fission

All toward more stable nucleus.

Nuclear Chemistry

1. Photographic plates(emulsion) darkening is proportional to radiation

2. Scintillation counter Emits light when absorbs beta particle or gamma ray : phosphorescencecan count number of light pulses.

3. Geiger counter Beta particle produces positive ions and electrons in a tube filled with gas.Then electric current is generated.

Detecting radioactivity

Nuclear Chemistry

is first order rate kinetics

then, Nt = No e-kt

Kinetics of radioactive decay

ln Nt

No= - kt

half life t1/2 = k

ln 2k

0.693=

0.693

t1/2t = ln Nt

No

Measured radioactivity (A) is the decay rate dt

dN = - = kN

0.693

t1/2t = ln

Ao

At

Radioactivity unit : becquerel (Bq) – 1 radioactive disintegration per second curie (Ci) – activity of 1g of radium is 1 Ci 1 Ci = 3.7 x 1010 Bq

Nuclear Chemistry

If we know half-life of a radioactive nuclide

Radioactive dating

1. for the age of uranium containing rock

0.693

t1/2t = ln

Ao

At

We should know Ao to find out the age of the sample.

Assumption : Pb in the rock came only from uranium decay.

Then Nt(Pb) = No(U) – Nt(U) = Nt(U) (ekt – 1) Where Nt = No e-kt

Nt(U)

Nt(Pb)ln + 1t =

Nuclear Chemistry

2. Radiocarbon dating : 14C decay

Assumption : 14C/12C ratio is constant during last 50,000 years.

This can change with burning of fossil fuel that increases 12C in the atm.

Produces 14C all the time and therefore

In nature N14

7 C14

6n1

0+ + H

1

1

0.693

t1/2t = ln

Ao

At

From this, we can deduce Ao of carbon source from fossils or dead ones.

And the time of its death.

This is the ratio in CO2 in the atm and thus is the raio in the living organisms !

Nuclear Chemistry

3. Radiation in biology and medicine

Damage is proportional to the quantity and energy of the particle Somatic damage – burning, cancer Genetic damage – DNA mutation

Safe level of exposure : ~ 100 mrem/yr. (from nature), ~ 100 mrem/yr. (from human activity)LD50 (in 30 days) = 500 rad

1 Gy(gray) = 100 rad, 1 Sv(sievert) = 100 rem

Effect of radiation : chemical changes in cellular molecules leads to change in cellular function and becomes cancerous

Measurement of radiation damage

1 rad of beta or gamma radiation = 1 rem, 1 rad of alpha radiation = 10 rem

rad ( radiation absorbed dose) : the amount of radiation that deposits 0.01J/Kg

rem ( roentgen equivalent in man) : effective dosages of radiation received by humans

RBE : relative biological effectiveness

rem = rad x RBE

Nuclear Chemistry

Radon : gaseous, generated from rocks and soils, especially from granite 222Rn decay quickly but its products are also radio active and can accumulate in the confined air.

Rn222

86+ He

4

2Po218

84

Biology : biosynthetic route of biomoleculesMedical use of radiation : X-ray – imaging, cancer therapy 131I – thyroid cancer PET(positron emission tomography) : diagnostic tool for cancer, neurological disease good tool to study brain function

e-0

1C

11

6 B11

5+ + T1/2 = 20.3 min.

e-0

1F

18

9 O18

8+ + T1/2 = 109.8 min.

Normal Brain Image of the brain of a 9 year old female with a history of seizures poorly controlled by medication. PET imaging identifies the area (indicated by the arrows) of the brain responsible for the seizures. Through surgical removal of this area of the brain, the patient is rendered "seizure-free".

Nuclear Chemistry

Fragmentation of an element by neutron bombardment

Nuclear Fission

U235

92 Kr94

36n1

0+ + Ba

139

56 n1

0+ 3

Nuclear Chemistry

Fragmentation of an element by neutron bombardment

Nuclear Fission

Self-propagating chain reaction

U235

92 Kr94

36n1

0+ + Ba

139

56 n1

0+ 3

Controllable by absorbing produced neutrons(112Cd or 10B)

235U, 233U, 239Pu can be used.

Nuclear power reactor

Fuel : 235U (~3% for “light water” reactor, 0.7~1.5% for “heavy water” reactor)

Moderator : slows down neutron and transfer heat to a steam generator: water, carbon

Controller: controls fission reaction rate by capturing neutrons

Thermonuclear reactions : requires very high kinetic energies

Nuclear Fusion

Requires 40,000,000K

H1

1 e+0

1

+ H1

1 H2

1+ +

Hydrogen burning

Helium burning starts at 108K

Nucleosynthesis : nuclear transmutation

In stars

H2

1+ H

1

1 He3

2+

+ H1

1He

3

2 He3

2+He

4

2 2

He4

2 Be8

42

He4

2Be8

4 + C12

6

He4

2N14

7 + O17

8 H1

1+

N(p)14

7 O17

8

Fusion reactor : requires very high temperature and control --- still a dream cf. cold fusion

14 장숙제 : 4, 14, 16, 22, 26, 38, 42, 48

제출기한 : 11 월 6 일

제출장소 : 자연과학동 4109 호