Nuclear Reactions

• Nuclear Reactions involve the nucleus of atoms

• When a nuclear reaction occurs, the element is changed completely into another element

• As a result, nuclear reactions are also known as Transmutations

The Nucleus

d10-13 cm

(d10-8 cm for an atom)

The Nucleons

mass (AMU) charge (esu)

proton 1 +1

neutron 1 0

In case you forgot...

many elements have several isotopes

U23892

U23592

92 protons143 neutrons

92 protons146 neutrons

Identical chemistries, different nuclear reactions

Chemical vs. Nuclear Energies

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

H = -896 kJ/mol

= -56 kJ/g

235Unuclear fission

other nuclei

H = -8.2 x 107 kJ/g

Why do nuclear reactions occur?

• Naturally, some atoms are stable while others are unstable

• Usually, when the ratio of neutron:proton is greater than 1.3, the nucleus is unstable

• Examples, 236U, 209Po, 14C, 230Th

• An unstable nucleus is radioactive and naturally emits certain radiations and is converted to a more stable isotope

proton (p)

neutron (n) the nuclear force

Forces in the Nucleuselectrostatic repulsion

Nuclear Stability

protons (Z)

neut

rons

(N

)

0 900

140

N/Z=1

.

.

.

...... . .

........

....................

. ............. ....................

..............

..............

............... .................. .. . ..

.......

..............

................

..

..............

...

“Island of Stability”

decay

EC decay

decay

neutronsneeded for stability

Determine if any of the following isotopes is stable or unstable

Isotope

Protons (P)

Neutrons (N)

N/P ratio Stable or unstable

218Po

14C

214Pb

206Pb

Types of RadiationThere are 4 main types of radiation:

Radiation Charge Mass Symbol

Alpha rays

(-particles)

+2 4 42He

Beta rays

(-particles)

-1 0 0-1

Gamma rays

(-rays)

0 0 00

Positrons +1 0 O+1

The Discovery of Radioactivity

U

-

+

+

Particles

positively charged

massive

accurate measurements 4He nuclei

2 protons2 neutrons

Emission of particles

• Some unstable nuclide decay by emitting only -particles

• Examples:• 226

88Ra → 22286Rn + 4

2He• 210

84Po → 20682Pb + 4

2He• 230

90Th → 22688Ra + 4

2He• Using Table N, identify 4 nuclide that

undergo alpha decay and write the nuclear equation

Decay

U238

92 He + 4

2

234

90 Th

nucleons are conserved (238)charge is conserved (92)

identities of atoms are not!

Emission of particles

• Some unstable nuclide decay by emitting only -particles

• Examples:• 239

92U → 23993Np + 0

-1• 239

93Np → 23994Pu + 0

-1• 14

6C → 147N + 0

-1• Using Table N, identify 4 nuclide that

undergo beta decay and write the nuclear equation

TABLE NSelected Radioisotopes

Particles

negatively charged small mass

accurate measurements electrons

Decay

234

90 Th e- + 0

-1234

91 Pa

i.e. a neutron is turned into a proton + an electron

90 p144 n

91 p143 n

Electron Capture

55

26 Fe + e-5525 Mn

26 p29 n

25 p30 n

i.e. a proton captures an electron and is turned into a neutron

Electron Capture (EC)++ ++

+++

+++

+ -

-

Decay Series

A B C D ... � non-radioactive nuclide

There are three such series:

A = 238U,A = 232Th,A = 235Uand

238U Decay Series

238U Ž 234Th Ž 234Pa Ž 234U Ž 230Th

226Ra

222Rn(g) Œ

218Po Œ

214Pb Œ

214Bi Ž 214Po Ž 210Pb Ž 210Bi

210Po206Pb Œ

non-radioactive

Nuclear Reactions

A + B Ž C

[mass (A) + mass (B)] mass (C)

e.g. Fe5626

26 p + 30 n Ž Fe5626

find mass before and after reaction

Nuclear Reactions

mass before = 26 mp + 30 mn

= 26(1.00728 amu) + 30(1.00866 amu)= 56.44908 amu

mass after = mass 56Fe atom - 26 me

= 55.9349 amu - 26(.0005486 amu)= 55.92070 amu

“mass defect” = 0.52838 amu

Nuclear Reactions

The mass is converted to

energy!

"It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing -- a somewhat unfamiliar conception for the average mind. Furthermore, the equation E is equal to m c-squared, in which energy is put

equal to mass, multiplied by the square of the velocity of light, showed that very small amounts of mass may be converted into a very large amount of energy and vice versa. The mass and energy were in fact equivalent, according to the formula mentioned before. This was demonstrated by Cockcroft and Walton in 1932, experimentally."

Nuclear Reactions

The mass is converted to energy!

E = mc2

m = 0.52838 amu / 56Fe nucleus = 0.52838 g/mol = 0.52838 x 10-3 kg/mol

c = 3.00 x 108 m/s

Nuclear Reactions

E = mc2

= 0.52838 x 10-3 kg/mol (3.00 x 108 m/s)2

= 4.75 x 1013 (kg m2 s-2) / mol

= 4.75 x 1013 J/mol

= 4.75 x 1010 kJ/mol

= the “binding energy” of 56Fe

Binding Energy

is a maximum at 56Fe

lighter elements become more stable upon fusion

heavier elements becomemore stable upon fission

Nuclear Fission

n

235Uunstable 236U

92Kr36

141Ba56

+ 3 1n0

Nuclear Fission

1n + 235U 141Ba + 92Kr + 3 1n

Many other fission “pathways” exist:

1n + 235U 137Te + 97Zr+ 2 1n

Exercises

• Page 814: questions 8, 9, 10, 11 and 12

Artificial Transmutations

Aim: What is Artificial Transmutation and how do Artificial Transmutations occur?

Do Now

Write Nuclear Equations for the following natural transmutations:

(a) U-235 → Th-234 (b) Th-230 → Ra-226

(c) Po-214 → Pb-210 (d) Pb-214 → Bi-214

(e) U-234 → Th-230 (e) Pa-234 → U-234

What is Artificial Transmutation?

• Artificial Transmutation is where a stable isotope is made to disintegrate

• This is usually done by bombardment with high speed particles

• Examples:

• 42He + 14

7N → 178O + 1

1p

• 2713Al + 4

2He → 3015P + 1

0n

• 3215P + 0

-1 → 3214Si

Individual Practice

• Page 816: Problems 15 and 16

• Page 837: Problems 73-76

Group Practice

• Examine the Figure on page 814. Write a series of nuclear equations showing the transmutations from U-238 to Po-214