NUCLEAR CHEMISTRY

Kenneth E. Schnobrich

General Facts

As we look at the Periodic Table we note both the atomic number (Z) and the atomic mass number (A).

In 6C12, the 6 represents the

The number of protons in the nucleus

The number of electrons associated with a neutral atom

The difference between A (the atomic mass number) and Z (the atomic number) represents the number of neutrons (N) in the nucleus.

6C12 and 6C14 would be identified as isotopes of the same element and differ in the number of neutrons in the nucleus.

General Facts

In the case of 6C12 and 6C14 they have the same number of protons in the nucleus but 6C14 has two addition neutrons in the nucleus.

Depending on the proton/neutron ratio, nuclei can be unstable and the nucleus breaks down by a process called radioactivity (nuclear decay).

Nuclear decay or half life can occur in fractions of a second or it can take hundreds of years. During this decay several of the common products are –

Alpha particles (He nuclei)

Beta particles (electrons)

Gamma radiation

General Facts

Alpha particles – have a mass of 4 amu and a relative charge of +2 – they are helium nuclei

Beta particles – have a mass of 0 amu and a relative charge of - 1 – they have all the properties of electrons

Gamma radiation – high energy photons with a very short wavelength (high frequency) – very destructive

Some isotopes occur naturally unstable – natural radioactivity

Some unstable isotopes are prepared in the lab – induced radioactivity or artificial radioactivity

Separating Radiations

Positive Plate (+)

Negative Plate (-)

Beta Particle

Alpha Particle

Gamma Particle

Radioactive sample

General Facts

In ordinary chemical reactions it is the electrons in the outer shells that are involved and energy is released or absorbed.

In nuclear reactions, the nucleus changes and the amount of energy involved can be hundreds of thousands times greater than a chemical reaction.

In Nuclear Fusion and Nuclear Fission the amounts of energy released are even greater than the amount released during a normal decay.

General Facts

Nuclear Fission – a process where the nucleus of a larger unstable atom breaks down into smaller nuclei and other subatomic particles with a tremendous release of energy (like an atom bomb).

Nuclear Fusion – a process where smaller nuclei are fused together to form a larger nucleus with the release of tremendous amounts of energy (like the hydrogen bomb or the reaction on the sun’s surface).

What Causes Instability?

The nucleus is about 10-15m in size, very small volume. In the nucleus the protons exhibit typical electrostatic repulsive forces, there are also very strong attractive forces that hold the protons together in the very small nucleus.

The neutrons act like a “nuclear glue” and are responsible for holding the protons in the nucleus together.

The stability of the nucleus then depends on the balance between the repulsive electrostatic forces between the protons and the “nuclear glue” forces of the neutrons.

The neutron/proton ratio is important for stability.

Instability

For the lighter elements the neutron/proton ratio is close to 1 which appears to be a very stable arrangement.

Any element beyond atomic number 83 (Bismuth) has no stable isotopes, they are all radioactive. It would appear that as the atomic number increases the number of neutrons needed for stability greatly increases. A ratio of 1 up to 1.51 seems to be stable (in 82Pb206 the nucleus is stable).

Particles Associated with Decay

In this table you see some of the particles normally associated with nuclear reactions. Unstable nuclei are generally trying to achieve stability by adjusting the neutron/proton ratio.

Table taken from NYS Chemistry Reference Tables

Instability

The neutron/proton ratio is 1

The neutron/proton ratio is 2 (unstable)

**Remember anything above atomic number 83 is unstable

Nuclear Equations

In writing a nuclear equation, it is important to remember that the sum of the subscripts on the left side of the arrow must equal the sum of the subscripts on the right side. The same can be said for the superscripts on the two sides of the equation.

Alpha Decay – an alpha particle is released

92U238 90Th234 + 2He4

Beta Decay – a beta particle is released

53I131 54Xe131 + -1e0

Nuclear Equations (cont.)

Positron Emission – a positron is released

6C11 5B11 + 1e0 (1p1 0n1 + 1e0)

Electron Capture – an electron is captured from an inner electron cloud surrounding the nucleus

37Rb81 + -1e0 36Kr81 (1p1 + -1e0 0n1)

Gamma radiation – the emission of this highly energetic radiation does not change the mass or the charge and is usually associated with emission of another subatomic particle.

Emissions• Nuclide – refers to the specific radioactive element and its mass number, the atomic number can be found on the Periodic table of the Elements.• Half-life (t1/2) – refers to the time it take for half of the given radioactive sample to decay and form another nuclide. The half-life is a constant value and is unaffected by variations in condition change such as temperature and pressure.• Decay Mode – describes the subatomic particle emitted during the nuclear change for that radioisotope.• Nuclide Name – the name of the element and its atomic mass number (A).

**Note – there is a key for the half-life at the bottom of the table

Table taken from NYS Chemistry Reference Tables

38Sr90 Half-life (t1/2)

0.0 28.8 57.6 86.4 115.20.00

5.00

10.00

15.00

20.00

25.00

20.00

10.00

5.00

2.501.25

Amount (g)

Am

ount

of

38Sr9

0

Years (t1/2 = 28.8 years)

Nuclear Fission

Fission can occur when a slow moving neutron strikes strikes a large nucleus causing it to split. U235, U233, and Pu239 are the most frequently radioisotopes used for this process.

0n1

56Ba142

36Kr91

92U2350n1

0n1

0n1

0n1

Nuclear Fission

During the fission process some of the neutrons produced can become involved in splitting other heavy nuclei and can lead to a chain reaction.

Subcritical mass – would indicate that there are not enough “effective” neutron/heavy nuclei collisions to sustain a reaction.

Critical mass – would indicate that there are enough “effective” neutron/heavy nuclei collisions to sustain a nuclear chain reaction.

In the atomic bomb two subcritical masses are forced to come together to produce a critical mass.

In a nuclear reactor, control rods are used to absorb neutrons and help control the fission reaction.

Nuclear Fusion

In the fusion reaction requires a great deal of energy to actually fuse two smaller nuclei. The products of fusion usually are not radioactive like those of fission. Temperatures as high as 4 x 107K are required for the fusion process.

1H2

1H3

2He4 + 0n1

Nuclear Fusion

Since extremely high temperatures are required to initiate the fusion reaction it is sometimes called a thermonuclear reaction.

Other small nuclei can be used in the fusion process.

Thermonuclear reactions occur on the surface of the sun and provide both the light and heat necessary to sustain life on earth.

Fusion reactions are not a practical source of energy at this point in our technological development because of the high energy needed for the fusion reaction.

The hydrogen bomb is a thermonuclear device it literally uses a fission reaction to initiate the fusion reaction.

Transmutation

In a transmutation reaction we can bombard a nucleus with a selected subatomic particle to produce a radioisotope.

The balancing of the subscripts and superscripts must be observed in a transmutation reaction.

7N14 + 2He4 8O17 + 1H1

26Fe58 + 0n1 26Fe59

Devices such as the cyclotron can be used to accelerate the subatomic particles used to bombard the target nuclei.

E = mc2

Using the famous equation above we can calculate the energy released in a nuclear reaction.

E – stands for energy

m – represents the mass loss (mass defect) in a nulcear reaction

c – represents the speed of light (3.00 x 108m/s)

Since the value for “c” is so large even a small mass change will produce a tremendous amount of energy.

Using Radioisotopes

In Medicine –I131 is used to diagnose thyroid problems

P32 can be used as a tracer for the eyes, liver, and tumors

Tc99 can be used as a tracer in the heart, bones, liver, and the lungs

Carbon DatingC14 levels can be used to date carbon based objects

C14 can also be used to trace metabolic pathways

Dating RocksThe ratio of U238 to Pb206 can be used to date uranium containing rocks

Using Radioisotopes

In agriculture –P31 can be used in fertilizers to trace uptake

In medicine –Co60 can be used in radiation treatments for cancer

Food supply –Both Co60 and Cs137 can be used to irradiate food supplies to kill bacteria like the anthrax bacilli

Radiation

Damage due to radiation occurs as a result of exposure to the more harmful forms of radiation such as beta radiation and gamma radiation.

Damage can occur in somatic cells (like the skin cells) and it interferes with the growth regulating process, cancer can be one of the results.

Damage can also occur in the reproductive cells and result in mutations that can be passed on to offspring.