Nuclear Chemistry

What is a Nuclear Reaction?

Nuclear reactions change an atom's nucleus. They start with unstable (radioactive) isotopes. These isotopes undergo changes to their nuclei to become more stable. These changes are not affected by variables like temperature, pressure, or catalysts.

Types of Radiation

Alpha Decay

Alpha particles are ejected helium atoms. They change the element.

Beta Decay

Beta particles are electrons. This also changes the element

Gama Radiation

Gamma radiation is not a particle. It is pure energy. It is often a part of alpha or beta decay.

Nuclear Notation Key

Other Types of Nuclear radiation

Nuclear Stability

The nucleus is held together by nuclear forces. This force counter acts the repulsion of electrical charges.

Emission increases stability

The ratio between protons and neutrons affects how stable a nucleus is. The ideal ratio is about 1:1. If there are too many neutrons the nucleus will undergo beta decay. If there are too many protons the nucleus may undergo alpha decay or positron emission. A positron has the mass of an electron but positive.

Band of Stability

Half-Life

Every radioactive isotope has a predictable rate of decay. This decay is measured in half-lives.

A half life is the amount of time it takes for half of the original atoms to decay into new elements.

Half lives follow an exponential decay model.

Mathematically can be expressed by:

A = A0 (1/2 )n

A = amount remaining

A0 = Initial amount

Example:

If there are 60 grams of Np-240 present, how much Np-240 will remain after 4 hours? (Np-240 has a half-life of 1 hour)

Solution:

how much mass remains = (60grams)

how much mass remains = A = (60g)(1/2 )4 = 3.75g

After 4 hours, only 3.75g of our original 60g sample would remain the radioactive isotope Np-240.

Radiometric dating

Because Radioisotopes decay at a constant rate we can use the ratio of the radioisotope to its products to determine age, i.e. by comparing the amount remaining to the initial amount. This is commonly done in radiocarbon dating.

Example:

How long will it take for 18.0 grams of Ra-226 to decay to leave a total of 2.25 grams? Ra-226 has a half-life of 1600 years.

Solution:

18.0g ⇒ 9.0g ⇒ 4.5g ⇒ 2.25g, this is three half-lives

how old (time)=3×1600years

This decay process takes 4800 years to occur.

Nuclear Fusion and Nuclear Fission

Fusion reactions occur when two atoms combine together.

Fission reactions occur when a single atom splits apart.

Modern nuclear reactors use nuclear fission to generate power. Fission causes a chain reaction to occur, which heats up water. The steam turns a turbine which generates electricity.

Chain reaction

When Uranium-235 splits it ejects 3 neutrons. These neutrons can collide with another uranium causing it to undergo fission, which also generates 3 more neutrons. This will continue until all the radioactive material decays.

Nuclear reactors use control rods to slow the chain reaction. Lead control rods block some of the neutrons and stop them from cascading.

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