## Nuclear Reactions

We can classify nuclear changes as a number of different types of nuclear reactions. Each has its own characteristics (and potential applications). In addition, like chemical reactions we can write down an infinite number of nuclear changes that could occur by simply constructing a balanced equation. However, we will focus on typical changes that occur in a number of nuclear chemistry applications. Rather than focus on predicting the outcomes of nuclear change, the goal will be to identify different types of reactions.

The four main reaction types that will be covered in this unit are:

1. Fission
2. Fusion
3. Nuclear Decay
4. Transmutation

#### Fission

We can classify a number of nuclear reactions.  The first important reactions are fission reactions.  In fission reactions, a heavy nucleus is "split" into two (or more) smaller nuclei. Generally, we discuss reactions which are downhill in energy (exothermic).  Fission reactions are exothermic that start with nuclei that are heavier than iron.

An example of an important fission reaction is

${\rm ^{235}_{\;92}U \;\;+\;\; ^1_0n \;\;\rightarrow ^{137}_{\;56}Ba \;\;+ \;\;^{97}_{36}Kr\;\; + \;\;2 ^1_0n}$

This is the fission of uranium-235 to make barium-137 and krypton-97 plus a couple of neutrons.  Note: there are neutrons on both sides of this reaction.  It is important to show them both in the reaction since the neutron instigates the reaction.  The fission is actually a uranium-236 nucleus that is created from the collision of a neutron and a uranium-235.

Fission reactions are widely used to generate electrical power using uranium as a fuel and generating a wide array of fission products.

#### Fusion

Fusion reactions is when two (or more) lighter nuclei come together to make a heavy nucleus.  For example

${\rm ^2_1H \;\;+\;\; ^3_1H \;\;\rightarrow \;\;^4_2He \;\;+\;\; ^1_0n}$

The fusion of four hydrogen atoms and two electrons into a single helium atom is the primary reaction in the sun (although it happens in a number of steps).  Fusion reactions are exothermic for nuclei smaller than iron.

Fusion reactions of light elements can be extremely exothermic.  And per mass generate by far the most energy.  Research is on going to maintain stable fusion reactions on earth.  Currently, reactions can be maintained for infinitesimally short times (or in uncontrolled reactions such as the hydrogen bomb).

#### Nuclear Decay

Nuclear decay is perhaps the most important process to understand in nuclear chemistry. This is the origin of "radioactivity" and is the basis of most applications of nuclear chemistry outside of the nuclear power industry. Nuclear decay is the process by which an unstable isotope of a particular element spontaneously transforms into a new element by emission of ionizing radiation. Later the details of the types of such decays as well as the types of radiation will be covered. In many ways, nuclear decay is similar to fission. The product elements are lighter than the reactant elements. However, unlike fission nuclear decay involves one element transforming into another rather than breaking up into two nuclei. Some nuclear decay involves the emission of a He-4 nucleus. Typically this is considered emission of a "particle" versus the nucleus breaking up into smaller pieces. Nuclear decay almost always involves large energy release in the form of radiation. An example is the electron capture reaction below that issued in the treatment of prostate cancer since the decay results in the emission of high energy gamma rays

${\rm ^{103}_{\;46}Pd \;\;+ \;\; ^{\phantom{-}0}_{-1}e \;\; \rightarrow \;\;^{103}_{\;45}Rh \;\;+ \;\;\gamma}$

#### Transmutation

Transmutation is essentially the reverse of nuclear decay. It is a non-spontaneous process where by one element is converted to another by the bombarding it with high energy radiation (or neutrons). This is generally an artificial process that allows the creation of radioactive isotopes. For example, the Pd-103 that is uses in the treatment of prostate cancer is made in laboratory is made by bombarding Pd-102 with high energy neutrons.

${\rm ^{102}_{\;46}Pd \;\;+ \;\; ^1_0n \;\; \rightarrow \;\;^{103}_{\;46}Pd}$

Transmutation involves increasing the mass of nuclei.

#### Concept Question

The following is what type of reaction?
${\rm ^{252}_{\;98}Cf \;\; \rightarrow \;\; ^{140}_{\;54}Xe + \;\;^{108}_{\;44}Ru \;\;+ \;\;4^1_0n}$