Nuclear Science

An updated version of this lesson is available at Visionlearning: Nuclear Chemistry

Up until now we have been describing how interactions between electrons affect chemical bonds and reactions. However, there is another branch of science that deals with changes in the number of protons and neutrons in an atom's nucleus. This branch of science is called nuclear science. As you move downward in the periodic table, the number of protons and neutrons in an atom's nucleus increases. An element like uranium (U) for example, has 92 protons and 146 neutrons to keep these protons stable. Even with all of these neutrons, large nuclei like that of uranium can become unstable. When this happens, the nuclei of large atoms can emit radiation in an effort to stabilize themselves.

Radiation
All of the elements heavier than bismuth (Bi) (and some lighter than bismuth) have natural radioactivity. Radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element. Unlike normal chemical reactions that form molecules, nuclear reactions involve sub-atomic particles and result in the transformation of one element into a different isotope or a different element altogether (remember that the number of protons in an element defines the element type). There are three types of radiation:

Alpha Radiation (a) is the emission of an alpha particle from an atom's nucleus. An a particle contains 2 protons and 2 neutrons ( and is also known as an He nucleus). When an atom emits an a particle, the atom's atomic mass will decrease by 4 (2 protons and 2 neutrons) and the atomic number will decrease by 2. The element will transmutate into another element that is 2 atomic numbers smaller. An example of this transmutation takes place when uranium decays into the element thorium (Th) as depicted in the following equation:

238
92

4
2

234
90

Beta Radiation (b) is the transmutation of 1 neutron into 1 proton and 1 electron (followed by the emission of the electron from the atom's nucleus). When an atom emits a b particle, the atom's atomic mass will not change however the atomic number will increase by 1. An example of this is the decay of the element carbon 14 into the element nitrogen:

14
6

0
-1

14
7

Gamma Radiation (g) involves the emission of electromagnetic energy (similar to light energy) from an atom's nucleus. No particles are emitted during gamma radiation, however g radiation is often emitted during, and simultaneous to, a or b radioactive decay.

Half-life
Radioactive decay proceeds according to a principal called the half-life. The half-life is the amount of time necessary for ½ of the radioactive material to decay. For example, the radioactive element bismuth (²¹⁴Bi) can undergo alpha decay to form the element thallium (²¹⁰Th) and this reaction has a half-life equal to 5 days. If we begin our example with 1 mole of bismuth in a sealed jar, then after 5 days we would have ½ mole of bismuth and ½ mole of thallium in the jar. After another 5 days (10 from the starting point), ½ of the remaining bismuth would decay and we would be left with ¼ mole of bismuth and ¾ moles of thallium in the jar. As illustrated, the reaction proceeds in ½'s. The fraction of original material that remains can be calculated using the equation:

Fraction remaining =

1
2ⁿ

where n is the number of half-lives elapsed

Artificial Nuclear Reactions
While many elements undergo radioactive decay naturally, nuclear reactions can also be stimulated artificially. There are 2 types of artificial nuclear reactions:

Fission: some elements can be stimulated to split into parts. Most commonly this is done be 'firing' a neutron at the nucleus of an atom. The energy of the neutron 'bullet' causes the target nucleus to split into 2 parts. An excellent simulation of this process has been constructed by AJ Software & Multimedia and can be viewed by clicking here (~250k movie. Protons = red, Neutrons = gray). The neutrons that are released during a fission reaction can go on to stimulate fission reactions in other atoms. This process, known as a chain reaction (~230 k movie, AJ Software & Multimedia), is at the core of the process of energy production in nuclear power plants. For more information on nuclear fission, visit the Atomic Archive Fission pages.
Fusion: some elements can be stimulated to 'fuse' together. If the nuclei of 2 small atoms are forced together, these atoms can combine with each other to form a larger atom. AJ Software & Multimedia has constructed an excellent simulation of the fusion of a deuterium atom (hydrogen w/1 neutron) with a tritium atom (hydrogen w/2 neutrons) to form a helium nucleus and a free neutron; this simulation can be viewed by clicking here (~270k quicktime movie. Protons = red, Neutrons = gray). Although many people think of the sun as a fireball, the sun, and all stars, generate power through nuclear fusion reactions. In the sun, light and heat are emitted by the fusion of hydrogen atoms into helium. For more information on nuclear fusion, visit the Atomic Archive Fusion pages.

Additional information on the history of the nuclear age can be found at the Atomic Archive pages. The Science Net also has additional information on radioactivity.