Introduction to Radiochemistry
The nucleus of an element contains protons and neutrons. We have already learned that an element’s identity and atomic number is found from the number of protons in a nucleus. However, around 1912, researchers found odd changes in samples of the same elements. Their atomic number was the same, but they had different atomic masses. Chemists couldn’t decide if they had discovered new elements or different forms of old ones.
Isotopes
In 1913, Frederick Soddy named these chemically identical elements with different atomic weights isotopes , from the Greek word meaning “same place,” since they were placed in the sample’s same spot on the Periodic Table.
Isotopes are chemically identical atoms of the same element but with different numbers of neutrons and different mass numbers.
The naturally occurring isotope of hydrogen, called deuterium , was discovered by Harold Urey in 1931. Urey found that certain samples of hydrogen were twice the weight of common hydrogen. His experiments on simulating early Earth’s atoms earned him the Nobel Prize for Chemistry in 1934.
Although most hydrogen atoms have a nucleus of only one proton and no neutrons, 1 out of every 5,000 hydrogen atoms have a nucleus of 1 proton and 1 neutron (deuterium). When common hydrogen (also known as protium) is measured, then, it is found to have a mass of one, while deuterium atoms have a mass of two. The even rarer radioactive isotope of hydrogen called tritium has a mass of three, with 1 proton and 2 neutrons. Figure 11.1 illustrates the three forms of hydrogen.
Fig. 11.1. Hydrogen has three different forms that are named separately.
It is interesting that deuterium and common hydrogen both react with oxygen to form water. Regular water has a mass of 18 grams, while “heavy water” has a mass of 20. Though less likely to form the same compounds as hydrogen and deuterium, tritium does enter into reactions.
Naming Isotopes
Hydrogen is the only element that has been given separate names for its different isotopes, perhaps because it is involved in so many different kinds of reactions.
In naming isotope forms of elements other than hydrogen, you basically have two methods to choose from. One way is to write the element name with a hyphen and then the mass number. The second isotope shorthand method uses the element symbol along with the atomic number (Z) as a subscript and the mass number as a superscript ( A ). Both are written to the left side of the element’s symbol. ( Note : atomic mass is not the same as atomic number.) Figure 11.2 shows both ways of writing isotopes for radioactive radon, which has over 20 different isotopes. Radon-222 is the longest-lived of these isotopes with a half-life of nearly four days.
Fig. 11.2. There are two different ways to name radioactive isotopes. Radioactive radon is shown.
Examples
Example 1
What are the atomic numbers of 90 Sr, 37 Cl, and 24 Mg? Did you get 38, 17, and 12?
Example 2
Can you name these elements: 
, and 
? Did you get carbon, uranium, and technetium?
Radioactivity
Radioactivity of chemical elements, sometimes called radiochemistry , was discovered in 1896 by Antoine Becquerel, when he found that a photographic plate never exposed to sunlight in his lab had become exposed. The only possible culprit was a nearby uranium salt sitting on the bench top.
The term radioactivity was first used by French scientist Marie Curie in 1898. Marie Curie and her physicist husband, Pierre, found that radioactive particles were emitted as either electrically negative (–) which were called beta ( β ) particles or positive (+) called alpha ( α ) particles.
The first years following the discovery of the special properties of radioactive elements, with all of their wonders, led Pierre and Marie Curie to plan more studies. In 1903, Becquerel and the Curies shared the Nobel Prize for Physics for their work in radioactivity.
In 1911, after the discovery of polonium and radium the year before, the Curies received another Nobel Prize, this time for chemistry for their continued work.
Nuclear Reactions
Most chemical reactions are focused on the outer electrons of an element, sharing, swapping, and bumping electrons into and out of the combining partners of a reaction. Nuclear reactions are different. They take place within the nucleus.
There are two types of nuclear reactions. The first is the radioactive decay of bonds within the nucleus that emit radiation when broken. The second is the “billiard ball” type of reaction where the nucleus, or a nuclear particle (like a proton), is struck by another nucleus or nuclear particle. It is easy to remember these with the following: element → decay → radiation (see Figure 11.3 ).
Fig. 11.3. Radioactive elements decay and lose energy in an ordered way.
Radioactive Decay
A radioactive element, like everything else in life, decays (ages). When uranium or plutonium decays over billions of years, they go through a transformation process of degrading into lower energy element forms until they settle into one that is stable.
When a radioactive element decays, different nuclear particles are given off. These radiation particles can be separated by an electric (magnetic field) and detected in the laboratory:
beta ( β ) particles = negatively (–) charged particles
alpha ( α ) particles = positively (+) charged particles
Gamma ( γ ) particles are electromagnetic radiation with no overall charge, similar to X-rays but with a shorter wavelength.
Decay of radioactive isotopes is affected by the stability of an element at a certain energy level. Bismuth (Bi), at atomic number 83, is the heaviest element in the Periodic Table with a minimum of one stable isotope. All other heavier elements are radioactive.
Magic Numbers
Radioactive decay seems to be a mathematical process. Experimenters have found that protons have the magic numbers of 2, 8, 20, 28, 50, 82, and 114. Neutrons have the same magic numbers as protons, plus the number 126. Radioactive uranium ( 238 U) decays eventually to lead ( 82 Pb).
Magic numbers are the number of nuclear particles in a completed shell of protons or neutrons.
Magic numbers seem to come from the fact that some combinations of protons and neutrons are stable (non-radioactive), while some are unstable (radioactive). The radioactive elements decay (lose energy) over time. During decay, the unstable nucleus tries to become more stable by emitting particles. This goes on in a stepwise manner until a final, stable configuration of protons and neutrons is achieved.
Half-life
All radioactive isotopes have a specific, set half-life . These time periods are not dependent on pressure, temperature, or bonding properties.
The half-life of a radioactive isotope is the time needed for half of an elemental sample to decay.
For example, the half-life of 
is 4.5 × 10 9 years. This is about the same as the age of the Earth. It is amazing to think that the uranium found today will be around for another four billion years.
Nuclear Bombardment (billiard Ball Reaction)
In 1919, Ernest Rutherford discovered he could turn one element into another element (change from its original energy level to the energy level of a different element) by colliding the nucleus of one element with the nucleus of another.
Transmutation is the process of one element changing into another element.
Transmutation occurs when one element changes into a different element by hitting the nucleus of the first element with nuclear particles (protons, neutrons) of another element and changing its nuclear content.
Rutherford used alpha particles to collide with nitrogen nuclei in the laboratory. Protons were thrown off in the reaction. The equation 14 N + 4 He → 17 O + 1 H shows this process. Currently, particle accelerators are used to speed up electrons, protons, and alpha particles to super speeds. These super speeds are needed to penetrate the nucleus of elements with large atomic numbers.
Think of it as tossing a tranquilizer dart at a rhinoceros. The rhino’s tough hide would cause the dart to bounce off. Very high speeds are needed to penetrate. Slower speeds would have little effect. In the same way, nuclear particles must be greatly accelerated to penetrate the tight core of another element’s nucleus.
Transuranium Elements
Elements with atomic numbers greater than uranium ( 92 U), the element with the highest atomic number found in nature, are called transuranium elements. All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the University of California at Berkeley or at Argonne National Laboratory. By colliding uranium ( 238 U) with neutrons, they produced uranium ( 239 U), which days later decayed to neptunium ( 237 Np).
Americium and curium were placed after actinium in the actinide series in the Periodic Table because their chemical properties were similar. Since that time, elements with atomic numbers to 118 have been reported by scientists around the world.
Radioactivity is often thought of as a terrible side effect of nuclear weapons and X-rays, but when properly shielded, radioactive elements have a variety of uses. The actinide series of elements are used as power supplies for pacemakers, nuclear satellites, and submarines. Americium–241 is used in home smoke detectors to control the conductivity of air that changes when smoke is present.
Practice problems for these concepts can be found at – Radiochemistry Practice Test
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