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.
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.
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.
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.
What are the atomic numbers of 90 Sr, 37 Cl, and 24 Mg? Did you get 38, 17, and 12?
Can you name these elements: , and ? Did you get carbon, uranium, and technetium?
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