Isotope Notation



Isotope Notation 

Subscripts and superscripts can be added to an elements symbol to specify a particular isotope of the element and provide other important information. The atomic number is written as a subscript on the left of the element symbol, the mass number is written as a superscript on the left of the element symbol, and the ionic charge, if any, appears as a superscript on the right side of the element symbol. If the charge is zero, nothing is written in the charge position. If the charge is +1 or -1, the convention is to write + or (without the 1) as a superscript on the right. If the charge is +2, +3, -2, or -3, we write 2+, 3+, 2-, or 3- as the superscripts. 

   

Examples are below.

Most abundant hydrogen isotope  

Most abundant isotope of uranium   

A sodium cation, Na+   

An aluminum cation, Al3+    

An iodine anion, I-    

An oxygen anion, O2-    

Because all of the isotopes of an element have the same atomic number, the atomic number is often left off the isotope notation. Another way of naming isotopes uses the name of the element followed by the isotopes mass number. For example,  or carbon-14 can be used in the place of .

All isotopes of an element have essentially the same chemical characteristics, and there is usually no need for the chemist to distinguish between them, but sometimes the differences between isotopes are very important. For example, although the iodine atoms found in nature are almost 100% iodine-127, iodine-131 can be formed in nuclear reactions. A major difference between these isotopes is that iodine-127 atoms are stable, and atoms of iodine-131 are unstable and undergo radioactive decay. Because these isotopes have virtually the same chemical properties, -1 ions of each are absorbed by our thyroid glands in the same way (thyroid tissue specifically absorbs and stores iodine, whereas other body tissues do not). A physician who suspects that a patient has a malfunctioning thyroid gland may perform a diagnostic test in which a very small amount of sodium iodide made with iodine-131 is administered. Instruments for detecting the levels and locations of the resulting radioactive emissions can then be used to study the thyroid glands activity.

When you are given an isotope symbol, use its subscripts and superscripts to find the atomic number, mass number, and ionic charge:

           

Number of protons = atomic number

Number of neutrons = mass number atomic number

In an uncharged atom,

Number of electrons = number of protons

In an ion,

Number of electrons = number of protons charge on ion

For example, the following equation calculates the number of electrons in a +2 magnesium cation, .

12 - (+2) = 10 electrons

The following equation calculates the number of electrons in a -3 nitrogen anion, .

7 - (-3) = 10 electrons

Charge = number of protons number of electrons 

Click here to see an example. 

Click here to see an exercise. 

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