5.5 Isotopes of the Elements
Learning Objectives
By the end of this section, you will be able to:
- Define isotopes and identify examples of isotopes for several elements
- Write and interpret symbols that depict the atomic number, mass number of isotopes
- Write isotope names using common naming methods
What is an Isotope?
Isotopes are various forms of the same element that have the same number of protons but a different number of neutrons. As the number of neutrons of an atom changes, so does its relative isotopic mass. The relative isotopic mass (also called mass number) is the sum of the protons and neutrons present in that isotope.
Mass Number (A) = Number of Protons + Number of Neutrons
Isotope symbols for elements are used to represent specific isotopes of atoms and include mass number (A) in superscript, atomic number (Z) in subscript, followed by the element symbol (X) in normal case (Figure 5.5a).
The number of protons in the nucleus of an atom is its atomic number (Z). This is the defining trait of an element. Its atomic number (Z) determines the identity of the atom. For example, any atom that contains six protons is the element carbon and has the atomic number 6, regardless of how many neutrons or electrons it may have. If you change the atomic number to 7, you are no longer dealing with carbon atoms, but nitrogen atoms. A neutral atom must contain the same number of positive and negative charges, so the number of protons equals the number of electrons. Therefore, the atomic number also indicates the number of electrons in an atom. The total number of protons and neutrons in an atom is called its mass number (A). The number of neutrons in that atom is therefore the difference between the mass number (A) and the atomic number (Z).
In summary:
[latex]\begin{array}{r @ {{}={}} l} \text{atomic number (Z)} & \text{number of protons} \\[1em] \text{mass number (A)} & \text{number of protons + number of neutrons} \\[1em] \text{A - Z} & \text{number of neutrons} \end{array}[/latex]
Examples of Isotopes and their Properties:
As mentioned above, the symbol for a specific isotope of any element is written by placing the mass number as a superscript to the left of the element symbol. The atomic number is sometimes written as a subscript preceding the symbol, but since this number defines the element’s identity (atomic number), as does its symbol, it is sometimes omitted, as shown in Figure 5.5b. The various isotopes for the element carbon and the number of each subatomic particle in that isotope are shown below:
- Carbon-12 (or 12C) has the atomic number 6 and mass number 12 (six protons and six neutrons). It contains six protons, six neutrons, and six electrons
- Carbon-13 (or 13C) has the atomic number 6 and mass number 13 (six protons and seven neutrons). It contains six protons, seven neutrons, and six electrons.
- Carbon-14 (or 14C) has the atomic number 6 and mass number 14 (six protons and eight neutrons). It contains six protons, eight ne