Chapter 4, Section 3
Mass Number
You know that most of the mass of an atom is concentrated in its nucleus and depends on the number of protons and neutrons. The total number of protons and neutrons in an atom is called the mass number. Look again at Table 4.2 and note the mass numbers of helium and carbon. A helium atom has two protons and two neutrons, so its mass number is 4. A carbon atom, which has six protons and six neutrons, has a mass number of 12.
If you know the atomic number and mass number of an atom of any element, you can determine the atom’s composition. Table 4.2 shows that an oxygen atom has an atomic number of 8 and a mass number of 16. Because the atomic number equals the number of protons, which equals the number of electrons, an oxygen atom has eight protons and eight electrons. The mass number of oxygen is equal to the number of protons plus the number of neutrons. The oxygen atom, then, has eight neutrons, which is the difference between the mass number and the atomic number (16 − 8 = 8). The number of neutrons in an atom is the difference between the mass number and atomic number.
Number of neutrons = mass number − atomic number
The composition of any atom can be represented in shorthand notation using atomic number and mass number. Figure 4.8 shows how an atom of gold is represented using this notation. The chemical symbol Au appears with two numbers written to its left. The atomic number is the subscript. The mass number is the superscript.
Figure 4.8 Au is the chemical symbol for gold. Applying Concepts How many electrons does a gold atom have?
You can also refer to atoms by using the mass number and the name of the element. For example,
4.1 Determining the Composition of an Atom
Problem-Solving 4.17 Solve Problem 17 with the help of an interactive guided tutorial.