Chapter 4, Section 3

Isotopes

Figure 4.9 shows that there are three different kinds of neon atoms. How do these atoms differ? All have the same number of protons (10) and electrons (10), but they each have different numbers of neutrons. Isotopes are atoms that have the same number of protons but different numbers of neutrons.  Because isotopes of an element have different numbers of neutrons, they also have different mass numbers. Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons, which are the subatomic particles responsible for chemical behavior.

Figure 4.9

There are three known isotopes of hydrogen. Each isotope of hydrogen has one proton in its nucleus. The most common hydrogen isotope has no neutrons. It has a mass number of 1 and is called hydrogen-1

or simply hydrogen. The second isotope has one neutron and a mass number of 2. It is called either hydrogen-2

or deuterium. The third isotope has two neutrons and a mass number of 3. This isotope is called hydrogen-3

or tritium.

4.2 Writing Chemical Symbols of Isotopes

Problem-Solving 4.20 Solve Problem 20 with the help of an interactive guided tutorial.