Explain what is meant by an isotope and state that an element may have more than one isotope

5.1.2 The Nucleus

Isotopes – Definition

An isotope is a variant of a chemical element that has the same number of protons (\$Z\$) but a different number of neutrons. Consequently, isotopes of the same element share the same atomic number but have different mass numbers (\$A = Z + N\$), where \$N\$ is the number of neutrons.

Key Points

• All atoms of an element have the same atomic number \$Z\$ (same number of protons).
• Isotopes differ in their neutron number \$N\$, giving different mass numbers \$A\$.
• Because chemical behaviour is determined by the electron configuration (which depends on \$Z\$), isotopes of an element exhibit essentially the same chemical properties.
• Physical properties such as mass, density, and nuclear stability can vary between isotopes.

Example: Carbon Isotopes

Carbon (\$Z = 6\$) has several naturally occurring isotopes. The most common are shown in the table below.

Why an Element Can Have More Than One Isotope

During nuclear reactions or natural processes, neutrons can be added to or removed from a nucleus without changing the number of protons. This creates nuclei with the same \$Z\$ but different \$N\$, giving rise to multiple isotopes of the same element. The stability of each isotope depends on the balance between the attractive nuclear force and the repulsive electrostatic force among protons.

Key Statement for Exam Answers

When answering exam questions, you can state:

“An isotope is a form of an element that has the same number of protons but a different number of neutrons, giving it a different mass number. Therefore, an element may have more than one isotope.”