Isotopes: Understanding Atomic Number and Mass

Isotopes are defined based on specific properties related to their atomic structure. Let's break down the definition:

Key Characteristics of Isotopes

• Same Atomic Number: Isotopes of an element always have the same number of protons. The number of protons determines the element's identity and is represented by the atomic number (Z).
• Different Atomic Mass: Isotopes differ in their number of neutrons. Since the atomic mass (A) is the sum of protons and neutrons ($A = Z + N$), a different number of neutrons results in a different atomic mass.

Analysis of Options

Based on the definition:

• Option 1 is incorrect because isotopes have the same atomic number.
• Option 2 is incorrect because isotopes have the same atomic number.
• Option 3 is incorrect because isotopes have different atomic masses.
• Option 4 is correct because it accurately states that isotopes have the same atomic number but different atomic mass.

In summary, isotopes are variants of a chemical element distinguished by having the same number of protons but a different number of neutrons in their nucleus, leading to variations in atomic mass.