5.1.2 The Nucleus - Isotopes

What is an Isotope?

An isotope is a form of an element that has the same number of protons but a different number of neutrons in its nucleus. This means isotopes of the same element have the same atomic number but different mass numbers.

The atomic number (Z) defines the element. The mass number (A) is the sum of protons and neutrons (A = Z + N, where N is the number of neutrons).

Isotopes of an Element

It is a fundamental concept in nuclear physics that an element may have more than one isotope. This is because the number of neutrons in the nucleus can vary even though the number of protons remains constant.

For example, Carbon (C) always has 6 protons (atomic number 6). However, Carbon-12 ($^{12}C$) has 6 protons and 6 neutrons, while Carbon-14 ($^{14}C$) has 6 protons and 8 neutrons. Both are isotopes of Carbon.

Table of Isotopes Example

Isotope	Atomic Number (Z)	Mass Number (A)	Number of Protons	Number of Neutrons
Carbon-12 ($^{12}C$)	6	12	6	6
Carbon-13 ($^{13}C$)	6	13	6	7
Carbon-14 ($^{14}C$)	6	14	6	8

Key takeaway: Isotopes of an element have the same chemical properties because they have the same number of electrons. However, they have different physical properties, such as mass and nuclear stability.