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. This means isotopes of an 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 in the nucleus.

For example, consider Carbon. Carbon always has 6 protons (Z=6). However, it can exist in three different forms (isotopes) with 6 neutrons, 7 neutrons, and 8 neutrons, giving mass numbers of 12, 13, and 14 respectively.

Isotopes of an Element

It is a fundamental concept in chemistry and 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.

The notation for an isotope includes the element symbol, the mass number (A) written as a superscript to the left of the symbol, and the atomic number (Z) written as a subscript to the left of the symbol.

For example:

• Carbon-12: $^{12}_{6}C$
• Carbon-13: $^{13}_{6}C$
• Carbon-14: $^{14}_{6}C$

Table summarizing Isotopes

Isotope	Mass Number (A)	Atomic Number (Z)	Protons	Neutrons
Carbon-12	12	6	6	6
Carbon-13	13	6	6	7
Carbon-14	14	6	6	8

The relative abundance of different isotopes of an element varies. Some isotopes are stable, while others are radioactive. Radioactive isotopes are used in various applications, such as carbon dating.