1.2 Atomic Structure and Isotopes

Understanding the basic building blocks of matter is the first step to comprehending radiation science.

The Structure of an Atom

All matter is made up of tiny particles called **atoms**. An atom consists of a dense, central **nucleus** surrounded by a cloud of negatively charged **electrons**. The nucleus itself is composed of positively charged **protons** and neutral **neutrons**.

Protons ($p^+$): Found in the nucleus, they have a positive charge. The number of protons determines the element (e.g., all carbon atoms have 6 protons).
Neutrons ($n^0$): Also in the nucleus, they have no charge. Their number can vary within an element, leading to isotopes.
Electrons ($e^-$): Orbiting the nucleus, they have a negative charge. In a neutral atom, the number of electrons equals the number of protons.

Atomic Number (Z) and Mass Number (A)

To describe an atom, two key numbers are used:

Atomic Number ($Z$): This is the number of protons in the nucleus. It is unique to each element and determines its chemical properties.
Mass Number ($A$): This is the total number of protons and neutrons in the nucleus. It represents the atom's approximate mass.

What Are Isotopes?

**Isotopes** are variants of a particular element. They have the same number of protons (and thus the same atomic number $Z$), but a different number of neutrons. This means they have the same chemical properties but different atomic masses.

For example, Carbon has a stable isotope, Carbon-12 ($^{12}$C, 6 protons, 6 neutrons), and a radioactive isotope, Carbon-14 ($^{14}$C, 6 protons, 8 neutrons). Both are carbon because they have 6 protons, but their different neutron counts make them distinct isotopes.

Isotopes that are unstable and decay by emitting radiation are known as **radionuclides**.