Define mass number.

At the center of every atom lies the atomic nucleus, a small yet dense region that houses the core components critical to the identity of an atom: protons and neutrons. This tiny powerhouse is pivotal for understanding the atomic structure and is essential in defining the element's characteristics.

The atomic nucleus is more than millions of times smaller than the atom itself, but it contains virtually all the atom's mass. Protons, which carry a positive charge, define the atomic number of the element and therefore its place on the periodic table. Neutrons, on the other hand, are neutral and add to the mass of the nucleus without affecting the atomic number.

A fascinating aspect of the nucleus is the strong nuclear force that holds protons and neutrons together in such a compact space despite the repulsive force among positively-charged protons. Without this force, the nucleus, and atoms as we know them, couldn't exist.

Delving into the composition of an atomic nucleus, we encounter protons and neutrons, commonly known as nucleons. These subatomic particles are heftier than electrons and play a significant role in determining the properties of an element.

Each proton carries a single positive charge and determines the element's identity by defining its atomic number – for example, hydrogen has one proton, hence an atomic number of 1. Neutrons, though charge-neutral, have a mass similar to protons and are crucial in stabilizing the nucleus by offsetting the electrostatic repulsion between protons.

The mass number is the sum of protons and neutrons in the nucleus and provides a glimpse into the isotopic nature of an element. By understanding these two particles and their interaction, we unlock the mystery of elements' diversity and the range of their atomic masses.

When we speak of isotopes, we're referring to the versions of an element that have the same number of protons but different numbers of neutrons in their nuclei. This variance in neutron count leads to different mass numbers, and hence the term 'isotopes' arises – 'iso' meaning same and 'topos' meaning place, as they occupy the same position on the periodic table.

Isotopes play a critical role in various fields, from medicinal applications, where they're used in imaging and treatment, to archaeology, through radiocarbon dating. Some isotopes are stable, while others are radioactive, meaning they decay over time, emitting radiation.

It is crucial to appreciate that isotopes of an element behave nearly identically in chemical reactions, as the chemical behavior is governed mostly by electrons. However, their physical properties, such as melting point and density, may differ due to the difference in mass number.