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The journey to understanding the core of matter begins with the atomic structure. At the heart of each atom is a nucleus, a tiny central core that holds most of the atom's mass. Electrons, nearly 2,000 times lighter than the smallest nucleus particle, orbit this nucleus. This pattern is somewhat analogous to planets revolving around the sun, but governed by the forces of quantum mechanics rather than gravity.

The nucleus itself is composed of protons and neutrons, with the former carrying a positive charge and the latter being neutral. The number of protons determines the element's identity – carbon atoms have six protons, oxygen atoms have eight, and so on. The arrangement of electrons around the nucleus dictates how atoms will interact with each other, forming the building blocks of molecules and, by extension, the material world around us.

Protons and neutrons form the bulk of an atom's mass and reside within the nucleus. Protons are positively charged particles, and their number in an atom is what defines the atomic number – it's essentially the ID card of an element. Neutrons, on the other hand, have no electric charge but play a crucial role in stabilizing the nucleus. Without neutrons, protons would repel each other due to their like charges, causing the nucleus to break apart. The closely packed protons and neutrons are held together by a force much stronger than the repulsion between the positive charges — this is known as the strong nuclear force.

Different isotopes of an element have the same number of protons but vary in the number of neutrons, which can affect the atom's stability and some physical properties. This subtle balance between protons and neutrons is what makes each element unique and leads to a vast diversity of matter in the universe.

Nucleons are the collective name for protons and neutrons, the particles that make up the atomic nucleus. Understanding nucleons is crucial when it comes to differentiating between atomic mass and mass number. While the electrons do play a role in the atom's mass, their contribution is so minute compared to nucleons that they are often overlooked in calculations pertaining to atomic mass.

The mass number, which is always a whole number, is simply the tally of nucleons in an atom's nucleus. It serves as a quick reference to approximate an atom's mass. Let's think of the nucleus as a 'bowl of nucleons' - by counting the protons and neutrons (the pieces of 'nuclear fruit'), we get the mass number. But to know the precise mass of the atom, we would have to 'weigh the bowl' with all its contents, including the 'lightweight electrons' surrounding the nucleus.

Delving into physical chemistry allows us to understand how fundamental principles govern the behavior and characteristics of matter. It's the terrain where physics meets chemistry, providing insight into the ways atomic-level interactions manifest in the physical world - be it through the form of material properties or chemical reactions.

In the context of atomic mass versus mass number, physical chemistry involves the study of isotopes and atomic weights. It's a discipline where knowledge of mass spectrometry, a technique used to measure the mass of atoms and molecules with high precision, becomes invaluable. Physical chemists seek to explain why certain isotopes are more stable than others based on mass differences and how these masses affect the physical properties of substances. By bridging the microscopic world of atoms and the macroscopic properties they confer to materials, physical chemistry illuminates the underpinnings of the periodic table and the essence of chemical compounds.