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Mass defect is a critical notion in the realm of nuclear physics, playing a pivotal role in understanding the binding energy of a nucleus. It can be somewhat perplexing, but let's simplify it. Imagine assembling a puzzle; the individual pieces symbolize protons and neutrons. The completed puzzle signifies the nucleus. Now, curiously, when you weigh the completed puzzle, it's lighter than the total weight of the individual pieces. This weight difference is what we call the 'mass defect'.

Mass defect is pivotal because it exhibits the mass converted into energy to bind the nucleons together in the nucleus. It's not lost but rather transformed in line with the concept of mass-energy equivalence. The calculation begins by noting the individual masses of protons and neutrons, then measuring the mass of the nucleus itself (which can often be found in tables of isotopic masses). The mass defect is the disparity between the total mass of protons and neutrons when separate and when they are part of the nucleus.

When discussing nuclear physics, it's important to understand that the 'nucleus mass' refers to the actual, observable mass of a nucleus of an atom. Contrary to the summed mass of its individual protons and neutrons (also termed nucleons), the nucleus mass, as previously noted with mass defect, is invariably less due to the binding energy involved.

The nucleus mass can be measured precisely using instruments like mass spectrometers. For the calculation of binding energy, accurate figures of the nucleus mass are paramount. In the case of textbook problems, these masses are typically provided. This differentiated mass from the sum of its constituent nucleons is proof of the binding energy's manifestation and is foundational to the stability of every atomic nucleus you encounter in nature.

Einstein's revolutionary equation, E=mc^2, is a cornerstone of modern physics, illustrating the interchangeability of mass and energy. This equation reveals that a tiny amount of mass can be converted into a significant amount of energy and vice versa. The 'E' stands for energy, 'm' for mass, and 'c' is the speed of light in a vacuum (about 299,792,458 meters per second).

For binding energy calculations, this equation is the magic formula that allows us to convert the mass defect into energy units, typically in megaelectronvolts (MeV). It explains why the nucleus mass being less than the combined mass of protons and neutrons results in a release of energy, which is the binding energy stabilizing the nucleus. Clearly understanding this equation is vital for students, as it is the turning point of many critical calculations in nuclear physics.

Binding energy per nucleon reveals a lot about the nucleus' stability. It is the quotient of the total binding energy of a nucleus and the total number of nucleons it contains. To digest this, consider it as a benchmark for stability—the higher the binding energy per nucleon, the more stable the nucleus. This concept is meaningful across various elements and isotopes, allowing scientists to compare which nuclei are more tightly bound.

When it's time to crunch the numbers, this ratio helps determine whether a nucleus will tend to bind together or if it might split apart or fuse with another nucleus - key to understanding nuclear reactions like fusion and fission. By diving into this measure, students gain insights into the relative strength of nuclear bonds across different elements. This is where the energy that powers stars, nuclear reactors, and even atomic bombs comes from. It's worth noting that the energy trends across the periodic table generally show an increase in binding energy per nucleon up to iron (Fe), beyond which it gradually decreases.