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About these units
Atomic Mass Unit (u)
The atomic mass unit (u) is defined as exactly 1/12 the mass of a neutral carbon-12 atom, which makes it approximately 1.66053906660 × 10⁻²⁷ kilograms. This definition allows scientists to express atomic and molecular masses in a convenient, intuitive scale that aligns with the structure of the periodic table. By anchoring the unit to carbon-12, chemists and physicists gained a universal reference point that harmonizes molecular weight, relative isotopic abundance, and mass spectroscopy results. The atomic mass unit effectively normalizes the complexity of atomic masses into whole-number or near-whole-number values for most nuclei. In chemistry, u is indispensable for calculating molar masses, reaction stoichiometry, and isotopic compositions. It is also used across nuclear physics, astrophysics, and biophysics, demonstrating the universality of atomic-scale measurement.
Stone (st)
The stone equals 14 pounds, or approximately 6.35029 kilograms, and is commonly used in the United Kingdom and Ireland for expressing human body weight. It has deep traditional roots, originating from stones used as counterweights in early commerce. Although largely replaced by metric units in most applications, the stone remains emotionally and culturally significant for personal weight expression in the UK. Many people find it more intuitive and relatable than kilograms due to long-standing social habits. The stone's endurance highlights how measurement systems reflect cultural identity as much as mathematical convenience. It is one of the few units still commonly used in conversation but not in scientific or industrial contexts.