Atomic Weight / Atomic Number
Also: proton number, relative atomic mass, mass number, nuclear charge number
The atomic number gives the proton count of an element in the periodic table, while the atomic weight (relative atomic mass) describes the average mass of its atoms in atomic mass units.
Atomic number and atomic weight (also: relative atomic mass) are two fundamental characteristics of every chemical element. The atomic number — also nuclear charge number or proton number — fixes the unique place of an element in the periodic table and fully determines its chemical behaviour. The atomic weight gives the mean mass of an atom in atomic mass units (u, formerly "amu"), averaged over the naturally occurring isotopes of an element.
Significance for precious metals
Every precious metal is uniquely defined by its atomic number. Gold (Au) carries atomic number 79 — its nucleus contains 79 protons — and has a relative atomic mass of 196.967 u. This high mass is partly responsible for the exceptional density of gold (19.32 g/cm³), which is used in density measurement after Archimedes for authenticity testing.
The platinum group metals — platinum (Pt, Z = 78), palladium (Pd, Z = 46), rhodium (Rh, Z = 45), iridium (Ir, Z = 77), osmium (Os, Z = 76) and ruthenium (Ru, Z = 44) — sit close together in the periodic table and therefore share similar physico-chemical properties such as high corrosion resistance and catalytic activity.
Atomic weight and fine weight — no direct link
The atomic weight influences the density and therefore indirectly the fine weight of a bar or coin, but is not equivalent to it. The fine weight describes the pure metal content of an object, independent of atomic mass. In the melt value calculator, the atomic weight enters implicitly via the density when volume measurements are used to estimate weight.
Formula: relative atomic mass (u) = Σ (isotope mass × natural abundance)
Example gold: 196.967 u (only 1 stable isotope: ¹⁹⁷Au, abundance 100%)
Atomic number as an authenticity feature
Analytical methods such as X-ray fluorescence analysis (XRF) identify elements precisely via their characteristic X-ray radiation, which results directly from the atomic number (electron configuration). A tungsten core (Z = 74, density 19.25 g/cm³) can deceptively imitate gold (Z = 79, density 19.32 g/cm³) in density — XRF and ultrasonic testing distinguish the two reliably, since the characteristic X-ray lines are bound directly to the atomic number and cannot be faked.
In brief
The atomic number is an element's atomic identity — unchangeable and counterfeit-proof. The atomic weight explains why equally sized bars of gold, platinum or silver differ in weight, and forms the physical basis of modern authenticity-testing methods.