Atomic Weight / Atomic Number
Also: proton number, relative atomic mass, mass number, nuclear charge number
The atomic number states the number of protons 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 called the nuclear charge number or proton number – fixes the unique position of an element in the periodic table and completely 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 unmistakably defined by its atomic number. Gold (Au) carries the 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 the Archimedes density measurement 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) – lie close together in the periodic table and therefore share similar physical and 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 thus indirectly the fine weight of a bar or coin, but it is not synonymous with it. The fine weight describes the pure metal content of an object, independent of the 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 imitate gold (Z = 79, density 19.32 g/cm³) deceptively in terms of density – XRF and ultrasonic testing distinguish the two reliably, because the characteristic X-ray lines are directly tied to the atomic number and cannot be faked.
In brief
The atomic number is the atomic identity of an element – immutable and forgery-proof. The atomic weight explains why bars of equal size made of gold, platinum or silver weigh differently, and forms the physical basis of modern authenticity testing methods.