Atomic Weight / Atomic Number
Also: Proton number, Relative atomic mass, Mass number, Nuclear charge number
The atomic number indicates the number of protons of an element in the periodic table; 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 characteristic quantities of every chemical element. The atomic number – also known as the nuclear charge number or proton number – determines the unique position of an element in the periodic table and completely governs its chemical behaviour. The atomic weight gives the average 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 unambiguously 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 for authenticity testing in the Archimedes density determination.
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) – are closely grouped 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 Relationship
The atomic weight influences density and thereby indirectly the fine weight of a bar or coin, but it is not synonymous with it. Fine weight describes the pure metal content of an object, independent of its atomic mass. In the melt value calculator, atomic weight feeds in implicitly through 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 through 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 mimic gold (Z = 79, density 19.32 g/cm³) in terms of density – but XRF and ultrasound testing reliably distinguish both, since the characteristic X-ray lines are directly tied to the atomic number and cannot be forged.
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 from gold, platinum, or silver differ in weight, and forms the physical basis of modern authenticity testing methods.