relative mass explained

B2. RELATIVE MASSES OF ATOMS, MOLECULES & FORMULA UNITS

2.1 THE KILOGRAM

The mass of any object expressed in any unit is measured relative to some STANDARD,
e.g., until just recently, the universal standard of mass was a block of Pt-Ir alloy, the

Formerly the global standard for the exact mass of 1 kg, the IPK is kept under lock and key at Saint-Cloud, Sèvres, Paris (FR).

standard kilogram (IPK - International Prototype Kilogram), kept under carefully controlled conditions in a glass case at Sèvres,(FR).

That this block of alloy is slowly shrinking was just one of the reasons why, in 2018, BIPM re-defined the kilogram in terms of a now fixed numerical value of the Planck constant, which is itself linked by updated definitions of the second and the metre based on invariable, atomic-scale properties.

In the everyday arena of a greengrocers, however, one buys three kilograms of potatoes: this is three times the mass of the standard kilogram and there are two different but nonetheless equivalent ways of expressing these:

‘the mass of the potatoes is 3 kg’;

‘on the kilogram scale the relative mass of the potatoes is 3’.

discovery of isotopes

mononuclidic elements

Ar from abundances

relative isotopic mass