The “SI Periodic Table” is a designation for the modern Periodic Table that fully integrates the International System of Units (SI) for all its quantitative data. The Periodic Table remains the fundamental organization of elements based on their atomic number and electron configuration. The SI designation signifies that all values, such as atomic weights, are derived from and consistent with the world’s most rigorous measurement standards. This standardization ensures global consistency and high precision for modern chemistry, physics, and international commerce.
Defining the International System of Units
The International System of Units (SI) is the globally accepted standard for measurement. Its purpose is to provide a unified and coherent system of units that eliminates ambiguity in scientific and technical work. The SI system is built upon seven base units: the meter (length), second (time), kilogram (mass), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity).
Historically, many units were defined by physical artifacts, such as the International Prototype Kilogram. This reliance on physical objects introduced the risk of drift or damage, compromising measurement stability. The 2019 redefinition of the SI established that all seven base units are now defined by fixing the numerical values of seven fundamental physical constants of nature.
For example, the second is defined by the fixed frequency of radiation corresponding to a specific transition in the cesium-133 atom, and the meter is defined by the fixed speed of light in a vacuum. By linking units to unchanging constants, the SI provides a universal and stable reference for all measurements. This shift ensures that the units can be realized anywhere in the world with the highest possible accuracy.
Standardizing Elemental Mass Measurements
The consistency of the non-integer mass values—the standard atomic weights—is directly established by the SI system’s definition of mass. Every atomic mass is measured relative to the unified atomic mass unit (\(u\)), also known as the dalton (Da). The unified atomic mass unit is defined as exactly one-twelfth of the mass of a single, unbound carbon-12 atom in its ground state.
The value of the unified atomic mass unit is fixed by the SI definition of the kilogram, the base unit of mass. The kilogram (kg) is defined in terms of the fixed value of the Planck constant (\(h\)), a fundamental constant of quantum mechanics. This definition ensures that the kilogram is no longer tied to a physical object, making the mass standard fundamentally stable and reproducible.
Because the kilogram is precisely defined by \(h\), the mass of the carbon-12 atom and thus the value of the unified atomic mass unit are traceable to this same constant. This traceability means the relative atomic masses listed on the SI Periodic Table are globally consistent and connected to the fundamental laws of physics. Atomic weights are continuously reviewed and updated by bodies like the International Union of Pure and Applied Chemistry (IUPAC) to reflect the highest-accuracy measurements.
The standard atomic weight for an element like silicon (Si), which is approximately \(28.085 u\), represents the weighted average of the atomic masses of all its naturally occurring isotopes. This value reflects the relative abundance of isotopes like silicon-28, silicon-29, and silicon-30 in typical terrestrial samples. The SI framework guarantees that this value, \(28.085 u\), is perfectly coherent with the SI kilogram standard.
The Mole and the Redefined Periodic Table
The mole (mol) is the SI base unit for the amount of substance, measuring the number of specified elementary entities, such as atoms or molecules. Before 2019, the mole was defined as the amount of substance that contained as many elementary entities as there were atoms in exactly \(0.012\) kilogram of carbon-12. This definition meant the Avogadro constant (\(N_A\)) was an experimentally determined value with slight uncertainty.
The 2019 SI redefinition fundamentally changed this by fixing the Avogadro constant to an exact number: \(6.02214076 \times 10^{23}\) reciprocal moles (\(\text{mol}^{-1}\)). The mole is now defined by this fixed numerical value, meaning one mole contains exactly that number of entities. This change transformed the mole from a unit dependent on the mass of a physical object to a unit dependent on a fixed, defined constant.
A significant consequence of fixing \(N_A\) is that the molar mass—the mass of one mole of a substance—is now perfectly consistent with the standard atomic weights listed on the Periodic Table. For example, the molar mass of carbon-12 is now known to be very close to, but no longer exactly, \(12\) grams per mole. This minor difference is negligible for practical purposes but represents a profound shift toward basing all chemical quantities on defined constants. This final step cemented the Periodic Table’s status as fully integrated with the SI, ensuring unprecedented measurement stability.