What Is the SI Periodic Table and How Is It Defined?

The term “SI Periodic Table” blends two distinct but deeply connected pillars of modern science. The SI, short for Système International d’Unités (International System of Units), is the globally agreed-upon measurement framework that defines units like the kilogram, the second, and the mole. The periodic table is the organized chart of chemical elements maintained by the International Union of Pure and Applied Chemistry (IUPAC). Where they meet is chemistry: the mole, one of the SI’s seven base units, is the bridge that links the atomic-scale world of the periodic table to the macroscopic measurements scientists and engineers rely on every day. Understanding how the SI is defined, especially after its landmark 2019 overhaul, sheds light on why the numbers printed on every periodic table carry the precision they do.

What the SI System Is and Why It Matters

The International System of Units is the measurement standard used by virtually every country and scientific discipline on Earth. It covers everything from the length of a fiber-optic cable to the temperature inside a fusion reactor. Since 2019, all seven of the SI’s base units have been defined not by physical objects sitting in a vault but by giving fixed numerical values to seven fundamental constants of nature. That shift was approved by the General Conference on Weights and Measures in 2018 and took effect on May 20, 2019, which is World Metrology Day.¹

The seven base units are the second (time), the metre (length), the kilogram (mass), the ampere (electric current), the kelvin (thermodynamic temperature), the mole (amount of substance), and the candela (luminous intensity). Before 2019, one of those units, the kilogram, was still tied to a single physical artifact: a platinum-iridium cylinder kept under three glass bell jars near Paris since 1889. The rest had already migrated to definitions rooted in physics, but that lone cylinder was a glaring holdout. The 2019 revision finally cut the last link to any physical object, making the entire system traceable to constants that are, as far as physicists can tell, the same everywhere in the universe.²

The Seven Defining Constants

Each base unit is now anchored to a specific constant whose numerical value has been fixed by international agreement. The approach is sometimes called “explicit-constant definition”: instead of saying “a kilogram is this particular lump of metal,” you say “the Planck constant has exactly this numerical value when expressed in SI units, and the kilogram is whatever mass makes that equation true.” The distinction between base and derived units still exists for teaching purposes, but it is no longer strictly necessary from a theoretical standpoint. Fixing seven constants is enough to define every SI unit, base or derived.³

The seven constants and the units they primarily anchor are:

Cesium hyperfine frequency: defines the second. The frequency of a specific transition in a cesium-133 atom is fixed at exactly 9,192,631,770 cycles per second.
Speed of light in vacuum: defines the metre. Fixed at exactly 299,792,458 metres per second.
Planck constant: defines the kilogram. Fixed at exactly 6.626 070 15 × 10⁻³⁴ joule-seconds.
Elementary charge: defines the ampere. Fixed at exactly 1.602 176 634 × 10⁻¹⁹ coulombs.
Boltzmann constant: defines the kelvin. Fixed at exactly 1.380 649 × 10⁻²³ joules per kelvin.
Avogadro constant: defines the mole. Fixed at exactly 6.022 140 76 × 10²³ per mole.
Luminous efficacy: defines the candela. Fixed at exactly 683 lumens per watt for a specific frequency of light.

Because these constants now have exact values, many other physical constants either became exact or had their uncertainties shrink dramatically. The 2018 CODATA adjustment of fundamental constants reflected this: the elementary charge, the Boltzmann constant, and the Avogadro constant all became exact, and a cascade of dependent constants became more precisely known as a result.⁴

How the Kilogram Got Rid of Its Artifact

The kilogram’s makeover is the most dramatic piece of the 2019 story and the easiest to explain. For over a century, the kilogram was whatever the International Prototype of the Kilogram (a polished cylinder kept at the Bureau International des Poids et Mesures outside Paris) happened to weigh. If that cylinder gained a few micrograms of contamination, the kilogram technically got heavier. If it lost atoms during cleaning, the kilogram got lighter. The entire measurement chain for mass worldwide depended on periodic comparisons against that single object and its official copies.

Replacing it required an instrument called a Kibble balance (named after the British physicist Bryan Kibble). A Kibble balance relates mass to the Planck constant through precise measurements of electrical power and velocity. By fixing the Planck constant’s numerical value, you turn the Kibble balance into a device that can realize the kilogram from scratch in any sufficiently equipped lab. The Bureau International des Poids et Mesures (BIPM) built its own Kibble balance and demonstrated that it could measure a 1 kg platinum-iridium artifact in terms of the Planck constant.⁵ National labs in other countries, including the Korea Research Institute of Standards and Science (KRISS), built independent Kibble balances that also realize the kilogram through the Planck constant, using both the Kibble balance method and an alternative technique involving ultra-pure silicon crystal spheres.⁶

For everyday life, nothing changed on May 20, 2019. Your bathroom scale did not need recalibrating. But for scientists measuring masses at microgram or nanogram precision, the new definition means they no longer depend on a single artifact’s stability. Any lab that builds a Kibble balance (or grows a nearly perfect silicon sphere and counts atoms) can independently realize the kilogram.

The Second and Its Future Redefinition

The unit of time is arguably the most precisely defined of all SI units, and it was the first to be anchored to a quantum property. Since 1967, the second has been defined by the hyperfine transition of the cesium-133 atom: one second equals exactly 9,192,631,770 periods of the radiation produced by that transition.⁷ Cesium fountain clocks currently realize this definition with uncertainties on the order of a few parts in ten thousand trillion.

That sounds incredibly precise, and it is. But optical atomic clocks, which operate at much higher frequencies using atoms like strontium, ytterbium, or aluminum, have already surpassed cesium clocks in stability by orders of magnitude. A strontium lattice clock, for example, has demonstrated stability at the level of 10⁻¹⁹, meaning it would neither gain nor lose a second over a timescale longer than the age of the universe.⁸ These clocks are so sensitive that they can detect the difference in the flow of time between two points separated by just a few centimeters of altitude, because general relativity says time passes slightly faster farther from Earth’s center.

The metrological community is actively working toward a redefinition of the second based on an optical transition, likely sometime in the late 2020s or early 2030s. When that happens, the SI will gain another leap in precision, and applications in navigation, telecommunications, and fundamental physics will benefit directly.

The Kelvin and the Boltzmann Constant

Before 2019, the kelvin was defined by the triple point of water: the exact temperature and pressure at which water can exist simultaneously as ice, liquid, and vapor, set by definition at exactly 273.16 kelvins. This was another artifact-like definition in the sense that it depended on the properties of a particular substance (highly purified water of a specific isotopic composition). The new definition fixes the Boltzmann constant at exactly 1.380 649 × 10⁻²³ joules per kelvin, which decouples the kelvin from any material reference.⁹

The practical consequence is subtle but real. The triple point of water is still used as a convenient calibration reference, and its temperature under the new system has an uncertainty of about 0.10 millikelvins, which is small enough that existing thermometers and calibration procedures did not need to change. The advantage shows up at extreme temperatures. At very high temperatures (inside furnaces, plasma experiments, or stars) and very low temperatures (near absolute zero in quantum-computing labs), the old water-based definition was awkward and limited. The new definition is valid across the full temperature range without special treatment at any point.

Where the SI Meets the Periodic Table

The periodic table and the SI intersect most directly through two things: the mole and standard atomic weights. The mole is the SI unit of “amount of substance,” and its 2019 redefinition was particularly meaningful for chemists. Previously, the mole was defined as the number of atoms in exactly 12 grams of carbon-12, which tied it to the kilogram (and therefore, indirectly, to that platinum cylinder in Paris). Now the mole is simply defined by fixing the Avogadro constant at exactly 6.022 140 76 × 10²³ entities per mole. The mole no longer depends on any particular element or any mass artifact.¹

This raised an interesting philosophical point that metrologists debated for years before the revision. The Avogadro constant is not a fundamental constant of nature in the way that the speed of light or the Planck constant is. It is a scaling factor, a human-chosen bridge between the atomic scale and the macroscopic scale. Fixing its value by convention is a deliberate choice to make the mole a clean, convenient unit for chemistry, rather than a reflection of some deep physical truth.¹⁰

The numbers printed beneath each element’s symbol on a standard periodic table are standard atomic weights, and those are maintained by IUPAC’s Commission on Isotopic Abundances and Atomic Weights. The commission regularly evaluates new isotopic-abundance measurements and publishes updated values in the Table of Standard Atomic Weights. In 2017, the commission changed the way it expresses uncertainty in those values, moving to a “±” notation to comply with the Guide to the Expression of Uncertainty in Measurement (GUM). The goal is that any scientist taking any natural sample of an element from commerce or research can expect the actual atomic weight of that sample to fall within the published standard atomic-weight value plus or minus its stated uncertainty.¹¹

So while the periodic table itself is not “part of the SI” in the formal sense, the measurement infrastructure underlying every number on it depends on SI definitions. The atomic masses are expressed in unified atomic mass units (related to the kilogram through the Avogadro and Planck constants), and the mole connects those atomic masses to the gram-scale quantities chemists weigh in their labs. When the SI was revised, the mole’s new definition propagated into how metrology for chemical measurements works, reinforcing the importance of accurate chemical standards.¹²

Why Constants Instead of Artifacts

The move from artifact standards to constant-based definitions was not a sudden decision. It unfolded over half a century. The metre was redefined in terms of the speed of light in 1983. The second was tied to the cesium atom in 1967. Each redefinition followed the same logic: physical artifacts drift, wear, and are accessible in only one place. A constant of nature, on the other hand, does not change, and any lab equipped to measure it can independently realize the unit without shipping a reference object around the world.²

The practical benefits are significant. Before 2019, if your national kilogram copy drifted relative to the international prototype, you might not know for years, until the next round of comparisons. Now any country that builds a Kibble balance or silicon-sphere experiment can check its mass standards independently. The same principle applies to every other unit: the definitions are universally accessible because they rest on physics, not on a particular object stored in a particular building.

There is also a future-proofing element. If a new experiment reveals that some constant was measured slightly off before it was fixed, the constant’s numerical value stays the same by definition and the affected unit shifts slightly in absolute terms. This sounds unsettling, but the shifts involved are far smaller than any practical application would notice. Meanwhile, the framework stays coherent and internally consistent, which is the whole point of a measurement system.

Bringing SI Traceability Out of the Lab

One of the more ambitious consequences of the constant-based SI is the possibility of miniaturized, deployable measurement standards. The U.S. National Institute of Standards and Technology (NIST) is developing a program called “NIST on a Chip,” which aims to create small, portable sensors based on fundamental properties of nature. Because these sensors derive their measurements directly from quantum phenomena tied to SI-defining constants, they are inherently traceable to the SI without needing to be compared against a centralized artifact. The program covers pressure, vacuum, temperature, and other quantities, with the broader goal of enabling accurate, SI-traceable measurements outside national metrology institutes entirely.¹³

NIST calls this approach “QSI,” or Quantum-based International System of units. The vision is that a factory floor, a remote environmental monitoring station, or even a spacecraft could carry a chip-scale device that performs measurements traceable to the SI through quantum standards, with no calibration chain leading back to a national lab. This is only possible because the SI’s definitions no longer require physical reference objects. When your unit of mass is defined by the Planck constant rather than a metal cylinder, a device that exploits quantum mechanics to link a measurement to the Planck constant is, in principle, just as authoritative as the most expensive balance in any national metrology institute.

How Atomic Weights on the Periodic Table Can Vary

One thing that surprises people about the periodic table is that the atomic weight listed for many elements is not a single fixed number but a range. Lithium, for instance, has a standard atomic weight given as an interval rather than a point value, because the relative abundance of its isotopes varies depending on where the sample comes from. A lithium sample from a Chilean salt flat may have a measurably different atomic weight than one from a North Carolina pegmatite deposit. IUPAC’s commission accounts for this by evaluating measurements from a broad cross-section of natural materials and reporting values, along with consensus uncertainties, that should encompass what you would find in any commercially or scientifically available sample.¹¹

The commission also serves as a gatekeeper against unreliable data. If a published measurement claims an implausibly narrow uncertainty for an element’s isotopic composition, the commission may decline to incorporate it. The result is a table that is conservative by design: the listed uncertainties are “decisional” rather than purely statistical, reflecting the commission’s collective judgment about how much variation real-world samples actually show. This cautious approach is why the atomic weights on a periodic table printed in 2025 may differ slightly from one printed in 2010. New measurements from better instruments or previously unstudied sample types can nudge the values.

Optical Clocks and What Comes After the Cesium Second

The cesium-based definition of the second has served remarkably well for nearly six decades, but its days are numbered. Optical clocks tick at frequencies hundreds of thousands of times higher than the microwave frequency of cesium, which gives them correspondingly finer resolution. Multiple national labs around the world are now operating strontium and ytterbium lattice clocks with performance that exceeds the best cesium fountains by roughly two orders of magnitude. The challenge is no longer building a clock that good but agreeing internationally on which optical transition to use, ensuring that clocks in different countries agree with each other at the required level, and establishing practical methods for comparing clocks over long distances using fiber-optic links or satellite transfers.

When the second is eventually redefined, it will follow the same template as the 2019 kilogram revision: a fixed numerical value will be assigned to the frequency of the chosen optical transition, and the second will become whatever duration makes that equation true. The ripple effects will be subtle for most people but profound for fields that depend on ultra-precise timing, including satellite navigation, gravitational-wave detection, and tests of whether fundamental constants drift over cosmological timescales.

1
PubMed Central. How to Define the Units of the Revised SI Starting from Seven Constants with Fixed Numerical Values
2
PubMed. The route to atomic and quantum standards
3
Metrologia. The revision of the SI—the result of three decades of progress in metrology
4
PubMed Central. CODATA recommended values of the fundamental physical constants: 2018
5
Metrologia. The BIPM Kibble balance for realizing the kilogram definition
6
Metrologia. Realization of the kilogram using the KRISS Kibble balance
7
Annalen der Physik. The Hyperfine Transition for the Definition of the Second
8
arXiv. A zero-dead-time strontium lattice clock with a stability at $10^{-19}$ level
9
Metrologia. The Boltzmann project
10
Metrologia. On the role of the Avogadro constant in redefining SI units for mass and amount of substance
11
PubMed Central. The Table of Standard Atomic Weights-An exercise in consensus
12
ACS Publications (Analytical Chemistry). Redefinition of the Mole in the Revised International System of Units and the Ongoing Importance of Metrology for Accurate Chemical Measurements
13
Sensor and Measurement Science International. NIST on a Chip: Photonic and Quantum-Based Sensors for Measurements of Pressure, Vacuum, Temperature and Beyond!