rontogram
An SI unit of mass equal to 10⁻²⁷ grams. Symbol: rg
rontogram: a unit for weighing individual atoms and molecules
A rontogram (symbol rg) is a unit of mass in the metric system equal to 10⁻²⁷ grams, or one septillionth of a gram. It was officially adopted by the International Bureau of Weights and Measures in 2022 as part of the SI base unit revision. The name derives from Wilhelm Röntgen, the physicist who discovered X-rays. At this scale, you are measuring the mass of individual atoms and small molecules: a single carbon-12 atom weighs approximately 20 rontograms, and a water molecule roughly 30 rontograms.
The rontogram exists because analytical chemistry and materials science increasingly demand precision at the atomic and molecular level. Mass spectrometry, for instance, routinely detects ions and neutral molecules in this range. A singly ionized oxygen atom weighs about 16 rontograms. Without a formal SI name and symbol, scientists and instrument makers had to rely on atomic mass units (u or amu), which are defined differently and tied to carbon-12 by convention rather than decimal logic. The rontogram integrates seamlessly with SI prefixes and the broader decimal system used in laboratories worldwide.
Where it matters in practice
High-resolution mass spectrometry instruments report results in rontograms or derived units like kilrontograms (10³ rg for polymer fragments and intact proteins). Nanotechnology research, X-ray crystallography, and pharmaceutical development all encounter masses in this range when analyzing drug molecules, protein complexes, or nanoparticles. The unit allows laboratories to avoid awkward conversions between atomic mass units and grams, streamlining data recording and reducing transcription errors in critical measurements.
The rontogram is so far removed from everyday experience that its practical use remains confined to instrumentation and published data. Most working chemists and physicists will encounter it in instrument readouts, published spectra, and international technical standards rather than in manual calculations. It represents a formalization of something the trade has needed for decades: a decimal-based, SI-compliant way to express masses that would otherwise require negative exponents or multiple unit conversions.