Chemical elements · Directory entry
Thorium
Element 90 · chemical symbol: Th
These Vrulum profiles mention Thorium by name. Read each profile for its specific context; a mention alone does not establish composition, performance or use of the pure element.
For periodic-table data and other properties, consult the Royal Society of Chemistry periodic table.
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Optical atomic clocks
Optical atomic clocks keep time by counting the oscillations of light that atoms or ions absorb, which occur at much higher frequencies than the microwaves used by today's caesium standard. Laboratory systems now report systematic uncertainties near 10⁻¹⁸, and the BIPM is preparing a redefinition of the SI second based on optical transitions, with CGPM adoption anticipated in 2030 or later.