Science and space glossary

The terms from our periodic-table, constant, compound, solar-system and launch pages, defined one by one, with the official source.

10 terms · The definitions are ours; where a term has an official definition, the source is named.

A laboratory analytical balance, with its glass draft shield closed over the pan — An analytical balance: an instrument like this is how a mole of a substance actually gets weighed. · Photo: US DEA — Public domain · file page on Wikimedia Commons

Jupiter photographed by the Hubble Space Telescope, with its cloud bands and the Great Red Spot — Jupiter: the solar system’s largest planet, used on the pages comparing planet sizes. · Photo: NASA/STSCI (S.T.A.R.S) — Public domain · file page on Wikimedia Commons

Atomic number
Atomic weight
Isotope
Group and period
s/p/d/f block
Mole
Standard uncertainty
Astronomical unit
Exoplanet
Launch window

Atomic number: The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.; Standard definition by IUPAC
Atomic weight: The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.; Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights
Isotope: Atoms of the same element (same number of protons) with a different number of neutrons, and therefore a different mass. Carbon-12 and carbon-14 are isotopes of carbon; only the second one is radioactive.; Standard definition by IUPAC
Group and period: The group is the periodic table’s column and shows how many electrons sit in the outer shell; the period is the row and shows how many shells the atom has. Elements in the same group tend to react in similar ways.; Standard definition by IUPAC
s/p/d/f block: A split of the periodic table by the type of orbital receiving the atom’s last electron. Alkali and alkaline-earth metals sit in the s block, most everyday elements in the p block, transition metals in the d block, and lanthanides and actinides in the f block.; Standard definition by IUPAC
Mole: The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.; Standard definition by BIPM — Bureau International des Poids et Mesures (SI)
Standard uncertainty: The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.; Standard definition by BIPM — Bureau International des Poids et Mesures (SI)
Astronomical unit: The solar system’s reference distance, fixed at exactly 149,597,870,700 metres since 2012, close to the average Earth–Sun distance. It saves writing every planet’s orbit in kilometres with ten digits.; Standard definition by IAU — International Astronomical Union
Exoplanet: A planet orbiting a star other than the Sun. Most were found indirectly, by the star’s faint dimming as the planet crosses in front of it (transit) or by the gravitational wobble it causes in the star (radial velocity), not by direct image.; Standard definition by NASA Exoplanet Archive
Launch window: The span of time in which a rocket can lift off and still reach its planned orbit or destination. For a trip to another planet that window can last only a few weeks and only returns at the next favourable alignment, so missing it costs months or years of waiting.