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Calcium-42
⁴²Ca
Stable calcium isotope ⁴²Ca. Mass number A = 42.
Calcium-42 is the stable A = 42 isotope of calcium (Z = 20). NIST lists relative atomic mass 41.95861783(16) and natural isotopic composition 0.00647(23). Among A = 42 isobars it is the stable endpoint.
What it supports
An exact mass number, distinct from molybdenum (Z = 42) and from cosmogenic ⁴²Ar already on the Index. Calcium is biologically essential. The number is the mass number, not a wink. Aston identified stable ⁴²Ca (Nature 133, 684, 1934). NNDC and ENSDF mark the ground state stable.
What it does not
Not Z = 42. Not argon-42. Not potassium-42, the radioactive parent. Not a mystical calcium-42 link. Nuclear charts contain A = 42 for many Z. The strength is that this one is stable, that NIST prints the abundance, and that it is the only stable A = 42 isobar.
ExactnessExact
Form42
UnitA
Strengthhard
When1934
Hung24 September 2026