Calcium ($\mathrm{Ca}$)

A calcium ion with one electron removed, leaving a closed 18-electron core and one valence electron. It is distinct from the divalent calcium ion used in much biological terminology. Its bound electronic levels and ionization continuum support the Ca II optical series.

In singly ionized calcium, $np{}^{2}P\to 4s{}^{2}S$ resonance doublets, $ns{}^{2}S\to 4p{}^{2}P$ doublets and $np{}^{2}P\to 3d{}^{2}D$ three-component groups approach the same ionization continuum from different lower terms. A three-line group need not have spin multiplicity three. The electric-dipole rule excludes $P_{1/2}\to D_{5/2}$.^[ref]

The divalent cation ${\mathrm{Ca}}^{2+}$. Changes in its free intracellular concentration participate in calcium signaling; total cellular calcium includes substantial bound and sequestered pools.

Calcium-40 is an even-even nucleus with the closed-shell values $Z=N=20$. Its extra stability illustrates nuclear shell corrections beyond a smooth semi-empirical mass formula.

Calcium hydroxide supplies calcium cations and hydroxide anions on dissolution in water. The ideal one-to-two stoichiometry relates molar solubility to the dissolved concentrations; solubility product measurement by titration explains how the hydroxide inventory is measured.

Calcium hydroxide prepared by hydrating calcium oxide.

A clear aqueous chemical solution of calcium hydroxide. Carbon dioxide can form a cloudy calcium carbonate precipitate in it.