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An alkaline earth metal is a group-2 element, including magnesium, calcium, and barium. A common oxidation state is $+2$, associated with loss of two outer-shell electrons.

The resulting salts can nevertheless have very different solubilities and lattice properties. Comparing magnesium and calcium carbonates or hydroxides shows why periodic similarity does not eliminate the role of ion size, hydration, and crystal structure.

Calcium is the chemical element with atomic number 20 and symbol $\mathrm{Ca}$. It is a alkaline earth metal. Its common ion is $\mathrm{C}{\mathrm{a}}^{2+}$. Calcium in calcium carbonate or a biological signaling system is present in chemically bound or solvated forms, rather than as elemental metal; this distinction is essential when connecting mineral and biological chemistry.^[ref]

Radium is the chemical element with atomic number 88 and symbol $\mathrm{Ra}$. It is a alkaline earth metal. All its isotopes are radioactive. Its chemistry resembles that of barium, often favoring the $+2$ state, while radioactive decay distinguishes its long-term behavior from an ordinary stable alkaline-earth salt.^[ref]

In 1925, Wolfgang Pauli connected two puzzles: alkali metal doublets and their anomalous Zeeman effect, and the capacities of closed electron shells. His paper assigned the extra two-valued property to the optically active electron, rather than an angular momentum of the closed core. Its Paschen-Back effect classification used four quantum numbers; these historical labels should not be identified literally with modern $n,\ell ,m_{\ell },m_s$. The exclusion rule forbade matching all four. This connected resolved one-electron states with closed-group populations. An additional test was the missing triplet counterpart of the equivalent two-$s$-electron ground state in alkaline earth metals. Pauli explicitly claimed counts of allowed states, not their energies or ordering.^[ref]

The modern question is especially concrete: if a Quantum Hamiltonian has a lowest one-particle state, why cannot every electron occupy it? Minimizing energy answers which of the allowed quantum states is preferred; the Pauli exclusion principle determines which quantum states are allowed in the first place. Spectral lines then test energy differences between allowed multiparticle quantum states. A line count is not itself a direct count of occupied spin orbitals: transitions also depend on selection rules and the prepared quantum state populations. The observed anomalous Zeeman effect supplied constraints for the historical classification; the two-particle antisymmetrization below supplies the modern mathematical explanation of exclusion.

Yan-Ling Li coauthored research on calcium.^[ref]

Calcium is the chemical element with atomic number 20 and symbol $\mathrm{Ca}$. It is a alkaline earth metal. Its common ion is $\mathrm{C}{\mathrm{a}}^{2+}$. Calcium in calcium carbonate or a biological signaling system is present in chemically bound or solvated forms, rather than as elemental metal; this distinction is essential when connecting mineral and biological chemistry.