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A chemical atom of an alkali metal.

An alkali metal is a group-1 metallic element, such as lithium, sodium, or potassium; hydrogen is in group 1 but is not ordinarily classified as an alkali metal. These metals commonly form singly charged positive ions.

Their outer-electron configuration helps explain related chemical behavior across the group. Physical properties and detailed reactivity still vary, so group membership is an organizing pattern rather than identity of all properties.

Caesium is the chemical element with atomic number 55 and symbol $\mathrm{Cs}$. It is a alkali metal. It readily forms $\mathrm{C}{\mathrm{s}}^{+}$. A specified hyperfine transition of caesium-133 defines the SI second, linking a reproducible atomic frequency with time measurement rather than with a mechanical clock.^[ref]

Potassium is the chemical element with atomic number 19 and symbol $\mathrm{K}$. It is a alkali metal. It commonly forms ${\mathrm{K}}^{+}$. The ion plays a major role in cellular electrical gradients, whereas elemental potassium is a reactive metal; nutritional or physiological statements about potassium normally concern its compounds and ions.^[ref]

Rubidium is the chemical element with atomic number 37 and symbol $\mathrm{Rb}$. It is a alkali metal. It commonly forms a singly charged positive ion. Its optical transitions are useful in atomic spectroscopy and laser-cooling experiments, while rubidium-87 adds an experimentally useful isotope-specific nuclear-spin structure.^[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.

A surface ionization detector detects a neutral atomic beam by converting incident chemical atoms into ions on a hot surface. For suitable alkali metals, a heated tungsten surface can favor loss of an electron: the competing surface work function and atomic ionization energy make the efficiency strongly dependent on species, surface condition and temperature. An electric field then collects the resulting ions, producing an electric current or a count after amplification. A response therefore requires an efficiency measurement calibration; it is not automatically equal to the incident particle flux.

A 2002 imaging experiment used a tungsten-coated heated ribbon followed by a microchannel plate detector and a phosphor screen. It recorded where the arriving neutral chemical atoms struck, rather than just their total rate. The reported approximately $20\mu \mathrm{m}$ spatial resolution and roughly $50\%$ detection efficiency belong to that apparatus and its conditions, rather than to every surface ionization detector.^[ref]