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Which electron arrangement do all alkali metals share?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

All alkali metals share the same electronic configuration in their outermost shell: each atom has exactly one valence electron located in an s orbital.12

Because this single electron is in the outermost shell and is only weakly bound to the nucleus, it is easily removed, which accounts for the high reactivity and similar chemical properties characteristic of the alkali metals group.12