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Which alkaline-earth metal has the lowest atomic number?

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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. — alkaline-earth metal Modern version of the periodic table of the elements (printable).

The alkaline-earth metal with the lowest atomic number is beryllium (Be), which has an atomic number of 4.12