What is the electron configuration of calcium?

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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 electron configuration of calcium is $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2}$ .1

This can also be written in shorthand notation as [Ar] $4 s^{2}$ , where [Ar] represents the stable electronic structure of the noble gas argon. Like other alkaline-earth metals, calcium possesses a pair of s electrons in its outermost orbital, which are relatively easily ionized to form the characteristic $C a^{2 +}$ ion.2