The element with atomic number Z = 20 refers to calcium (Ca). Calcium belongs to the group 2 elements in the periodic table and is categorized as an alkaline earth metal.

Alkaline earth metals are found in group 2 of the periodic table and include elements like beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These metals have two valence electrons and are reactive, but less so compared to the alkali metals in group 1.

Option 1, an alkali metal, is incorrect because alkali metals are found in group 1 of the periodic table and include elements like lithium (Li), sodium (Na), potassium (K), and so on.

Option 3, a halogen, is incorrect because halogens are found in group 17 of the periodic table and include elements like fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

Option 4, an inert gas, is incorrect because inert gases (or noble gases) are found in group 18 of the periodic table and include elements like helium (He), neon (Ne), argon (Ar), and so on.