In the compound mgcl2, the subscript 2 indicates that
a. there are two magnesium ions for each ion of chlorine
b. the chloride ion is twice the size of the magnesium ion.
c. magnesium and chlorine form a double covalent bond.
d. there are two chloride ions for each magnesium ion.

The correct answer is answer choice D, there are two chloride ions for each magnesium ions. Subscripts always indicate the number of atoms, or ions, present in the molecule. The 2 refers to the element directly to its left, so answer choice A is reversed; there are 2 chloride ions for each magnesium ion, not the other way around. Answer choice B and C are incorrect because the subscript refers to the number of atoms, not the size of the atoms or the type of bond.