Compared to the atomic radius of a sodium atom, the atomic radius of a magnesium atom is smaller. The smaller radius is primarily a result of the magnesium atom having A) more principal energy levels B) a larger nuclear charge C) fewer principal energy levels D) a smaller nuclear charge

 B) a larger nuclear charge

Explanation:

Atomic radius of an atom is defined as the total distance from the nucleus to the outermost shell of the atom.

Sodium and magnesium lies in the same period and magnesium lies to the right of sodium. They have same number of principle shell.

As moving from left to right in a period, more and more electrons get added up in the same principle shell and the attraction between the last electron and nucleus increases , which results in the larger nuclear charge and thus shrinkage in the size of an atom. Thus, decreasing the atomic radii of the atom on moving towards right of the periodic table.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.