Which element has a smaller ionic radius than magnesium (Mg)? A) Rb
B) Al
C) Ca
D) Na

Na has atomic number 11

which is less than magnesium has atomic number 12

ANSWER IS D

Explanation:

aluminum or Al is atomic 13 and Ca or calcium is atomic number 20 and rubidium or Rb is atomic number 37

Since chlorine's 17 protons are greater than magnesium's 12 protons, chlorine will have a greater effective nuclear charge to draw chlorine's valence electrons closer to the nucleus and, thus, chlorine is expected to have the smaller atomic radius, while magnesium with the lower effective nuclear charge is expected  ,so the answer is Na

Explanation:

helium

has the smallest ionic radius

As can be seen in the figures below, the atomic radius increases from top to bottom in a group, and decreases from left to right across a period. Thus, helium is the smallest element, and francium is the largest

i think   the answer is c hope i'm right