A the electron configuration around radium atom is 7s2, belonging to IIA group, so A is wrong;

B. the metallicity of the same family increases from top to bottom. Radium is more metallic than Ba, and Ba(OH)2 is a strong alkali, so Ra(OH)2 is more alkaline than Ba(OH)2, so Ra(OH)2 should not be amphoteric, so B is wrong;

C magnesium carbonate is slightly soluble, while calcium carbonate and barium carbonate are insoluble, and their congeneric properties are similar. The carbonate of radium may be insoluble in water, so c is correct;

D. The radioactivity of an element means that the nucleus releases particles or rays and eventually decays into a stable element, while the element atom loses electrons, so the element atom is easy to lose electrons, which means that the element is metallic, so D is wrong;

So choose: C.