B

Analysis of test questions: A. According to the information obtained in one cell of the periodic table of elements, it is known that the relative atomic mass of elements is 24.31, and the unit of relative atomic mass is "1", not "gram", so the option statement is wrong; B, according to the information obtained in one cell of the periodic table of elements, the atomic number of the element is 24; According to atomic number = nuclear charge number, the nuclear charge number of this element is 12; Or from the structural diagram of magnesium atom, its nuclear charge number is 12, so the option statement is correct; C, according to the information obtained from a grid in the periodic table of elements, the name of the element is magnesium, which ends with the metal element, so the option statement is wrong; D, the outermost electron number of magnesium atom is 2, and it is easy to lose 2 electrons in the chemical reaction to form a cation, so the option statement is wrong, so B is chosen.