Final answer:
Sodium forms a 1+ ion and needs one atom to balance the charge of an anion, while calcium forms a 2+ ion and needs two anions to achieve charge neutrality in an ionic compound, as dictated by the law of conservation of matter.
Step-by-step explanation:
Sodium (Na) needs only one atom to make an ionic compound because it has one electron to donate, forming a 1+ cation. For instance, in sodium sulfate (Na2SO4), two sodium atoms donate one electron each to balance the 2- charge of the sulfate (SO42-) ion. Conversely, calcium (Ca) needs two atoms to make an ionic compound because it forms a 2+ cation, and thus requires two anions to balance its charge, as seen in calcium nitrate (Ca(NO3)2).
The law of conservation of matter requires that the number of electrons gained by anions equals the number of electrons lost by cations. An example is the synthesis of calcium chloride from elemental Ca and Cl, where Ca donates two electrons to two Cl atoms to form CaCl2. In forming ionic compounds, it is crucial to adjust the ratios of anions and cations so that the compound is electrically neutral, without writing the charges in the final formula.