Final answer:
Sodium (from Group 1A) forms a 1+ cation, requiring one sodium atom to balance one anion's charge, while calcium (from Group 2A) forms a 2+ cation, requiring two anions to balance the charge for electrical neutrality in the ionic compound.
Step-by-step explanation:
The reason why sodium needs one atom to make an ionic compound, and calcium needs two, is related to their positions on the periodic table and their electron configurations. Sodium, which is in Group 1A, has one valence electron and forms a 1+ cation. When sodium forms an ionic compound with a sulfate ion (SO42-), two sodium ions are required to balance the 2- charge of the sulfate ion, resulting in the formula Na2SO4.
Calcium, on the other hand, is in Group 2A and has two valence electrons, forming a 2+ cation when it reacts. For instance, when calcium combines with nitrate (NO3) to form an ionic compound, two nitrate anions are needed to balance the 2+ charge of a single calcium ion, leading to the formula Ca(NO3)2. This balance ensures that the ionic compound is electrically neutral, adhering to the law of conservation of matter.
In summary, the need for the number of electrons lost to be equal to the number of electrons gained is what determines the ratio of cations to anions in an ionic compound. For example, when forming calcium chloride (CaCl2), calcium donates two electrons, one to each chlorine atom, forming a Ca2+ cation and two Cl- anions, resulting in a neutral compound.