Question 4.5: Determine the chemical formula for the compound that is form......
Determine the chemical formula for the compound that is formed when each of the following pairs of ions interact.
a. Na^{+} and P^{3-} b. Be^{2+} and P^{3-}
a. The Na^{+} and P^{3-} ions combine in a three-to-one ratio because this combination causes the charges to add to zero. Three Na^{+} ions give a total positive charge of 3. One P^{3-} ion results in a total negative charge of 3. Thus the chemical formula for the compound is Na_{3}P.
b. The numbers in the charges for these ions are 2 and 3. The lowest common multiple of 2 and 3 is 6 (2 × 3 = 6). Thus we need 6 units of positive charge and 6 units of negative charge. Three Be^{2+} ions are needed to give the 6 units of positive charge, and two P^{3-} ions are needed to give the 6 units of negative charge. The combining ratio of ions is three to two, and the chemical formula is Be_{3}P_{2}.
\quad\quadThe strategy of finding the lowest common multiple of the numbers in the charges of the ions always works, and it is a faster process than that of drawing the Lewis structures.