91Ó°ÊÓ

We can find the charges of the elements by using the periodic table and looking at their groups. Elements in Group 1 form ions with a +1 charge, Group 2 with a +2 charge, Group 13 with a +3 charge, Group 15 with a -3 charge, Group 16 with a -2 charge, and Group 17 with a -1 charge. (a) \(\mathrm{Al}\) and \(\mathrm{F}\): \(\mathrm{Al}\) is in Group 13, so it forms a +3 ion. \(\mathrm{F}\) is in Group 17, so it forms a -1 ion. (b) \(\mathrm{K}\) and \(\mathrm{S}\): \(\mathrm{K}\) is in Group 1, so it forms a +1 ion. \(\mathrm{S}\) is in Group 16, so it forms a -2 ion. (c) \(\mathrm{Y}\) and \(\mathrm{O}\): \(\mathrm{Y}\) (yttrium) is in Group 3, so it forms a +3 ion. \(\mathrm{O}\) is in Group 16, so it forms a -2 ion. (d) \(\mathrm{Mg}\) and \(\mathrm{N}\): \(\mathrm{Mg}\) is in Group 2, so it forms a +2 ion. \(\mathrm{N}\) is in Group 15, so it forms a -3 ion.

Now, we will determine the chemical formula by satisfying the electrical neutrality in the compound by ensuring that the total positive charge equals the total negative charge. (a) \(\mathrm{Al}\) and \(\mathrm{F}\): We need 3 \(\mathrm{F}\) ions (-1 each) for every \(\mathrm{Al}\) ion (+3) to satisfy neutrality: \(\mathrm{AlF_3}\) (b) \(\mathrm{K}\) and \(\mathrm{S}\): We need 2 \(\mathrm{K}\) ions (+1 each) for every \(\mathrm{S}\) ion (-2) to satisfy neutrality: \(\mathrm{K_2S}\) (c) \(\mathrm{Y}\) and \(\mathrm{O}\): We need 2 \(\mathrm{Y}\) ions (+3 each) for every 3 \(\mathrm{O}\) ions (-2 each) to satisfy neutrality: \(\mathrm{Y_2O_3}\) (d) \(\mathrm{Mg}\) and \(\mathrm{N}\): We need 3 \(\mathrm{Mg}\) ions (+2 each) for every 2 \(\mathrm{N}\) ions (-3 each) to satisfy neutrality: \(\mathrm{Mg_3N_2}\) We have the chemical formulas of the ionic compounds for each pair of elements.