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In order to find the ionic compound formed when two elements combine, we first need to determine the charges of the ions formed by each element. For this, we will use their positions in the periodic table and their group number. (a) Barium and fluorine Barium (Ba) is in Group 2, so it forms a +2 ion: Ba^2+ Fluorine (F) is in Group 17, so it forms a -1 ion: F^- (b) Cesium and chlorine Cesium (Cs) is in Group 1, so it forms a +1 ion: Cs^+ Chlorine (Cl) is in Group 17, so it forms a -1 ion: Cl^- (c) Lithium and nitrogen Lithium (Li) is in Group 1, so it forms a +1 ion: Li^+ Nitrogen (N) is in Group 15, so it forms a -3 ion: N^3- (d) Aluminum and oxygen Aluminum (Al) is in Group 13, so it forms a +3 ion: Al^3+ Oxygen (O) is in Group 16, so it forms a -2 ion: O^2-

Now that we have determined the charges of the ions formed by each element, we can combine them in such a way that the resulting compound is electrically neutral. This is done by ensuring that the total positive charge equals the total negative charge. (a) Barium and fluorine Ba^2+ and F^- Since 1 Ba^2+ ion has a charge of +2 and 1 F^- ion has a charge of -1, we need two F^- ions to balance the charge of one Ba^2+ ion. The compound is therefore BaF2. (b) Cesium and chlorine Cs^+ and Cl^- Since both ions have charges of +1 and -1, we need one Cl^- ion to balance the charge of one Cs^+ ion. The compound is therefore CsCl. (c) Lithium and nitrogen Li^+ and N^3- Since 1 Li^+ ion has a charge of +1 and 1 N^3- ion has a charge of -3, we need three Li^+ ions to balance the charge of one N^3- ion. The compound is therefore Li3N. (d) Aluminum and oxygen Al^3+ and O^2- Since 1 Al^3+ ion has a charge of +3 and 1 O^2- ion has a charge of -2, we need two Al^3+ ions and three O^2- ions to make the charges balance. The compound is therefore Al2O3. Therefore, the ionic compounds expected to form from combining the given pairs of elements are: (a) BaF2 (b) CsCl (c) Li3N (d) Al2O3