Writing Ionic Formulas

Ions: Ca+2 O-2 Formula: CaO Ions: Al+3 N-3 Formula: AlN There is an easy way to determine the ratio of an ionic formula when you are given the ions: 1. If the charges of the ions are the same, then the ratio is 1:1. Ions: Ca+2 O-2 Formula: CaO Ions: Al+3 N-3 Formula: AlN

Ions: Mg+2 Cl- Formula: MgCl2 Ions: K+ S-2 Formula: K2S 2. If the charges of the ions are different, then the ratio can be determined by criss-crossing the charges. Ions: Mg+2 Cl- Formula: MgCl2 Ions: K+ S-2 Formula: K2S

Ions: Mg+2 Cl- Formula: MgCl2 Notice that we write the charge as a superscript with a + or – sign. Ions: Mg+2 Cl- But notice that the ratios are written as a subscript without any signs. Formula: MgCl2

Ions: Al+3 Br- Formula: AlBr3 This is why we say that we criss-cross the numbers. We don’t just move them over, we also move them down! Ions: Al+3 Br- Formula: AlBr3

For each combination of ions, write the formula for the compound they would make. (Be careful to make sure that if they have the same charge, you put them in a 1:1 ratio!) HF K2O MgCl2 CuI2 BaO CaCl2 ZnI2 SrF2 MgO FeBr2 Fe2O3 SrS

There are also ions that form after elements have shared electrons There are also ions that form after elements have shared electrons. These ions are known as polyatomic ions, and there is one special rule when you write them in a formula. SO4-2 CO3-2 NH4+ NO3- OH- PO4-3

This is NO DIFFERENT than before! 1. If the charges of the ions are the same, then the ratio is 1:1. Ions: Ca+2 SO4-2 Formula: CaSO4 Ions: Al+3 PO4-3 Formula: AlPO4 This is NO DIFFERENT than before!

Ions: Mg+2 OH- Formula: Mg(OH)2 MgOH2 2. If the charges are different, then the ratio can be determined by criss-crossing the charges. 2. If the charges are different, then the ratio can be determined by criss-crossing the charges and add parentheses only if we need more than one polyatomic ion. Ions: Mg+2 OH- Formula: Mg(OH)2 MgOH2 Problem: The formula looks like we have 1 Mg, 1 O, and 2 H’s when we really meant to write that we had 2 OH’s. To fix this, we add parentheses around the polyatomic ion.

Notice that we only need parentheses around the polyatomic ions when there is more than one in the formula. Ions: Ca+2 SO4-2 Ions: Al+3 SO4-2 Formula: CaSO4 Formula: Al2(SO4)3 Ions: Li+ NO3- Ions: Mg+2 NO3- Formula: LiNO3 Formula: Mg(NO3)2 Ions: K+ PO4-3 Ions: Sr+2 PO4-3 Formula: K3PO4 Formula: Sr3(PO4)2

It is important that you are aware that any ion with more than one CAPITAL letter has more than one element and is a polyatomic ion. Ions: Na+ OH- Ions: Al+3 OH- Formula: NaOH Formula: Al(OH)3 Ions: H+ CN- Ions: Ca+2 CN- Formula: HCN Formula: Ca(CN)2

(Be sure to use parentheses appropriately!) For each combination of ions on your notes, write the formula for the compound they would make. (Be sure to use parentheses appropriately!) Ca(OH)2 Ba(NO3)2 Al2 (CO3) 3 NH4Cl Mg(CN)2 Ca(NO3) 2 Fe2 (SO4)3 Zn(NO3) 2