Writing Ionic Compounds - Online Tutor, Practice Problems & Exam Prep

When writing ionic compounds, start by identifying the ions from the compound's name. For instance, aluminum nitride indicates aluminum has a +3 charge (Al³⁺) and nitride, as a non-metal with an "ide" ending, signifies nitrogen with a -3 charge (N^3-). If the ion charges are equal, as in the case of aluminum (+3) and nitrogen (-3), they cancel each other out, resulting in the formula AlN for aluminum nitride. However, if the charges differ, such as barium (+2) and phosphate (PO₄^3-), use the crisscross method to balance the charges, leading to the formula Ba₃(PO₄)₂ for barium phosphate. Remember, this crisscross method only applies to the numbers, not the charges themselves. Understanding the charges of elements based on their periodic table group is crucial, and practicing these steps with various compounds will solidify your grasp of forming correct ionic compound formulas.

Writing Ionic Compounds

Here we have the rules for writing ionic compounds. Now, step one, we write the ions involved in the compound from the provided name. Step 2, you use these ions to write the formula of the ionic compound. So if we take a look here, we have aluminum nitride and barium phosphate.

All right, So we're going to say aluminum is in Group 3A. Because of that, its charge is Al3+ or three plus. That's why you write it as Al3+ nitride. I'd would mean that it is a nonmetal and it had its ending change to I'd Nitrogen is in Group 5A so its charge is N3-. Now if you don't quite remember the charges associated with the different groups of the periodic table, make sure you go back and take a look at my videos on the periodic table charges. OK, that'll help you see all the elements of the periodic table and what are the common types of charges based on the location of the element.

Now for this first one, when numbers and charges are the same, so here the numbers are three and three. OK. So the numbers are three and three. They cancel out to combine the elements. So since they both have three in their charges, they just cancel out. So aluminum nitride will be AlN. Now barium phosphate. Barium is in Group 2A, so its charge is Ba2+. Phosphate is one of our polyatomic ions. It's one of our common Tetra oxides. Its formula is PO43-.

Now when the numbers and charges are different, here this is 2 and this is 3, then they Criss cross. They Criss cross to combine the elements. So what I mean by crisscross is that the two from here would come here and the three from here would come here. That would give me. If we look, it would give me Ba3. The three came down here. Now this two is for the entire polyatomic ion for both phosphorus and oxygen. So because it's for both of them, I have to put it in parentheses PO4)2. So that would represent barium phosphate.

These are the things you must do when given the name of an ionic compound. Write out its ions and then look to see are the numbers and the charges the same? If so, they cancel out. Are they different? If so, crisscross them. Now, doing this crisscrossing motion, we don't have to worry about crisscrossing the charges, so you're only crisscrossing the numbers. Doing this type of action takes care of the charges on their own, so just ignore them. OK, so these would be our two formulas for these ionic compounds.

Now that we've gotten down the basics, we'll move on to some problems input to what we just learned into practice.

Writing Ionic Compounds Example 1

Here in this example question, it says provide the molecular formula for the following compound. We have magnesium sulfate all right. So Magnesium is MG. Magnesium is in Group 2A, so its charge is +2 or two plus. Now, sulfate 8 is a good indicator that this is a polyatomic ion. If you remember, sulfate is one of our common types of Tetra oxides, meaning that we have 4 oxygens involved.

All right. So SOLVE means we're dealing with sulfur. It is one of our common Tetra oxides, so it has four oxygens and it has a charge of 2 minus. The polyatomic ions can be a little bit of annoying, but again, go back and review the polyatomic ions if you don't remember them. Now we look at our Mg2+ and our SO42-. Look at the numbers within the charges.

Here we have a two and here we have a two. Remember when the numbers are the same, they just cancel out and then you combine the elements together so the two plus and the 2 minus cancel out. So what we'll have left is MgSO4, so magnesium sulfate is represented by MgSO4.

Provide the molecular formula for the following compound:Copper (I) nitrate

• A. CuNO₃
• B. Cu(NO₃)₂
• C. Cu(NO₃)₃
• D. CuNO₂
• E. Cu(NO₂)₂

Provide the molecular formula for the following compound:Sodium dichromate

• A. NaCr₂O₇
• B. Na(Cr₂O₇)₂
• C. NaCrO₄
• D. Na₂Cr₂O₇
• E. Na₂CrO₄

Here’s what students ask on this topic:

What are the steps to write the formula for an ionic compound?

To write the formula for an ionic compound, follow these steps: First, identify the ions involved from the compound's name. For example, in aluminum nitride, aluminum has a +3 charge (Al³⁺) and nitride has a -3 charge (N^3-). Second, if the ion charges are equal, they cancel each other out, resulting in the formula AlN. If the charges differ, use the crisscross method to balance the charges. For instance, in barium phosphate, barium has a +2 charge (Ba²⁺) and phosphate has a -3 charge (PO₄^3-). Crisscross the charges to get Ba₃(PO₄)₂. This method only applies to the numbers, not the charges themselves.

How do you determine the charge of an ion from the periodic table?

The charge of an ion can be determined based on its group in the periodic table. For example, elements in Group 1A have a +1 charge, Group 2A have a +2 charge, and Group 3A have a +3 charge. Nonmetals in Group 5A have a -3 charge, Group 6A have a -2 charge, and Group 7A have a -1 charge. Transition metals can have multiple charges, so it's important to refer to specific rules or additional information for those elements. Understanding these charges is crucial for writing correct ionic compound formulas.

What is the crisscross method in writing ionic compounds?

The crisscross method is used to balance the charges of ions in an ionic compound. When the charges of the ions are different, you crisscross the numbers of the charges to balance them. For example, in barium phosphate, barium has a +2 charge (Ba²⁺) and phosphate has a -3 charge (PO₄^3-). By crisscrossing, the 2 from barium becomes the subscript for phosphate, and the 3 from phosphate becomes the subscript for barium, resulting in the formula Ba₃(PO₄)₂. This method ensures the compound is electrically neutral.

Why do you need to use parentheses in some ionic compound formulas?

Parentheses are used in ionic compound formulas when you have a polyatomic ion that needs to be multiplied by a subscript. For example, in barium phosphate (Ba₃(PO₄)₂), the phosphate ion (PO₄^3-) is a polyatomic ion. Since the subscript 2 applies to the entire phosphate ion, parentheses are used to indicate that both phosphorus and oxygen are included in the multiplication. This ensures the correct ratio of ions in the compound.

How do you write the formula for aluminum nitride?

To write the formula for aluminum nitride, first identify the ions involved. Aluminum is in Group 3A and has a +3 charge (Al³⁺). Nitride, derived from nitrogen in Group 5A, has a -3 charge (N^3-). Since the charges are equal and opposite, they cancel each other out. Therefore, the formula for aluminum nitride is AlN.