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

The formula unit of an ionic compound represents the simplest ratio of ions that results in a neutral charge, forming a crystal lattice rather than isolated ionic pairs. To write ionic compounds, identify the ions from the name, determine their charges, and either cancel or crisscross the charges to derive the formula. For example, aluminum nitride is represented as AlN, while barium phosphate is Ba₃(PO₄)₂. Understanding these principles is essential for mastering ionic compounds and their properties.

Writing Formula Units of Ionic Compounds Concept 1

Now the formula unit represents the empirical, what we call the simplest ratio of ions in an ionic solid that combine to give a neutral charge. We're going to say here in reality an ionic solid doesn't exist as an ionic pair. So it's not just a positive ion and a negative ion combining. It's not that ionic pair, but instead it's a crystal lattice. Now, a crystal lattice is a 3D arrangement of several cations and anions together that form a stable pattern. So for example here, we have the ions of sodium ion and chloride ion. They have opposing charges which makes them attracted to one another. So that's why they're kind of combined together. In reality, though, the ionic compound is not just a single positive ion and a single negative ion. It's a collection of all of them together. This helps to make my crystal lattice. So in reality, this is my ionic solid. It's a bunch of positive ions and negative ions combined together. Now if we were to take one of these positive ions and one of these negative ions, pull them out, that would represent the formula unit, which is the simplest portion of my ionic solid. So here it would be sodium chloride which would be NaCl. So just remember, when we're talking about ionic pairs, that's just a formula unit of an ionic compound. It is not the true representation of that ionic solid. The true representation is a collection of those positive and negative ions together which form a crystal lattice.

Writing Formula Units of Ionic Compounds Concept 2

Here we have the rules for writing ionic compounds. Now step 1, we write the ions involved in the compound from the provided name. Step 2, you use these ions to write the formula of the ion compound. So if we take a look here, we have aluminum, nitride, and barium phosphate. Alright. So we're gonna say aluminum is in group 3A. Because of that, its charge is plus 3 or 3 plus. So that's why you write it as Al³⁺. Nitride, ide would mean that it is a nonmetal, and it had its ending changed to ide. Nitrogen is in group 5A, so its charge is 3 minus. 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. Okay? 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 in charges are the same, so here the numbers are 33. Okay. So the numbers are 33, they cancel out to combine the elements. So since they both have 3 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 2+. Phosphate is one of our polyatomic ions. It's one of our common tetraoxides. Its formula is PO₄^3- Now, when the numbers in charges are different, here this is 2 and this is 3, then they crisscross. They crisscross to combine the elements. So what I mean by crisscross is that the 2 from here would come here, and the 3 from here would come here. That would give me, if we look, it'd give me Ba₃(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 in 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. Okay? So you're only crisscrossing the numbers. Doing this type of action takes care of the charges on them on their own, so just ignore them. Okay? 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 and put what we just learned into practice.

Writing Formula Units of Ionic Compounds Example 1

Provide the formula unit for the compound formed by the following ions. So we have magnesium ion and sulfate ion. Alright. So remember, when the numbers in the charges are the same, they just simply cancel out. So Mg²⁺ SO₄^2-, just combine the Mg with the SO₄. The charges completely cancel out, so ignore the 2, ignore the positive negative charges. So it'd just be MgSO₄ as our newly created formula unit for our ionic solid.

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 is a formula unit in an ionic compound?

A formula unit in an ionic compound represents the simplest ratio of ions that results in a neutral charge. Unlike a single ionic pair, an ionic compound forms a crystal lattice, which is a 3D arrangement of cations and anions. For example, in sodium chloride (NaCl), the formula unit is NaCl, indicating a 1:1 ratio of sodium ions (Na⁺) to chloride ions (Cl^-). This ratio is the smallest repeating unit that maintains the compound's overall neutrality.

How do you write the formula for an ionic compound?

To write the formula for an ionic compound, follow these steps: 1) Identify the ions from the compound's name. 2) Determine the charges of the ions. 3) If the charges are the same, they cancel out. If different, crisscross the charges to balance them. For example, for aluminum nitride, aluminum (Al) has a charge of 3⁺ and nitride (N) has a charge of 3^-. Since the charges are the same, they cancel out, resulting in AlN. For barium phosphate, barium (Ba) has a charge of 2⁺ and phosphate (PO₄) has a charge of 3^-. Crisscrossing the charges gives Ba₃(PO₄)₂.

What is the significance of a crystal lattice in ionic compounds?

A crystal lattice in ionic compounds is significant because it represents the true structure of the compound. It is a 3D arrangement of cations and anions that maximizes attraction and minimizes repulsion, leading to a stable structure. This lattice is responsible for many properties of ionic compounds, such as high melting and boiling points, and electrical conductivity in molten or dissolved states. The formula unit is a simplified representation of this complex structure, showing the smallest ratio of ions that maintains electrical neutrality.

Why do we crisscross charges when writing formulas for ionic compounds?

We crisscross charges when writing formulas for ionic compounds to ensure that the total positive and negative charges balance out, resulting in a neutral compound. This method involves taking the magnitude of the charge of one ion and using it as the subscript for the other ion. For example, in barium phosphate, barium (Ba) has a charge of 2⁺ and phosphate (PO₄) has a charge of 3^-. Crisscrossing these charges gives Ba₃(PO₄)₂, ensuring the compound is electrically neutral.

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

The charge of an ion can be determined from its position in the periodic table. Elements in Group 1A have a charge of 1⁺, Group 2A have 2⁺, and so on. Nonmetals in Group 5A have a charge of 3^-, Group 6A have 2^-, and Group 7A have 1^-. Transition metals can have multiple charges, so their specific charge is often given in the compound's name. For example, aluminum (Group 3A) has a charge of 3⁺, while nitrogen (Group 5A) has a charge of 3^-.