Writing Ionic Compounds: Videos & Practice Problems

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.

Rules for Writing Ionic Compounds

Understanding how to write ionic compounds involves a systematic approach to identifying the ions and their respective charges. The first step is to determine the ions from the name of the compound. For example, in the case of aluminum nitride and barium phosphate, we start by identifying the elements involved.

Aluminum, which is located in group 3A of the periodic table, has a charge of +3, represented as \( \text{Al}^{3+} \). Nitride, derived from nitrogen in group 5A, has a charge of -3, denoted as \( \text{N}^{3-} \). When the charges of the ions are equal, as in this case where both charges are 3, they effectively cancel each other out. Therefore, the formula for aluminum nitride is simply \( \text{AlN} \).

Next, we analyze barium phosphate. Barium, found in group 2A, carries a charge of +2, written as \( \text{Ba}^{2+} \). Phosphate is a polyatomic ion with the formula \( \text{PO}_4^{3-} \). When the charges differ, we employ a crisscross method to combine the elements. Here, the +2 charge from barium and the -3 charge from phosphate lead us to crisscross the numbers: the 2 from barium becomes the subscript for phosphate, and the 3 from phosphate becomes the subscript for barium. This results in the formula \( \text{Ba}_3(\text{PO}_4)_2 \), where the parentheses indicate that the subscript applies to the entire phosphate ion.

In summary, when writing ionic compounds, first identify the ions and their charges. If the charges are the same, they cancel out; if they differ, use the crisscross method to determine the subscripts. This method simplifies the process of writing correct formulas for ionic compounds.

Writing Ionic Compounds Example

To determine the molecular formula for magnesium sulfate, we start by identifying the components involved. Magnesium (Mg) is located in group 2A of the periodic table, which gives it a charge of +2 (denoted as Mg²⁺). On the other hand, sulfate (SO₄^2-) is a polyatomic ion, specifically a tetraoxide, meaning it contains four oxygen atoms bonded to a sulfur atom. The sulfate ion carries a charge of -2.

When combining these two ions, we need to balance their charges. The +2 charge from magnesium and the -2 charge from sulfate are equal in magnitude but opposite in sign. This allows them to cancel each other out, resulting in a neutral compound. Therefore, when we combine the elements, we simply write them together without any additional subscripts, leading us to the molecular formula for magnesium sulfate: 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

Answer: (A)

Provide the molecular formula for the following compound:Sodium dichromate

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

Answer: (D)

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.