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Ch.2 - Atoms, Molecules, and Ions
Chapter 2, Problem 61e

Predict the chemical formulas of the ionic compound formed by (e) In3+ and O2-.

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Identify the charges of the ions involved: In^{3+} has a charge of +3 and O^{2-} has a charge of -2.
To form a neutral ionic compound, the total positive charge must balance the total negative charge.
Determine the least common multiple of the charges: The least common multiple of 3 and 2 is 6.
Calculate the number of each ion needed to achieve charge balance: You need 2 In^{3+} ions (2 x +3 = +6) and 3 O^{2-} ions (3 x -2 = -6) to balance the charges.
Write the chemical formula by placing the number of each ion as subscripts: The formula is In_2O_3.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons, resulting in the formation of positively charged cations and negatively charged anions. The electrostatic attraction between these oppositely charged ions leads to the creation of a stable compound. Understanding the charges of the ions involved is crucial for predicting the correct formula of the resulting ionic compound.
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Charge Balance

In ionic compounds, the total positive charge must balance the total negative charge to achieve electrical neutrality. This means that the ratio of cations to anions in the formula must be such that the sum of their charges equals zero. For example, if a cation has a charge of +3, it will require three anions with a charge of -2 to balance the charges appropriately.
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Chemical Formula Representation

The chemical formula of an ionic compound is represented by the symbols of the constituent elements, with subscripts indicating the number of each type of ion present. The formula reflects the simplest whole-number ratio of the ions that results in a neutral compound. For instance, the formula for the compound formed by In3+ and O2- will be determined by the least common multiple of their charges.
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