Textbook Question
Consider the following Lewis symbols for elements X and Y:
d. What would be the formula of a compound of X and Y?
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3. Ionic Compounds
Writing Formula Units of Ionic Compounds
2:36 minutesProblem 17e
Textbook Question
Write the correct ionic formula for the compound formed between each of the following pairs of ions:
e. Al3+ and S2-
Verified step by step guidance1
Identify the charges of the ions: Aluminum (Al) has a charge of +3 (Al³⁺), and Sulfide (S) has a charge of -2 (S²⁻).
Determine the least common multiple (LCM) of the charges to balance the total positive and negative charges. The LCM of 3 and 2 is 6.
Calculate the number of each ion needed to achieve charge neutrality. To balance the charges, you need 2 Al³⁺ ions (2 × +3 = +6) and 3 S²⁻ ions (3 × -2 = -6).
Write the formula by placing the number of each ion as subscripts after the element symbols. The formula is written as Al₂S₃, where the subscripts indicate the ratio of aluminum to sulfide ions.
Double-check the formula to ensure the total positive and negative charges are equal and the subscripts are in the simplest whole-number ratio.
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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 positively charged ions (cations) and negatively charged ions (anions) bond together through electrostatic forces. The overall charge of the compound must be neutral, meaning the total positive charge from cations must balance the total negative charge from anions.
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Charge Balance
Charge balance is a fundamental principle in forming ionic compounds. Each ion carries a specific charge, and when combining ions, the total positive and negative charges must equal zero. For example, in the case of Al³⁺ (with a +3 charge) and S²⁻ (with a -2 charge), the charges must be balanced to determine the correct ratio of ions in the formula.
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Empirical Formula
The empirical formula represents the simplest whole-number ratio of ions in an ionic compound. To derive the empirical formula from the ions Al³⁺ and S²⁻, one must find the least common multiple of their charges, which leads to the formula Al₂S₃, indicating that two aluminum ions combine with three sulfide ions to achieve charge neutrality.
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