Textbook Question
Consider an ion with the symbol X2+ formed from a representative element.
d. What is the formula of the compound formed from X and the nitride ion?
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3. Ionic Compounds
Writing Formula Units of Ionic Compounds
0:57 minutesProblem 20c
Textbook Question
Write the symbols for the ions, and the correct formula for the ionic compound formed by each of the following:
c. sodium and phosphorus
Verified step by step guidance1
Identify the ions formed by each element: Sodium (Na) is an alkali metal in Group 1 of the periodic table, so it loses one electron to form a cation with a charge of +1, written as Na⁺. Phosphorus (P) is a nonmetal in Group 15, so it gains three electrons to form an anion with a charge of -3, written as P³⁻.
Determine the ratio of cations to anions needed to balance the charges: To create a neutral ionic compound, the total positive charge must equal the total negative charge. Since sodium has a +1 charge and phosphorus has a -3 charge, three sodium ions (Na⁺) are needed to balance the charge of one phosphorus ion (P³⁻).
Write the formula for the ionic compound: Combine the ions in the ratio determined in the previous step. The formula is written with the cation first, followed by the anion, and subscripts are used to indicate the number of each ion. In this case, the formula is Na₃P.
Double-check the charge balance: Verify that the total positive charge from the sodium ions (3 × +1 = +3) equals the total negative charge from the phosphorus ion (-3). Since the charges balance, the formula is correct.
Confirm the naming convention: The compound formed is named sodium phosphide, following the rule that the cation name (sodium) is written first, and the anion name (phosphide) is derived from the root of the nonmetal's name (phosphorus) with the suffix '-ide'.
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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 through the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). In this case, sodium (Na) acts as a cation, while phosphorus (P) can form an anion, typically as phosphate (PO4^3-) or phosphide (P^3-). Understanding how these ions combine is essential for writing the correct formula.
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Naming Ionic Compounds
Ion Symbols
Ion symbols represent the charged forms of elements. Sodium, for example, has the symbol Na^+ as it loses one electron to become a cation. Phosphorus can form different anions, but in the context of ionic compounds, it is important to identify the specific ion it forms, such as P^3- for phosphide, which is crucial for determining the compound's formula.
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Charge Balance
Charge balance is a fundamental principle in forming ionic compounds, where the total positive charge must equal the total negative charge. For sodium (Na^+) and phosphorus (P^3-), three sodium ions are needed to balance one phosphide ion, resulting in the formula Na3P. This concept ensures that the resulting compound is electrically neutral.
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