Question

Write the formula for each of the following compounds, which contain polyatomic ions. Be sure to enclose the polyatomic ion in parentheses if more than one such ion is needed to balance the oppositely charged ion(s). a. silver(I) perchlorate (usually called silver perchlorate) b. cobalt(III) hydroxide c. sodium hypochlorite d. potassium dichromate e. ammonium nitrite f. ferric hydroxide g. ammonium hydrogen carbonate h. potassium perbromate

Short answer

a. Ag(ClO₄) b. Co(OH)₃ c. NaClO d. K₂Cr₂O₇ e. NH₄NO₂ f. Fe(OH)₃ g. NH₄HCO₃ h. KBrO₄

Step-by-step solution

Identify the ions and their charges

Silver(I) is Ag+ and perchlorate is ClO4-. To form a neutral compound, the charges must balance out.

Write the formula for the compound

Since the charges of silver(I) and perchlorate are +1 and -1 respectively, the compound's formula is Ag(ClO4) without parentheses as the charges balance without a need for multiple ions. b. cobalt(III) hydroxide:

Identify the ions and their charges

Cobalt(III) is Co3+ and hydroxide is OH-.

Write the formula for the compound

The charges of Co3+ and OH- are balanced when we have three hydroxide ions for each cobalt ion. Hence, the formula is Co(OH)₃. c. sodium hypochlorite:

Identify the ions and their charges

Sodium is Na+ and hypochlorite is ClO-.

Write the formula for the compound

Their charges balance out one-to-one, so the formula for sodium hypochlorite is NaClO. d. potassium dichromate:

Identify the ions and their charges

Potassium is K+ and dichromate is Cr2O7²-.

Write the formula for the compound

Since the charge of K+ is +1 and Cr2O7²- is -2, two potassium ions are needed to balance the charges, and the formula is K2Cr2O7. e. ammonium nitrite:

Identify the ions and their charges

Ammonium is NH4+ and nitrite is NO2-.

Write the formula for the compound

Their charges balance out one-to-one, so the formula for ammonium nitrite is NH4NO2. f. ferric hydroxide:

Identify the ions and their charges

Ferric is Fe³+ and hydroxide is OH-.

Write the formula for the compound

Since the Fe³+ charge is +3 and OH- charge is -1, three hydroxide ions are needed to balance the charges. Hence, the formula is Fe(OH)₃. g. ammonium hydrogen carbonate:

Identify the ions and their charges

Ammonium is NH4+ and hydrogen carbonate is HCO3-.

Write the formula for the compound

Their charges balance out one-to-one, so the formula for ammonium hydrogen carbonate is NH4HCO3. h. potassium perbromate:

Identify the ions and their charges

Potassium is K+ and perbromate is BrO4-.

Write the formula

Their charges balance out one-to-one, so the formula for potassium perbromate is KBrO4.

Key concepts

Understanding Chemical Formulas

Chemical formulas represent the types and numbers of atoms that constitute a particular chemical compound. At its most basic level, a chemical formula gives students a clear picture of the elements involved and the proportions in which they combine to form compounds. When dealing with polyatomic ions—ions made up of two or more atoms bonded together—it's crucial to know both the ion's name and its chemical formula, as they act as a single unit in forming compounds.

For example, in the compound sodium hypochlorite (NaClO), the formula indicates that one sodium (Na) ion combines with one hypochlorite (ClO-) ion. It is vital to remember, especially when compounds consist of ions like ammonium (NH4+) combining with nitrite (NO2-) to form ammonium nitrite (NH4NO2), that the positive and negative charges must balance out for the compound to be stable.

The Importance of Charge Balance in Compounds

Charge balance is the key to understanding how ionic compounds form. It's all about achieving electrical neutrality. This means the total positive charge in the compound must equal the total negative charge. While a metal ion like silver in silver perchlorate (AgClO4) might carry a +1 charge, the polyatomic perchlorate ion (ClO4-) carries a -1 charge. When these ions come together, their opposite charges cancel out, resulting in a neutral compound.

Visualizing charge balancing

Imagine you have a seesaw and on one side there's a +3 charge as with cobalt(III) in cobalt(III) hydroxide (Co(OH)3). To balance the seesaw, you would need three -1 charges, like those of hydroxide ions (OH-), on the other side. This visual can help students understand the necessity of adding three hydroxide ions to neutralize the charge from one cobalt(III) ion.

Ionic Compounds: Formation and Structure

Ionic compounds are formed when positively charged ions, which are usually metals, and negatively charged ions, generally non-metals or polyatomic ions, bond together. The strength of this electrostatic attraction creates the bond that holds the ions together in a crystal lattice structure. For instance, when potassium ions (K+) combine with dichromate ions (Cr2O7²-), you get potassium dichromate (K2Cr2O7), where each pair of potassium ions balances the -2 charge of the dichromate ion.

Properties of ionic compounds

Students should recognize that ionic compounds have high melting and boiling points, are typically solid at room temperature, and can conduct electricity when melted or dissolved in water—this is because the ions are free to move in liquid form.

Naming Chemical Compounds with Polyatomic Ions

Naming chemical compounds correctly requires an understanding of the rules that govern chemical nomenclature. For compounds with polyatomic ions, the name of the positive ion (the cation) comes first, followed by the name of the negative ion (the anion). If the cation can have multiple charges, like iron in ferric hydroxide (Fe(OH)3), its charge is specified in Roman numerals in parentheses immediately after the element's name.

Special naming conventions

Some polyatomic ions have their own unique names, and it’s vital to learn these to correctly name compounds. For example, ammonium hydrogen carbonate (NH4HCO3) includes the polyatomic ions ammonium (NH4+) and hydrogen carbonate (HCO3-). Through practice, students can become proficient in using these naming conventions seamlessly in their chemistry studies.