Polyatomic Ions: Videos & Practice Problems

Polyatomic ions are groups of atoms that carry a charge, with oxyanions being a key focus. Negatively charged trioxides, like borate (B_O3^3-), contain three oxygens, while tetraoxides, such as sulfate (S_O4^2-), have four. The naming convention changes with oxygen count, where reducing oxygens alters the suffix from -ate to -ite. Additionally, positively charged ions like ammonium (NH₄⁺) and mercury(I) (Hg₂²⁺) are notable exceptions. Understanding these ions is crucial for mastering chemical reactions and stoichiometry.

Polyatomic Oxyanions

Polyatomic ions are groups of atoms bonded together that carry an overall charge, which can be either positive or negative. This summary focuses on negatively charged polyatomic ions known as oxyanions, specifically those that contain oxygen. Oxyanions can be categorized into two main types: trioxides and tetraoxides.

Trioxides are characterized by having three oxygen atoms. A helpful mnemonic to remember this is the letter "T," which can be visualized on the periodic table. The oxyanions that fall into this category include borate (\( \text{BO}_3^{3-} \)), carbonate (\( \text{CO}_3^{2-} \)), nitrate (\( \text{NO}_3^{-} \)), and silicate (\( \text{SiO}_3^{2-} \)). Each of these ions ends with the suffix "-ate," indicating the presence of three oxygen atoms.

On the other hand, tetraoxides contain four oxygen atoms, as suggested by the prefix "tetra," which means four. The common tetraoxides include phosphate (\( \text{PO}_4^{3-} \)) and sulfate (\( \text{SO}_4^{2-} \)). Similar to trioxides, these ions also end with the suffix "-ate," but they possess four oxygen atoms.

Understanding the structure and naming conventions of these polyatomic oxyanions is crucial for recognizing their chemical behavior and charges. The next step involves exploring how these oxyanions are assigned their specific charges, which will further enhance comprehension of their roles in chemical reactions.

Polyatomic Ion Charges

Understanding the charges of polyatomic ions, specifically trioxides and tetraoxides, is essential in chemistry. Trioxides, characterized by having three oxygen atoms, follow a specific pattern based on their position in the periodic table. For instance, elements in group 3A typically exhibit a charge of -3, leading to the formula for boron trioxide as \( \text{B}_2\text{O}_3^{3-} \). As we move through the groups, the charges decrease: group 4A elements have a charge of -2, while group 5A elements show a charge of -1, exemplified by nitrate \( \text{NO}_3^{-} \).

On the other hand, tetraoxides contain four oxygen atoms. The same periodic trend applies here, starting again from -3 for group 3A, as seen in phosphate \( \text{PO}_4^{3-} \). For sulfate, which is a tetraoxide, the charge is -2, represented as \( \text{SO}_4^{2-} \). Additionally, elements in group 7A can also exhibit a -1 charge, which will be explored further in subsequent discussions.

In summary, recognizing the number of oxygen atoms in these polyatomic ions and their corresponding charges is crucial for understanding their chemical behavior and interactions. This foundational knowledge sets the stage for exploring more complex polyatomic ions in future studies.

Polyatomic Oxyanion Names

In the study of polyatomic ions, understanding the structures and names of trioxides and tetraoxides is essential. Trioxides contain three oxygen atoms, while tetraoxides consist of four. For instance, the trioxide borate, represented as \( \text{BO}_3^{3-} \), indicates the presence of boron. Similarly, carbonate, with the formula \( \text{CO}_3^{2-} \), is associated with carbon, and nitrate, \( \text{NO}_3^{-} \), corresponds to nitrogen. Silicate, represented as \( \text{SiO}_3^{2-} \), involves silicon.

Moving on to tetraoxides, phosphate is a key example, with the formula \( \text{PO}_4^{3-} \), indicating phosphorus. Sulfate, represented as \( \text{SO}_4^{2-} \), is linked to sulfur. These polyatomic oxyanions are among the most common and are crucial for understanding chemical reactions and compounds. Recognizing their structures and associated charges allows for the identification of new polyatomic ions and their corresponding names, facilitating further exploration in chemistry.

Polyatomic Ion Variations

Understanding polyatomic ions involves recognizing how variations in the number of oxygen atoms can lead to the formation of new ions with distinct names and properties. When the number of oxygen atoms in a polyatomic ion is decreased by one, the suffix changes from "ate" to "ite," while the overall charge remains unchanged. For instance, sulfate (SO₄^2-) is a tetraoxide, containing four oxygen atoms and carrying a charge of -2. When one oxygen is removed, it transforms into sulfite (SO₃^2-), maintaining the same charge but adopting the new "ite" suffix.

This systematic alteration in naming reflects a broader principle in chemistry: the manipulation of oxygen atoms in polyatomic ions can yield a variety of new ions, each with unique characteristics. Recognizing these patterns is crucial for mastering the nomenclature of polyatomic ions and understanding their roles in chemical reactions and compounds.

Naming Polyatomic Oxyanion Example

To determine the formal name of the polyatomic ion \( \text{PO}_3^{3-} \), it is essential to understand the relationship between the number of oxygen atoms and the naming conventions for polyatomic ions. The ion in question contains phosphorus and is derived from the phosphate ion, which is represented as \( \text{PO}_4^{3-} \). The name for this ion is phosphate.

When the number of oxygen atoms is decreased by one, the resulting ion is \( \text{PO}_3^{3-} \). According to the naming rules for polyatomic ions, reducing the number of oxygen atoms from four to three changes the suffix from "ate" to "ite." Therefore, the systematic name for \( \text{PO}_3^{3-} \) is phosphite.

This naming convention is crucial for understanding how variations in the number of oxygen atoms affect the names of polyatomic ions. Remembering the tetra- and trioxides, along with their corresponding names, allows for easy identification and naming of related ions.

Give the systematic name for the following polyatomic ion.
NO₂^–

• A. nitrate
• B. nitrite
• C. nitrous
• D. pernitrate

Answer: (B)

Give the systematic name for the following polyatomic ion.
CO₂^2–

• A. hypocarbonate
• B. carbonate
• C. carbonite
• D. carbonide
• E. carbide

Answer: (C)

Give the systematic name for the following polyatomic ion.
AsO₄^3–

• A. arsenate
• B. arsenite
• C. arsenic
• D. hypoarsenate
• E. arsenide

Answer: (A)

The formula for the sulfate ion, SO₄^2–. If the term of 'thio' means the replacement of an oxygen by a sulfur, what is the formula for the thiosulfate ion?

• A. SO₄^3-
• B. SO₃^3-
• C. SO₃^2-
• D. S₂O₃^2-
• E. S₂O₃^-

Answer: (D)

Halogenoxy Anions Naming

Halogenoxy anions, also known as oxyhalogens, are polyatomic ions that consist of oxygen and halogens, which are found in Group 17 of the periodic table. These ions are characterized by a common charge of -1. The naming of halogenoxy anions is influenced by the number of oxygen atoms present, which affects both the prefix and suffix of their names.

The base name of these anions is derived from the halogen's name, with specific modifications. For example, the base names are as follows: fluorine becomes "fluor," chlorine becomes "chlor," bromine becomes "brom," and iodine becomes "iod." The naming conventions change based on the number of oxygen atoms:

• If there are four oxygen atoms, the prefix "per-" is added to the base name, followed by the suffix "-ate." For instance, with bromine and four oxygens, the name is "perbromate" (BrO₄^-).
• With three oxygen atoms, the prefix "per-" is dropped, and the suffix "-ate" remains. For example, chlorine with three oxygens is named "chlorate" (ClO₃^-).
• For two oxygen atoms, the suffix changes to "-ite." Thus, iodine with two oxygens is called "iodite" (IO₂^-).
• Finally, with one oxygen atom, the prefix "hypo-" is used, followed by the base name and the suffix "-ite." For example, fluorine with one oxygen is named "hypofluorite" (FO₁^-).

In summary, the naming of halogenoxy anions is determined by the number of oxygen atoms, which influences both the prefix and suffix, while the type of halogen dictates the base name. Understanding these conventions is essential for accurately identifying and naming these polyatomic ions.

Naming Halogen Oxyanion Example

When naming halogen oxyanions, the number of oxygen atoms present in the compound determines the prefix and suffix used in the name. For example, consider the compound ClO₄^-. This compound contains four oxygen atoms, which leads to the use of the prefix "per-" combined with the base name of chlorine, resulting in the name perchlorate. The perchlorate ion carries a charge of -1.

Now, let's examine the second compound, BrO₂^-. In this case, there are two oxygen atoms. For halogens, when there are two oxygens, no prefix is used. Instead, we take the base name of bromine, which is "brom," and apply the suffix "-ite." Therefore, BrO₂^- is named bromite.

In summary, the naming of halogen oxyanions is influenced by the number of oxygen atoms: four oxygens lead to the "per-" prefix and "-ate" suffix, while two oxygens result in the base name with the "-ite" suffix. Understanding these naming conventions is essential for correctly identifying and communicating about these chemical species.

Polyatomic Cations

In the study of polyatomic ions, it is essential to recognize that while most polyatomic ions carry a negative charge, there are notable exceptions that possess positive charges. The two primary positively charged polyatomic ions to be aware of are the ammonium ion (NH₄⁺) and the mercury(I) ion (Hg₂²⁺).

The ammonium ion, NH₄⁺, is the only major polyatomic ion with a +1 charge that students need to memorize. This ion plays a significant role in various chemical reactions and is commonly encountered in both organic and inorganic chemistry.

On the other hand, the mercury(I) ion, represented as Hg₂²⁺, can be a bit confusing due to its notation. The subscript '2' indicates that there are two individual mercury ions (Hg⁺) that combine to form a diatomic ion. Each mercury ion carries a +1 charge, and when they combine, their total charge results in +2. Thus, it is important to understand that the charge of +2 arises from the combination of two +1 charged mercury ions.

In summary, while the majority of polyatomic ions are negatively charged, the ammonium ion and the mercury(I) ion are key examples of positively charged polyatomic ions that are crucial for understanding various chemical processes.

Other Polyatomic Ions

Polyatomic ions can be complex, especially those that do not fit into predictable patterns and must be memorized. Among these, the tetraoxides are notable, including permanganate, chromate, and oxalate. Each of these ions contains four oxygen atoms, but they are categorized as "others" due to the distinct elements involved: manganese, chromium, and carbon. Their formulas are as follows: permanganate is represented as \(\text{MnO}_4^{-1}\), chromate as \(\text{CrO}_4^{2-}\), and oxalate as \(\text{C}_2\text{O}_4^{2-}\).

In addition to tetraoxides, there are other polyatomic ions that do not conform to the same classification. For instance, cyanide, with the formula \(\text{CN}^{-}\), and hydroxide, \(\text{OH}^{-}\), are examples of ions that may not contain multiple oxygen atoms. Peroxide, represented as \(\text{O}_2^{2-}\), consists of two oxygen atoms, each carrying a -1 charge, resulting in a total charge of -2. Dichromate, which is similar to chromate, is denoted as \(\text{Cr}_2\text{O}_7^{2-}\), indicating a doubled structure.

Furthermore, cyanate, which is related to cyanide, has the formula \(\text{OCN}^{-}\), distinguishing it by the presence of an oxygen atom. Lastly, the acetate ion can be expressed in two common forms: \(\text{C}_2\text{H}_3\text{O}_2^{-}\) or \(\text{CH}_3\text{COO}^{-}\). Both representations are widely accepted, and familiarity with either form is beneficial as you progress in chemistry.

Polyatomic Ion Example

To understand the structure of the thiocyanate ion, it is essential to first recognize the relationship between thiocyanate and its related polyatomic ion, cyanate. The cyanate ion is represented as OCN^-. The prefix "thio" indicates that one oxygen atom in the cyanate ion is replaced by a sulfur atom. Therefore, when constructing the thiocyanate ion, we substitute the oxygen in cyanate with sulfur, resulting in the formula SCN^-.

This transformation highlights the importance of recognizing prefixes in chemical nomenclature, as they provide critical information about the composition of the ions. In summary, the thiocyanate ion is structured as SCN^-, where the sulfur atom replaces the oxygen from the cyanate ion while keeping the remaining components intact.

Give the systematic name for the following polyatomic ion.
FO^–

• A. hypofluorite
• B. fluorite
• C. fluoride
• D. fluorate
• E. hypofluorate

Answer: (A)

Give the systematic name for the following polyatomic ion.
IO₃^–

• A. iodite
• B. hypoiodate
• C. iodide
• D. iodate
• E. periodate

Answer: (D)

The silicate ion is the silicon version of the carbonate ion. Based on this description, provide the structure of the silicate ion.

• A. SiO₃^4-
• B. SiO₃^3-
• C. SiO₃^2-
• D. SiO₂^2-
• E. SiO₂^3-

Answer: (C)

Here’s what students ask on this topic:

What are polyatomic ions and how are they classified?

Polyatomic ions are groups of atoms covalently bonded together that carry an overall charge, either positive or negative. They are classified based on their composition and charge. Negatively charged polyatomic ions are more common and include oxyanions, which contain oxygen. Oxyanions are further categorized into trioxides (e.g., NO₃^-, CO₃^2-) and tetraoxides (e.g., SO₄^2-, PO₄^3-) based on the number of oxygen atoms. Some polyatomic ions, like NH₄⁺ (ammonium), are positively charged. Others, such as cyanide (CN^-) and hydroxide (OH^-), do not fit into predictable patterns and must be memorized.

How do the names of polyatomic ions change when the number of oxygen atoms varies?

The names of polyatomic ions change based on the number of oxygen atoms they contain. For oxyanions, the suffix '-ate' is used for the base form (e.g., SO₄^2-, sulfate). If one oxygen atom is removed, the suffix changes to '-ite' (e.g., SO₃^2-, sulfite). For halogen oxyanions, prefixes are also used: 'per-' indicates one more oxygen than the '-ate' form (e.g., ClO₄^-, perchlorate), and 'hypo-' indicates one less oxygen than the '-ite' form (e.g., ClO^-, hypochlorite). The overall charge remains unchanged during these variations.

What are the charges of common trioxides and tetraoxides, and how are they determined?

The charges of common trioxides and tetraoxides are determined based on their position in the periodic table. For trioxides, such as borate (BO₃^3-), carbonate (CO₃^2-), and nitrate (NO₃^-), the charges decrease from -3 to -1 as you move across the periodic table. For tetraoxides, such as phosphate (PO₄^3-) and sulfate (SO₄^2-), the charges also follow a similar pattern. These charges are consistent with the group numbers of the central atoms in the periodic table.

What are halogen oxyanions, and how are they named?

Halogen oxyanions are polyatomic ions that contain a halogen (group 7A/17 elements) and oxygen. Their names depend on the number of oxygen atoms. For example, ClO₄^- is perchlorate ('per-' prefix and '-ate' suffix for 4 oxygens), ClO₃^- is chlorate ('-ate' suffix for 3 oxygens), ClO₂^- is chlorite ('-ite' suffix for 2 oxygens), and ClO^- is hypochlorite ('hypo-' prefix and '-ite' suffix for 1 oxygen). All halogen oxyanions carry a -1 charge, and the base name of the halogen (e.g., chlor for chlorine) is used in the name.

What are the exceptions to negatively charged polyatomic ions?

While most polyatomic ions are negatively charged, there are exceptions. The ammonium ion (NH₄⁺) is a positively charged polyatomic ion with a +1 charge. Another exception is the mercury(I) ion (Hg₂²⁺), which consists of two Hg⁺ ions bonded together, resulting in an overall +2 charge. These positively charged polyatomic ions are less common but important to recognize in chemistry.

What are some polyatomic ions that do not fit into predictable patterns?

Some polyatomic ions do not fit into predictable patterns and must be memorized. Examples include cyanide (CN^-), hydroxide (OH^-), peroxide (O₂^2-), and acetate (C₂H₃O₂^-). Additionally, dichromate (Cr₂O₇^2-) and permanganate (MnO₄^-) are notable examples. These ions often have unique structures or compositions, making them harder to categorize systematically.