Polyatomic positive ions often have common names ending with the suffix ______.
onium
Understanding Polyatomic Ion Naming
When studying chemistry, understanding how ions are named is crucial. Ions are atoms or groups of atoms that have gained or lost electrons, resulting in a net electrical charge. They can be simple (consisting of a single atom) or polyatomic (consisting of multiple atoms bonded together).
What are Polyatomic Ions?
Polyatomic ions are charged species composed of two or more atoms covalently bonded together. These groups of atoms act as a single unit when forming ionic compounds or participating in reactions.
Naming Conventions for Polyatomic Ions
The naming of polyatomic ions follows specific rules. While many polyatomic anions (negatively charged ions) have names ending in '-ate' or '-ite' (like sulfate, nitrate, sulfite, nitrite), polyatomic cations (positively charged ions) have a different common naming convention.
The question specifically asks about the common suffix for polyatomic positive ions. Let's examine the options:
- ic: This suffix is often used in the naming of simple metal cations to indicate a higher oxidation state of the metal (e.g., Ferric ion, $\text{Fe}^{3+}$). It is not typically used for polyatomic positive ions.
- ide: This suffix is commonly used for simple monatomic anions (like chloride, $\text{Cl}^-$; oxide, $\text{O}^{2-}$) and some common polyatomic anions (like hydroxide, $\text{OH}^-$; cyanide, $\text{CN}^-$). It indicates a negative charge, not a positive one.
- onium: This suffix is characteristic of many common polyatomic positive ions. It signifies a positively charged polyatomic species, often formed by adding a proton ($\text{H}^+$) to a neutral molecule.
- ous: This suffix is also used in the naming of simple metal cations to indicate a lower oxidation state of the metal (e.g., Ferrous ion, $\text{Fe}^{2+}$). It is not used for polyatomic positive ions.
Based on chemical nomenclature rules, the suffix 'onium' is the common ending for many polyatomic positive ions. Examples include:
- Ammonium ion: $\text{NH}_4^+$ (formed from ammonia, $\text{NH}_3$, plus $\text{H}^+$)
- Hydronium ion: $\text{H}_3\text{O}^+$ (formed from water, $\text{H}_2\text{O}$, plus $\text{H}^+$)
- Phosphonium ion: $\text{PH}_4^+$ (formed from phosphine, $\text{PH}_3$, plus $\text{H}^+$)
- Sulfonium ion: E.g., $(\text{CH}_3)_3\text{S}^+$ (formed from dimethyl sulfide plus $\text{CH}_3^+$)
Therefore, polyatomic positive ions often have common names ending with the suffix 'onium'.
| Suffix | Typical Use | Example |
|---|---|---|
| -ic | Simple metal cations (higher oxidation state) | Ferric ($\text{Fe}^{3+}$) |
| -ide | Simple anions, some polyatomic anions | Chloride ($\text{Cl}^-$), Hydroxide ($\text{OH}^-$) |
| -onium | Common polyatomic positive ions | Ammonium ($\text{NH}_4^+$), Hydronium ($\text{H}_3\text{O}^+$) |
| -ous | Simple metal cations (lower oxidation state) | Ferrous ($\text{Fe}^{2+}$) |
Revision Table: Naming Ions
| Ion Type | Description | Common Suffix/Naming | Examples |
|---|---|---|---|
| Monatomic Cation | Single atom, positive charge | Name of element | Sodium ion ($\text{Na}^+$), Calcium ion ($\text{Ca}^{2+}$) |
| Monatomic Cation (Variable Charge) | Single atom, positive charge, multiple possible charges | Element name (Roman numeral) OR old name (-ous/-ic) | Iron(II) ion or Ferrous ion ($\text{Fe}^{2+}$), Iron(III) ion or Ferric ion ($\text{Fe}^{3+}$) |
| Polyatomic Cation | Group of atoms, positive charge | Often ends in -onium | Ammonium ion ($\text{NH}_4^+$), Hydronium ion ($\text{H}_3\text{O}^+$) |
| Monatomic Anion | Single atom, negative charge | Element name + -ide | Chloride ion ($\text{Cl}^-$), Oxide ion ($\text{O}^{2-}$) |
| Polyatomic Anion | Group of atoms, negative charge | Often ends in -ate or -ite, sometimes -ide | Sulfate ion ($\text{SO}_4^{2-}$), Nitrite ion ($\text{NO}_2^-$), Hydroxide ion ($\text{OH}^-$) |
Additional Information: Formation of Onium Ions
Polyatomic positive ions ending in '-onium' are typically formed when a molecule with a lone pair of electrons accepts a proton ($\text{H}^+$). This process is a type of acid-base reaction where the neutral molecule acts as a Brønsted-Lowry base. The accepted proton adds a positive charge and often changes the structure slightly, but the core group of atoms remains together as a single charged entity. For instance, ammonia ($\text{NH}_3$) has a lone pair on the nitrogen atom. When it accepts a proton, it forms the ammonium ion ($\text{NH}_4^+$), which is positively charged. Similarly, water ($\text{H}_2\text{O}$) with lone pairs on oxygen forms the hydronium ion ($\text{H}_3\text{O}^+$) by accepting a proton.
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