Which one of the following is a cation?
Ammonium
Understanding Cations in Chemistry
In chemistry, ions are atoms or molecules that have gained or lost electrons, resulting in a net electrical charge. Ions are classified into two main types:
- Cations: These are positively charged ions. They are formed when an atom or molecule loses one or more electrons. The number of protons is greater than the number of electrons.
- Anions: These are negatively charged ions. They are formed when an atom or molecule gains one or more electrons. The number of electrons is greater than the number of protons.
The question asks to identify which of the given options is a cation. Let's examine each option and determine its charge.
Analyzing the Options
Carbonate Ion
The carbonate ion is a polyatomic ion with the chemical formula $\text{CO}_3^{2-}$. It consists of one carbon atom bonded to three oxygen atoms, carrying a net charge of $-2$. Since it has a negative charge, it is an anion, not a cation.
Nitrate Ion
The nitrate ion is another polyatomic ion with the chemical formula $\text{NO}_3^{-}$. It consists of one nitrogen atom bonded to three oxygen atoms, carrying a net charge of $-1$. Since it has a negative charge, it is an anion, not a cation.
Ammonium Ion
The ammonium ion is a polyatomic ion with the chemical formula $\text{NH}_4^{+}$. It consists of one nitrogen atom bonded to four hydrogen atoms, carrying a net charge of $+1$. Since it has a positive charge, it is a cation.
Hydroxide Ion
The hydroxide ion is a polyatomic ion with the chemical formula $\text{OH}^{-}$. It consists of one oxygen atom bonded to one hydrogen atom, carrying a net charge of $-1$. Since it has a negative charge, it is an anion, not a cation.
Summary of Ions and Charges
Let's summarize the ions from the options and their respective charges in a table:
| Ion Name | Chemical Formula | Charge | Type |
|---|---|---|---|
| Carbonate | $\text{CO}_3^{2-}$ | $-2$ | Anion |
| Nitrate | $\text{NO}_3^{-}$ | $-1$ | Anion |
| Ammonium | $\text{NH}_4^{+}$ | $+1$ | Cation |
| Hydroxide | $\text{OH}^{-}$ | $-1$ | Anion |
Conclusion
Based on the analysis of the charges of each ion, the ammonium ion ($\text{NH}_4^{+}$) is the only one with a positive charge. Therefore, the ammonium ion is a cation.
The correct option is Ammonium.
Revision Table: Key Ion Concepts
| Concept | Description | Charge | Example |
|---|---|---|---|
| Ion | Atom or molecule with an electrical charge | Positive or Negative | $\text{Na}^{+}$, $\text{Cl}^{-}$ |
| Cation | Positively charged ion | Positive ($>0$) | $\text{NH}_4^{+}$, $\text{K}^{+}$, $\text{Mg}^{2+}$ |
| Anion | Negatively charged ion | Negative ($<0$) | $\text{NO}_3^{-}$, $\text{SO}_4^{2-}$, $\text{Br}^{-}$ |
| Polyatomic Ion | An ion composed of more than one atom covalently bonded together, carrying a net charge | Positive or Negative | $\text{NH}_4^{+}$, $\text{CO}_3^{2-}$, $\text{OH}^{-}$ |
Additional Information: Formation of Ions
Ions are formed through processes involving the transfer of electrons:
- Formation of Cations: Atoms of metals typically lose valence electrons to achieve a stable electron configuration, becoming positively charged cations. For example, a sodium atom ($\text{Na}$) loses one electron to form a sodium ion ($\text{Na}^{+}$). $\text{Na} \rightarrow \text{Na}^{+} + \text{e}^{-}$. Polyatomic cations like ammonium ($\text{NH}_4^{+}$) are formed through different chemical reactions where a neutral molecule (like ammonia, $\text{NH}_3$) gains a proton ($\text{H}^{+}$). $\text{NH}_3 + \text{H}^{+} \rightarrow \text{NH}_4^{+}$.
- Formation of Anions: Atoms of nonmetals typically gain electrons to achieve a stable electron configuration, becoming negatively charged anions. For example, a chlorine atom ($\text{Cl}$) gains one electron to form a chloride ion ($\text{Cl}^{-}$). $\text{Cl} + \text{e}^{-} \rightarrow \text{Cl}^{-}$. Polyatomic anions like carbonate ($\text{CO}_3^{2-}$) are formed from the dissociation of acids or salts in water.
Understanding the nature of ions is fundamental to studying ionic compounds, electrolytes, and many chemical reactions.
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