Which one of the following ions has the largest size in aqueous solution?

Options

A. `Rb^(+)`
B. `Na^(+)`
C. `K^(+)`
D. `Li^(+)`

Answer

To determine which ion has the largest size in aqueous solution, we need to consider the effects of hydration on the ionic radii of the alkali metal ions. Here’s a step-by-step solution:

### Step 1: Understand Ionic Sizes
- The size of ions generally increases as we move down a group in the periodic table. For alkali metals, the order of ionic sizes in the gas phase is:
\[ \text{Li}^+ < \text{Na}^+ < \text{K}^+ < \text{Rb}^+ \]
- This means that rubidium ion (Rb⁺) is the largest in size among these ions when not in an aqueous solution.

**Hint for Step 1:** Remember that ionic size increases down a group in the periodic table due to the addition of electron shells.

### Step 2: Consider Hydration Effects
- In an aqueous solution, ions are surrounded by water molecules, which can affect their effective size. The extent of hydration depends on the charge density of the ion.
- Charge density is determined by the charge of the ion divided by its volume. Smaller ions with higher charge densities attract more water molecules.

**Hint for Step 2:** Charge density influences how many water molecules surround an ion, affecting its size in solution.

### Step 3: Analyze Hydration of Alkali Metal Ions
- Lithium ion (Li⁺) is smaller than the other alkali metal ions, but it has a high charge density. This means it can attract a larger number of water molecules compared to larger ions.
- As a result, the hydration shell around Li⁺ is larger than those around Na⁺, K⁺, and Rb⁺.

**Hint for Step 3:** Smaller ions with higher charge density tend to have larger hydration shells.

### Step 4: Compare Hydrated Sizes
- The order of ionic sizes in aqueous solution is influenced by hydration:
\[ \text{Li}^+ > \text{Na}^+ > \text{K}^+ > \text{Rb}^+ \]
- Therefore, in aqueous solution, the lithium ion (Li⁺) is larger than sodium (Na⁺), potassium (K⁺), and rubidium (Rb⁺) due to its extensive hydration.

**Hint for Step 4:** Remember that the order of ionic sizes can change when considering hydration effects.

### Conclusion
- The ion with the largest size in aqueous solution is **Li⁺** due to its strong hydration effect, despite being the smallest ion in the gas phase.

**Final Answer:** The correct option is **Li⁺**.