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 among lithium (Li⁺), sodium (Na⁺), potassium (K⁺), and rubidium (Rb⁺), we can follow these steps:

### Step 1: Understand the trend in ionic size
- In the periodic table, as you move down a group, the size of the atoms and their corresponding ions increases. This is due to the addition of electron shells, which increases the distance between the nucleus and the outermost electrons.

### Step 2: Identify the ions and their positions in the periodic table
- The ions in question are:
- Lithium ion (Li⁺)
- Sodium ion (Na⁺)
- Potassium ion (K⁺)
- Rubidium ion (Rb⁺)

- These ions are from Group 1 (alkali metals) of the periodic table. Their sizes increase as you go down the group:
- Li⁺ < Na⁺ < K⁺ < Rb⁺

### Step 3: Consider the effect of hydration in aqueous solution
- In aqueous solution, ions are surrounded by water molecules. The extent of hydration depends on the size of the ion and its charge.
- Smaller ions with a higher charge density (charge per volume) tend to attract more water molecules, leading to greater hydration.

### Step 4: Analyze the hydration of the ions
- Li⁺ is the smallest ion and has the highest charge density, which means it will attract more water molecules and become highly hydrated.
- Conversely, Rb⁺, being the largest ion, will be less hydrated because its charge density is lower, leading to fewer water molecules surrounding it.

### Step 5: Determine the effective size in aqueous solution
- Although Rb⁺ is the largest ion in terms of ionic radius, its effective size in solution is reduced due to hydration.
- Li⁺, despite being the smallest ion, has a significant increase in effective size due to extensive hydration.

### Conclusion
- Therefore, the ion with the largest effective size in aqueous solution is **Li⁺** due to its high hydration.

### Summary of the Answer
The ion with the largest size in aqueous solution is **Li⁺**.

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