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 the given options (rubidium ion, sodium ion, potassium ion, and lithium ion), we can follow these steps:

### Step 1: Understand the trend in ionic size
The size of ions generally increases as you move down a group in the periodic table. This is because with each successive element, an additional electron shell is added, 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 we are considering are:
- Lithium ion (Li⁺)
- Sodium ion (Na⁺)
- Potassium ion (K⁺)
- Rubidium ion (Rb⁺)

These ions belong to Group 1 (alkali metals) of the periodic table.

### Step 3: Compare the ionic sizes based on their positions
- **Lithium ion (Li⁺)**: Smallest size due to being at the top of the group.
- **Sodium ion (Na⁺)**: Larger than Li⁺ but smaller than K⁺ and Rb⁺.
- **Potassium ion (K⁺)**: Larger than Na⁺.
- **Rubidium ion (Rb⁺)**: Largest among these ions due to being at the bottom of the group.

### Step 4: Consider the effect of hydration in aqueous solution
In aqueous solution, the size of the hydrated ion can be influenced by the ion's charge density. A higher charge density can lead to stronger interactions with water molecules, resulting in a more significant hydration shell.

- **Lithium ion (Li⁺)** has a high charge density due to its small size, which means it attracts more water molecules and becomes highly hydrated.
- **Sodium ion (Na⁺)**, **Potassium ion (K⁺)**, and **Rubidium ion (Rb⁺)** have lower charge densities compared to Li⁺, leading to less hydration.

### Step 5: Conclude which ion has the largest size in aqueous solution
Despite Rb⁺ being the largest ion in terms of ionic radius, when considering hydration, the lithium ion (Li⁺) becomes more hydrated due to its high charge density. Therefore, in aqueous solution, the effective size of Li⁺ is larger than that of the other ions due to its hydration shell.

**Final Answer**: The lithium ion (Li⁺) has the largest size in aqueous solution.

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