Arrange the following in increasing order of their atomic radius: Na, K, Mg, Rb

Options

A. `Mg lt K lt Na lt Rb`
B. `Mg lt Na lt K lt Rb `
C. `Mg lt Na lt Rb lt K`
D. `Na lt K lt Rb lt Mg`

Answer

To arrange the elements Na (sodium), K (potassium), Mg (magnesium), and Rb (rubidium) in increasing order of their atomic radius, we can follow these steps:

### Step 1: Identify the Group and Period of Each Element
- **Sodium (Na)**: Group 1 (Alkali metals), Period 3
- **Potassium (K)**: Group 1 (Alkali metals), Period 4
- **Magnesium (Mg)**: Group 2 (Alkaline earth metals), Period 3
- **Rubidium (Rb)**: Group 1 (Alkali metals), Period 5

### Step 2: Understand the Trend in Atomic Radius
- Atomic radius increases as you move down a group in the periodic table because additional electron shells are added.
- Atomic radius decreases as you move from left to right across a period due to the increase in effective nuclear charge, which pulls the electrons closer to the nucleus.

### Step 3: Compare the Atomic Radii
- **Rubidium (Rb)** is in Period 5 and has the largest atomic radius because it is the furthest down in Group 1.
- **Potassium (K)** is in Period 4 and has a smaller atomic radius than Rb but larger than Na and Mg.
- **Sodium (Na)** is in Period 3 and has a smaller atomic radius than K but larger than Mg.
- **Magnesium (Mg)** is also in Period 3 but is in Group 2, which means it has a smaller atomic radius than Na due to the higher effective nuclear charge.

### Step 4: Arrange the Elements in Increasing Order of Atomic Radius
Based on the above analysis, the increasing order of atomic radius is:
1. **Magnesium (Mg)** - smallest radius
2. **Sodium (Na)**
3. **Potassium (K)**
4. **Rubidium (Rb)** - largest radius

### Final Answer
The increasing order of atomic radius is: **Mg < Na < K < Rb**

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