The energy required to pull the most loosely bound electron form an atom is known as ionizatino potential it is expressed in electron volts. The value of ionization potntial depends on three factors: (i) the charge on the nucleus (ii) the atomic radius and (iii) the screening effect of inner electron shells.
Q. Which of the following elements has the least ionization potential?

Options

A. Lithium
B. Cesium
C. Magnesium
D. Calcium

Answer

To determine which element has the least ionization potential among lithium, cesium, magnesium, and calcium, we can follow these steps:

### Step 1: Understand Ionization Potential
Ionization potential (or ionization energy) is the energy required to remove the most loosely bound electron from an atom. It is influenced by three main factors:
1. The charge on the nucleus (Z)
2. The atomic radius
3. The screening effect of inner electron shells

### Step 2: Analyze the Periodic Trends
- **Charge on the Nucleus**: As the positive charge (nuclear charge) increases, the ionization potential generally increases because the attraction between the nucleus and the outermost electron becomes stronger.
- **Atomic Radius**: As you move down a group in the periodic table, the atomic radius increases due to the addition of electron shells. This increase in radius leads to a decrease in ionization potential because the outermost electrons are further from the nucleus and are less tightly held.
- **Screening Effect**: Inner electrons can shield the outermost electrons from the full effect of the nuclear charge, reducing the effective nuclear charge felt by the outermost electron.

### Step 3: Compare the Elements
- **Lithium (Li)**: Group 1, Period 2
- **Cesium (Cs)**: Group 1, Period 6
- **Magnesium (Mg)**: Group 2, Period 3
- **Calcium (Ca)**: Group 2, Period 4

### Step 4: Group Analysis
1. **Group 1 Elements (Li and Cs)**:
- Cesium (Cs) is further down the group than lithium (Li), meaning it has a larger atomic radius and therefore a lower ionization potential.
2. **Group 2 Elements (Mg and Ca)**:
- Calcium (Ca) is below magnesium (Mg) in the periodic table, which means it also has a larger atomic radius and a lower ionization potential compared to magnesium.

### Step 5: Compare Across Groups
- Between lithium and cesium, cesium has a lower ionization potential due to its larger atomic radius.
- Between magnesium and calcium, calcium has a lower ionization potential.
- However, when comparing cesium and calcium, cesium is the largest and has the lowest ionization potential among all four elements.

### Conclusion
Thus, among the given options, **cesium (Cs)** has the least ionization potential.

### Final Answer
**Cesium (Cs)** has the least ionization potential.

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