Which one of the following has the largest ionisation energy? Ar Cl K Al

Options

A. Ar
B. Cl
C. K
D. Al

Explanation

To determine which of the given elements has the largest ionization energy, we need to analyze the properties of each element: Argon (Ar), Chlorine (Cl), Potassium (K), and Aluminum (Al).

### Step-by-Step Solution:

**Step 1: Understand Ionization Energy**
- Ionization energy is the minimum amount of energy required to remove an electron from the outermost shell of a neutral gaseous atom.

**Hint:** Remember that elements with stable electron configurations (like noble gases) tend to have higher ionization energies.

**Step 2: Consider the Position of Elements in the Periodic Table**
- Argon (Ar) is a noble gas located in Group 18.
- Chlorine (Cl) is a halogen located in Group 17.
- Aluminum (Al) is a post-transition metal located in Group 13.
- Potassium (K) is an alkali metal located in Group 1.

**Hint:** Noble gases have complete valence shells, which makes them more stable and less likely to lose electrons.

**Step 3: Compare Atomic Sizes**
- Atomic size generally decreases across a period from left to right and increases down a group.
- Potassium has the largest atomic size, followed by Aluminum, Chlorine, and Argon, which has the smallest atomic size.

**Hint:** Smaller atomic size usually correlates with higher ionization energy because the electrons are held more tightly by the nucleus.

**Step 4: Analyze the Electron Configuration**
- Argon has the electron configuration of [Ne] 3s² 3p⁶ (complete octet).
- Chlorine has the configuration of [Ne] 3s² 3p⁵.
- Aluminum has the configuration of [Ne] 3s² 3p¹.
- Potassium has the configuration of [Ar] 4s¹.

**Hint:** Elements with a complete outer shell (like Argon) are more stable and require more energy to remove an electron compared to those with incomplete outer shells.

**Step 5: Determine Ionization Energy Trends**
- Ionization energy increases across a period and decreases down a group. Therefore, Argon, being a noble gas with a complete octet, will have the highest ionization energy among the four elements.

**Hint:** Remember that ionization energy is highest for noble gases due to their stable electron configurations.

**Step 6: Conclusion**
- Based on the analysis, Argon (Ar) has the largest ionization energy among the options provided.

### Final Answer:
**Argon (Ar) has the largest ionization energy.**