Which of the following atomic species has mximum ionisation energy:

Options

A. `O^(-)`
B. `S^(-)`
C. `Se^(-)`
D. `Te^(-)`

Answer

To determine which of the given atomic species has the maximum ionization energy among O-, S-, Se-, and Te-, we can follow these steps:

### Step-by-Step Solution

1. **Understand Ionization Energy**: Ionization energy is the energy required to remove the most loosely bound electron from an atom or ion. Generally, the higher the ionization energy, the more energy is required to remove an electron.

2. **Write Electronic Configurations**:
- For O- (Oxygen anion): The electronic configuration is 1s² 2s² 2p⁵.
- For S- (Sulfur anion): The electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁵.
- For Se- (Selenium anion): The electronic configuration is [Ar] 4s² 3d¹⁰ 4p⁵.
- For Te- (Tellurium anion): The electronic configuration is [Kr] 5s² 4d¹⁰ 5p⁵.

3. **Analyze the Last Orbitals**:
- O- has its outermost electrons in the 2p orbital.
- S- has its outermost electrons in the 3p orbital.
- Se- has its outermost electrons in the 4p orbital.
- Te- has its outermost electrons in the 5p orbital.

4. **Consider Atomic Size and Shielding**: The ionization energy generally decreases down a group in the periodic table because the outer electrons are farther from the nucleus and experience more shielding from inner electrons. Thus, O- (with 2p electrons) will have a higher ionization energy than S-, Se-, and Te-.

5. **Compare Ionization Energies**:
- O- has the smallest atomic size and highest effective nuclear charge acting on its outermost electrons in the 2p orbital.
- S- has a larger atomic size compared to O-, and the electrons in the 3p orbital are more shielded.
- Se- and Te- have even larger atomic sizes and more shielding, leading to lower ionization energies.

6. **Conclusion**: Based on the above analysis, the order of ionization energies from highest to lowest is:
- O- > S- > Se- > Te-
Therefore, the species with the maximum ionization energy is **O-**.

### Final Answer
The atomic species with the maximum ionization energy is **O-**.