The correct order of energy of 2s orbitals of following is :

Q: The correct order of energy of 2s orbitals of following is :

The correct order of energy of 2s orbitals of following is :

Question

Accepted Answer

The correct order of energy of 2s orbitals for the elements given in Option 3, which is $H>Li>Na>K$, can be explained based on the concept of effective nuclear charge and the penetration effect of orbitals.

1. **Explanation for Option 3 ($H>Li>Na>K$):**
   - **Hydrogen (H):** In hydrogen, the 2s orbital is not occupied in the ground state, but if it were, it would experience the full nuclear charge of +1 without any electron shielding. Therefore, the energy level is relatively high.
   - **Lithium (Li):** In lithium, the 2s orbital is occupied, and the effective nuclear charge experienced by the 2s electron is reduced due to the shielding effect of the 1s electrons. However, since lithium has a higher nuclear charge than hydrogen, the 2s orbital is lower in energy compared to hydrogen.
   - **Sodium (Na):** In sodium, the 2s orbital is further shielded by the inner 1s and 2p electrons, reducing the effective nuclear charge experienced by the 2s electron even more than in lithium. This results in a lower energy level for the 2s orbital compared to lithium.
   - **Potassium (K):** In potassium, the 2s orbital is even more shielded due to the presence of additional inner electron shells. The effective nuclear charge experienced by the 2s electron is significantly reduced, making the 2s orbital energy the lowest among the elements listed.

2. **Why Other Options are Incorrect:**
   - **Option 1 ($H>Na>K>Li$):** This option incorrectly places lithium at the lowest energy level. As explained, lithium's 2s orbital should have higher energy than sodium and potassium due to less shielding.
   - **Option 2 ($Na>K>Li>H$):** This option suggests that sodium and potassium have higher 2s orbital energies than lithium, which contradicts the shielding effect and effective nuclear charge considerations. The 2s orbital in sodium and potassium is more shielded, resulting in lower energy compared to lithium.
   - **Option 4 ($K>Na>Li>H$):** This option incorrectly suggests that potassium has the highest 2s orbital energy. Due to extensive shielding in potassium, its 2s orbital energy is actually the lowest among the elements listed.

In conclusion, Option 3 ($H>Li>Na>K$) correctly reflects the order of 2s orbital energies based on the principles of effective nuclear charge and electron shielding.