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

Question

Accepted Answer

The correct order of energy of 2s orbitals for the given elements is $H>Li>Na>K$, as stated in Option 2. Let's explore why this is the correct order and why the other options are incorrect.

### Explanation for Option 2: $H>Li>Na>K$

1. **Hydrogen (H):** In hydrogen, the 2s orbital is not occupied in the ground state, but when considering excited states or comparing with other elements, it is important to note that hydrogen has the simplest atomic structure with only one electron. The energy levels in hydrogen are determined solely by the principal quantum number (n), and there is no electron shielding or electron-electron repulsion to consider. Therefore, the energy of the 2s orbital in hydrogen is relatively high compared to other elements with more electrons.

2. **Lithium (Li):** In lithium, the 2s orbital is occupied by the third electron. The presence of additional electrons compared to hydrogen introduces electron-electron repulsion and shielding effects, which lower the energy of the 2s orbital compared to hydrogen.

3. **Sodium (Na):** Sodium has more electrons than lithium, and its 2s orbital is further shielded by the inner electrons. This increased shielding effect further lowers the energy of the 2s orbital compared to lithium.

4. **Potassium (K):** Potassium has even more electrons, and the 2s orbital is even more shielded by the inner electrons. This results in the lowest energy for the 2s orbital among the elements listed.

### Why Other Options are Incorrect:

1. **Option 1: $H>Na>K>Li$**
   - This option incorrectly places sodium before lithium. As explained, lithium's 2s orbital has higher energy than sodium's due to less shielding and fewer electron-electron interactions.

2. **Option 3: $K>Na>Li>H$**
   - This option suggests that potassium has the highest energy 2s orbital, which is incorrect. Potassium's 2s orbital is the most shielded and thus has the lowest energy among the elements listed.

3. **Option 4: $Na>K>Li>H$**
   - This option incorrectly places sodium and potassium before lithium and hydrogen. As discussed, lithium's 2s orbital has higher energy than both sodium and potassium due to less shielding, and hydrogen's 2s orbital has the highest energy due to the absence of electron shielding and repulsion.

In conclusion, Option 2 correctly reflects the order of energy of the 2s orbitals based on the principles of electron shielding and electron-electron repulsion, making it the correct choice.