The completely filled and the completely half-filled sub-shells are stable due to

Options

A. symmetrical distribution of electrons
B. exchange energy
C. Both (1) and (2)
D. None of the above

Answer

To answer the question regarding the stability of completely filled and completely half-filled subshells, we can break down the explanation into a step-by-step solution:

### Step-by-Step Solution:

1. **Understanding Electron Configuration**:
- In an atom, electrons are arranged in various energy levels and subshells (s, p, d, f). Each subshell can hold a specific number of electrons. A completely filled subshell has the maximum number of electrons it can hold, while a half-filled subshell has exactly half of its capacity filled.

2. **Stability of Electron Configurations**:
- Completely filled and half-filled subshells are known to be more stable than other configurations. This stability arises from two main factors: symmetry and exchange energy.

3. **Symmetry in Electron Distribution**:
- A completely filled or half-filled subshell exhibits a symmetrical distribution of electrons. For example, in a half-filled p subshell, each of the three p orbitals contains one electron, leading to a balanced and symmetrical arrangement. This symmetry contributes to the overall stability of the atom.

4. **Exchange Energy**:
- When electrons are arranged in a symmetrical manner, they can exchange positions without increasing the energy of the system. This phenomenon is known as exchange energy. In a completely filled or half-filled subshell, the electrons can exchange their positions, which leads to a decrease in potential energy.

5. **Lower Energy State**:
- Systems tend to be more stable when they are in a lower energy state. The symmetrical arrangement and the resulting exchange energy lower the potential energy of the atom, making it more stable.

6. **Conclusion**:
- Therefore, the stability of completely filled and half-filled subshells is due to the symmetrical distribution of electrons and the exchange energy that arises from this arrangement. Both factors contribute to a lower energy state, enhancing the stability of the atom.

### Final Answer:
The completely filled and completely half-filled subshells are stable due to:
1. Symmetrical distribution of electrons.
2. Exchange energy resulting from this symmetry.

Thus, the correct answer is both 1 and 2.