Which of the following principles/limits the maximum number of electrons in an orbital to two ?

Options

A. Aufbau's principle
B. Pauli's exclusion principle
C. Hund's rule of maximum multiplicity
D. Heisenberg uncertainly principle

Explanation

To solve the question of which principle limits the maximum number of electrons in an orbital to two, we can analyze each of the given options step by step.

### Step-by-Step Solution:

1. **Understanding the Options**:
- **Option A: Aufbau Principle** - This principle states that electrons fill atomic orbitals in order of increasing energy levels. It does not limit the number of electrons per orbital.
- **Option B: Pauli Exclusion Principle** - This principle states that no two electrons in an atom can have the same set of four quantum numbers. This means that an orbital can hold a maximum of two electrons, provided they have opposite spins.
- **Option C: Hund's Rule of Maximum Multiplicity** - This rule states that electrons will fill degenerate orbitals (orbitals of the same energy) singly before pairing up. It does not limit the number of electrons in a single orbital.
- **Option D: Heisenberg Uncertainty Principle** - This principle relates to the limits of measuring the position and momentum of particles and does not pertain to the number of electrons in an orbital.

2. **Identifying the Correct Answer**:
- From the analysis, we can see that the **Pauli Exclusion Principle** is the only principle that specifically states that an orbital can hold a maximum of two electrons.

3. **Conclusion**:
- Therefore, the correct answer to the question is **Option B: Pauli Exclusion Principle**.