Introduction:
The arrangement of electrons in the atomic orbitals of actinoids follows a specific pattern according to the Aufbau principle, which states that electrons occupy the lowest energy levels first before moving to higher energy levels. However, there are exceptions to this rule due to the nature of electron configurations and the principle quantum numbers associated with different orbitals.

Explanation:
1. Energy Levels: The energy levels of orbitals are determined by the principal quantum number (n). In actinoids, both the 5f and 6d orbitals have similar energy levels, as they belong to the same principal quantum number (n=5). However, the 6d orbital has a slightly lower energy than the 5f orbital due to shielding effects and effective nuclear charge.

2. Shielding and Effective Nuclear Charge: The 5f orbitals experience more shielding and less effective nuclear charge compared to the 6d orbitals. This is because the 5f orbitals are more diffuse and farther from the nucleus, making them more shielded from the nuclear charge by the inner electron shells. On the other hand, the 6d orbitals are closer to the nucleus and experience a higher effective nuclear charge.

3. Order of Filling: According to Hund's rule, when orbitals of equal energy (degenerate orbitals) are available, electrons occupy different orbitals with the same spin before pairing up. This rule applies to both the 5f and 6d orbitals in actinoids.

4. Exceptions to Aufbau Principle: While the Aufbau principle suggests that the 5f orbitals should be filled before the 6d orbitals, there are exceptions in actinoids. This is because the slightly lower energy and higher effective nuclear charge of the 6d orbitals make them more favorable for electron occupation.

5. Stability and Energy Considerations: The energy of an electron configuration is influenced by factors such as stability, electron-electron repulsions, and electron-nuclear attractions. The arrangement of electrons in the 6d orbitals allows for a more stable and lower energy configuration compared to filling the 5f orbitals first.

6. Experimental Observations: Experimental observations of actinoids' electron configurations support the filling of 6d orbitals before the 5f orbitals. The observed electron configurations align with the concept of filling lower energy levels first, taking into account shielding effects and effective nuclear charge.

Conclusion:
In actinoids, the 6d orbitals are filled before the 5f orbitals due to their slightly lower energy levels, higher effective nuclear charge, and the stability considerations of the electron configuration. While the Aufbau principle generally guides the filling of orbitals, exceptions occur when the energy and stability factors favor a different order of occupation.