Answer :
Electrons enter the 4s orbital before entering the 3d orbital because the 4s orbital is lower in energy than the 3d orbital.
The order of electron filling in atoms is determined by the Aufbau principle, which states that electrons occupy the lowest energy orbitals first. In the case of transition metals, the 3d and 4s orbitals are close in energy. However, the 4s orbital has a slightly lower energy level, so it is filled before the 3d orbital. This is an exception to the general rule of filling orbitals based solely on the principal quantum number (n). Once the 4s orbital is filled with its maximum of two electrons, the electrons then begin to fill the 3d orbital. It's important to note that this electron filling order may vary for some elements with specific electron configurations, leading to exceptions where electrons can enter the 3d orbital before the 4s orbital, but this is not the general trend.
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Final answer:
Electrons enter the 4s orbital before the 3d orbital because the 4s orbital is lower in energy. According to the Aufbau Principle, electrons fill up the orbitals from the lowest to highest energy. Once the 3d orbital starts filling, it drops below the 4s orbital in energy level.
Explanation:
The reason electrons enter the 4s orbital before the 3d orbital is because the 4s orbital is lower in energy. According to the Aufbau Principle, electrons fill up the orbitals from the lowest to highest energy. Therefore, the correct answer to your question would be 'a. the 4s orbital is lower in energy than the 3d orbital'.
It's important to note that despite the 4s orbital being filled before the 3d orbital, once the 3d orbital begins to populate, it actually drops below the 4s in energy level. This can lead to a common misinterpretation that the 3d orbital has a higher energy level than 4s, which is not the case when an atom is in its ground state.
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