The 4s orbitals are filled first because, according to our theory, they have a lower energy than the 3d orbitals. We are aware that the 4s electrons are lost first when an atom ionizes.
The highest energy level electrons that are lost first are those that are most far from the nucleus' influence. A higher energy must therefore exist in the 4s orbital than in the 3d orbitals.
The electron is a subatomic particle with the symbol e or with an electric charge of one elementaryly negative charge. Due to their lack of known components or substructure, electrons, which are part of the first generation of the lepton particle family, are typically considered to be elementary particles.
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