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The orbital that is the first to be filled in any atom is the 1s orbital. This is because the 1s orbital has the lowest energy level among all the orbitals in an atom. In the ground state of an atom, electrons will fill the lowest energy level orbitals first before moving to higher energy levels.
The electronic configuration of an atom is determined by the Aufbau principle, which states that electrons fill orbitals starting from the lowest energy level to the highest. So, when building up the electronic configuration of an atom, electrons will enter the 1s orbital before filling any other orbital.
For example, let's consider the electronic configuration of hydrogen (H) which has one electron. The electron configuration of hydrogen is 1s¹, indicating that the electron occupies the 1s orbital before any other orbital in the atom.
I hope this explanation helps you understand why the 1s orbital is the first to be filled in any atom. If you have any more questions or need further clarification, feel free to ask!
