The correct order of orbital filling according to Aufbau principle is
(a) 4s 4p 4d
According to the Aufbau principle, electrons must occupy all of an atom's accessible orbitals, working their way up from the lowest energy level to the highest, to ensure that both the electrons and the atom have the least amount of energy possible and are stable.
As a result, electrons begin to occupy orbits in increasing order of "n" value, starting with the first orbit (n = 1).
If any orbital's primary quantum number is "n,"
n2 is the number of orbitals, and 2n2 is the number of electrons in any n-shell.
The orbital's's' sub-orbital count is 1′. P is three, while D is five.
There can be a total of two electrons in each orbital 0f an atom
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