Answer:
4s orbital is filled first because
it has lower energy. The energies of the orbitals can be compared by their \[n+l\]values.
For 4s orbital,\[n+l\,(4+0)\] value is 4 while for 3d orbital, \[n+l\,(3+2)\] value
is 5. Therefore 4s orbital is filled before 3d orbital. However, this is true
only for multi electron atom. In the excited states of one electron species(\[H,\,H{{e}^{+}},\,\,L{{i}^{2+}}\]),
3d orbital will be filled before 4s orbital.
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