sir please answer very ergent !!!!!!!!!!!!!!!!!!!!!!!!!:-) why & how do we put 3d orbitals after filling4s orbitals ? And why do we say copper &cromium are exceptional elements?
Raising n raises orbital energy. Electrons are attracted to the nucleus. To pull an electron farther away from the nucleus, you have to work against that attraction. That means an electron farther from the nucleus has more energy than electron closer in; energy is required to move the electron out, and energy can be released when the electron moves in. So we expect outer shells to have higher energies than inner shells, because increasing n increases the average distance between the nucleus and the electron. For atoms heavier than copper this effect dominates, and 4s electrons have higher energy than 3d electrons.
Raising l raises orbital energy. Higher l values result in orbitals with more nuclear nodes (a node being a place where the probability of finding the electron is zero). We say high-l orbitals are "less penetrating" because their electrons have a lower probability of being found at or near the nucleus. That gives high-l orbitals (like d orbitals) more energy than low-l orbitals (like s orbitals) within the same shell. This effect causes 4s orbitals to have lower energy than 3d orbitals for elements lighter than copper. (Although for hydrogen, the unoccupied 4s and 3d orbitals have nearly identical energies).
We say that copper and chromium are exceptional elements because in these the 3d orbital gets filled up before 4s making it upto d5 electrons in Cr and d10 in case of copper.