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When filling up electron orbitals to determine an element's electronic configuration, we follow the Aufbau principle and Madelung's rule. We fill up lower energy levels first before moving on to higher energy levels. We also fill in the s orbitals before the preceding d orbitals because they are slightly lower in energy. But in reality I believe that the s and d orbitals of adjacent principle quantum numbers are very close in energy values. The s is slightly lower, hence we fill it first, but when it comes to ionizing transition metals, we remove the s electrons first before removing the d electrons. I believe that this is due to electron density in relation to the distance from the nucleus. d orbitals are more complicated in their shape and have more of their electron density closer to the nucleus. The s orbital, on the other hand, is much simpler in shape with a more uniform distance from the nucleus. This larger probability of greater distance from the nucleus is what makes the s electrons the first to be removed when ionizing transition metals.
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