Explain why chromium (Cr) and copper (Cu) have unusual electron configurations compared to other elements in their respective periods.
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Chromium (Cr) and copper (Cu) have unusual electron configurations compared to other elements in their respective periods due to the stability achieved by half-filled or fully-filled d orbitals.
In a typical electron configuration, electrons fill up available energy levels and orbitals in a specific order. The order is determined by the Aufbau principle, which states that electrons fill the lowest energy levels first before moving to higher energy levels.
In the case of chromium, the electron configuration should follow the pattern of 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d4. However, instead of filling the 3d orbital with the remaining four electrons, one electron is removed from the 4s orbital and placed into the 3d orbital. Therefore, the electron configuration of chromium is 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1. This configuration is atypical because it allows for a half-filled 3d orbital, which provides additional stability. The energy required to remove an electron from the 4s orbital and place it in the 3d orbital is lower than the potential energy difference it would have if it remained in the 4s orbital.
Similarly, copper has the electron configuration 1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s1 instead of the expected configuration of 1s2, 2s2, 2p6, 3s2, 3p6, 3d9, 4s2. Here, again, one electron is removed from the 4s orbital and placed into the 3d orbital. This unusual configuration provides the copper atom with a fully-filled 3d orbital, which offers greater stability. The energy required to remove an electron from the 4s orbital and place it in the 3d orbital is lower than the potential energy difference it would have if it remained in the 4s orbital.
The stability associated with half-filled and fully-filled d orbitals in chromium and copper is due to the exchange and correlation effects in quantum mechanics. These effects lower the overall energy of the system, making it more favorable.
In summary, the unusual electron configurations of chromium and copper, deviating from the expected filling order, occur to attain the additional stability conferred by a half-filled and fully-filled d orbitals, respectively.
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