Question

Explain the reason for exceptional electronic configuration of copper and chromium.

Answer 1

Hint: We know that electronic configuration is based on the Aufbau principle. Aufbau principle states that electrons occupy the orbitals in the increasing order of energy levels and the orbitals with lowest energy are occupied first.

Complete step by step solution:
We know that the order of filling orbitals is $1s,2s,2p,3s,3p,4s,3d,4p,5s,4d$ and so.
The order of filling of electrons occupying the 3d subshell gets concerned in chromium and copper and because of distress in 3d subshell, these elements possess exceptional configuration.
The completely filled subshells are $s^2,p^6,d^{10},f^{14}$ and half-filled subshells are $s^1,p^3,d^5,f^7$ containing additional stability. We have to remember that the atomic number of chromium is $24$and predicted electronic configuration of chromium must be $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{4}4s^2$. In chromium, the $4s$ subshell is completely filled whereas the $3d$ subshell is either half-filled (or) completely filled. Because of the electronic repulsion forces one $4s$ electrons gets transferred to $3d$ subshell to attain the stable configuration as $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^1$. In this configuration, the subshells of $4s$ and $3d$ have additional stability because of the orbitals that are half filled and this also contributes to the additional stability of the atom.
Likewise, we can write the atomic number of copper is $29$ and predicted electronic configuration copper as $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{9}4s^2$. Due to the interelectronic repulsions one electron present in $4s$ enters to $3d$ subshell. We can write stable electronic configuration of copper is written as $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^1$. There is an additional stability because of the completely occupied $3d$ subshell and half-filled $4s$ subshell.

Note:While attempting questions related to electronic configuration one should always remember Aufbau principle and the order by which electrons fill the orbitals. Also, we have to remember that the exceptional electronic configuration of chromium and copper is only valid when the element is present in its ground state.
The compound $C_2{H}_4$ undergoes a Hydroformylation reaction to form ethanol. Ethene is reacted with carbon monoxide and hydrogen in presence of cobalt to form aldehyde, the aldehyde is then hydrogenated to form alcohol. Hydroboration of $C_2{H}_4$ also forms ethanol.