Stability associated with completely filled and half-filled subshells

 Stability associated with completely filled and half-filled subshells, especially in elements like copper (Cu) and chromium (Cr):

1. Ground State Electronic Configuration:

• The ground state electronic configuration of an atom represents the lowest total electronic energy state for that element.
• In most atoms, electronic configurations follow the standard rules based on the aufbau principle, Pauli's exclusion principle, and Hund's rule, as discussed earlier.

2. Deviation in Copper (Cu) and Chromium (Cr):

• In certain elements like copper (Cu) and chromium (Cr), there is a deviation from the expected electron configurations based on energy levels.
• Both copper and chromium have subshells (4s and 3d) with slightly different energies.
• In these elements, one electron shifts from the subshell of lower energy (4s) to the subshell of higher energy (3d), contrary to the usual electron filling pattern.

3. Completely Filled and Half-Filled Subshells:

• The reason for this deviation is the phenomenon of extra stability associated with completely filled and half-filled subshells.
• When all the orbitals in a subshell are either completely filled or half filled, the electronic configuration exhibits extra stability.
• In the case of chromium (Cr), it adopts the electronic configuration 3d⁵ 4s¹ instead of the expected 3d⁴ 4s². This configuration has a half-filled 3d subshell, which is considered more stable.
• Similarly, copper (Cu) adopts the electronic configuration 3d¹⁰ 4s¹ instead of 3d⁹ 4s². This configuration has a completely filled 3d subshell, which is also associated with extra stability.

4. Exceptional Stability:

• The stability associated with completely filled and half-filled subshells is attributed to the symmetrical arrangement of electrons in these configurations.
• Symmetry is often linked to lower energy levels, making these configurations more energetically favorable.

Elements like copper (Cu) and chromium (Cr) deviate from their expected electron configurations by shifting electrons between slightly different energy subshells. This deviation occurs because the resulting configurations have either completely filled or half-filled subshells, which are known to be exceptionally stable due to their symmetrical arrangements. This extra stability is a notable feature of these elements in the periodic table.