Electron Configuration Exceptions and Stability

Electrons in Atoms and the Periodic Table

Electron Orbital Stability

Electron configurations describe the arrangement of electrons in an atom's orbitals. Certain configurations are more stable due to symmetry and electron repulsion effects.

• d subshell orbitals are most stable when they are half-filled or fully-filled with electrons. This is due to symmetrical distribution and exchange energy stabilization.

• Half-filled (d5) and fully-filled (d10) configurations are especially stable.

Symmetrical Distribution Example:

• For the d subshell: d5 (half-filled) and d10 (fully-filled) are particularly stable.

Exceptions to Electron Configurations

Some elements do not follow the expected order of filling orbitals, especially in the transition metals. These exceptions are due to the extra stability associated with half-filled and fully-filled d subshells.

• Starting from chromium (Cr, Z = 24) and beyond, exceptions to the standard filling order are observed.

• For example, instead of the expected configuration [Ar] 4s2 3d4, chromium adopts [Ar] 4s1 3d5 to achieve a half-filled d subshell.

• Similarly, copper (Cu, Z = 29) has the configuration [Ar] 4s1 3d10 instead of [Ar] 4s2 3d9.

Periodic Table Reference

The periodic table can be used to determine electron configurations by following the order of filling (Aufbau principle), but exceptions must be memorized for certain elements.

Examples of Electron Configuration Exceptions

• Chromium (Cr, Z = 24): Expected: [Ar] 4s2 3d4 Actual: [Ar] 4s1 3d5

• Copper (Cu, Z = 29): Expected: [Ar] 4s2 3d9 Actual: [Ar] 4s1 3d10

• Molybdenum (Mo, Z = 42): Actual: [Kr] 5s1 4d5

• Silver (Ag, Z = 47): Actual: [Kr] 5s1 4d10

Practice Example

Write the condensed electron configuration for the silver atom (Ag, Z = 47):

• Expected: [Kr] 5s2 4d9

• Actual (exception): [Kr] 5s1 4d10

Practice Questions

• Which of the following is the correct electron configuration for gold (Au)?

• Compare the electron configurations of palladium (Pd) and silver (Ag):

  • Ag has 2 more d electrons and the same number of s electrons as Pd.

  • Ag has 1 more d electron and 1 less s electron than Pd. (Correct)

  • Ag has 1 more s electron and 1 less d electron than Pd.

  • Ag has 1 more s electron and 1 more d electron than Pd.

Summary Table: Common Electron Configuration Exceptions

Element	Expected Configuration	Actual Configuration
Cr (Z = 24)	[Ar] 4s2 3d4	[Ar] 4s1 3d5
Cu (Z = 29)	[Ar] 4s2 3d9	[Ar] 4s1 3d10
Mo (Z = 42)	[Kr] 5s2 4d4	[Kr] 5s1 4d5
Ag (Z = 47)	[Kr] 5s2 4d9	[Kr] 5s1 4d10

Key Equation:

General electron configuration order (Aufbau principle):

$1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p \rightarrow 5s \rightarrow 4d \rightarrow 5p \rightarrow 6s \rightarrow 4f \rightarrow 5d \rightarrow 6p \rightarrow 7s$

Additional info: These exceptions are important for understanding chemical reactivity and properties of transition metals, as the electron configuration influences oxidation states and bonding behavior.