Write the electron configuration of the following ions using the spdf notation:
a) Cr2+ , Cr3+ , Cr6+ ; b) Mo3+ , Mo4+ , Mo6+ ; Se4+ , Se6+ , Se2-
The Correct Answer and Explanation is :
Electron Configurations:
a) Chromium ions:
- Cr²⁺: [Ar] 3d⁴
- Cr³⁺: [Ar] 3d³
- Cr⁶⁺: [Ar] (all valence electrons are removed, leaving the noble gas core)
b) Molybdenum ions:
- Mo³⁺: [Kr] 4d³
- Mo⁴⁺: [Kr] 4d²
- Mo⁶⁺: [Kr] (all valence electrons are removed, leaving the noble gas core)
c) Selenium ions:
- Se²⁻: [Ar] 3d¹⁰ 4s² 4p⁶ (equivalent to krypton, [Kr])
- Se⁴⁺: [Ar] 3d¹⁰ 4s² 4p²
- Se⁶⁺: [Ar] 3d¹⁰ 4s² (4p electrons fully removed)
Explanation:
The electron configurations are based on the removal or addition of electrons following the Aufbau principle, Hund’s rule, and the periodic table trends.
- Transition Metals (Cr and Mo):
- Chromium (Cr) and molybdenum (Mo) are transition metals with anomalous electron configurations in their neutral states:
Cr: [Ar] 3d⁵ 4s¹
Mo: [Kr] 4d⁵ 5s¹. - When they form ions, electrons are removed first from the 4s or 5s orbital, then from the 3d or 4d orbitals. For higher oxidation states, such as Cr⁶⁺ or Mo⁶⁺, all valence electrons are removed, leaving only the core configuration.
- Nonmetals (Se):
- Selenium (Se) is a group 16 element with a neutral configuration of [Ar] 3d¹⁰ 4s² 4p⁴.
- When it forms anions like Se²⁻, it gains two electrons to complete its p orbital (4p⁶), becoming isoelectronic with krypton.
- For cations, electrons are removed from the 4p orbital first, then from the 4s orbital. In Se⁴⁺, two 4p electrons are lost; in Se⁶⁺, all four valence electrons are removed.
- Stability of Configurations:
- The configurations reflect the stability associated with half-filled and fully filled subshells. For example, the Cr²⁺ ion ([Ar] 3d⁴) and Cr³⁺ ion ([Ar] 3d³) highlight stability through d-electron arrangements.