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Electron Configuration of Monatomic Ions

It is possible to assign electronic structures to monatomic ions, formed from atoms by gaining or losing electrons. In general, when a monatomic ion is formed from an atom, electrons are added to or removed from sublevels in the highest principle energy level, n.

Ions with Noble –Gas Structures

Elements close to a noble gas in the periodic table form ions that have the same number of electrons as the noble-gas atom. This means that the atoms have noble-gas electron configurations.

Example: Nonmetal atoms achieve this by gaining electrons to form anions:

          ₇N (1s² 2s² 2p³) + 3e^- → ₇N (1s² 2s² 2p⁶ )

          ₈O (1s² 2s² 2p4) + 2e^- → ₈O (1s² 2s² 2p⁶ )

           ₉F (1s² 2s² 2p5) + e^- → ₉F (1s² 2s² 2p⁶ )

Example: Metal atoms achieve this by losing electrons to form cations:

          ₁₁Na (1s² 2s² 2p⁶ 3s¹) → ₁₁Na⁺ (1s² 2s² 2p⁶ ) + e^-

          ₁₂Mg (1s² 2s² 2p⁶ 3s²) → ₁₂Mg²⁺ (1s² 2s² 2p⁶ ) + 2e^-

          ₁₃Al (1s² 2s² 2p⁶ 3s² 3p¹) → ₁₃Al³⁺ (1s² 2s² 2p⁶ ) + 3e^-

The species N^3-, O^2-, F^{-, Ne, Na⁺, Mg²⁺, and Al³⁺ are said to be isoelectric, that is they all have the same electron configuration.}

Transition Metal Cations

Applying the principle that, in forming cations, electrons are removed from the sublevel if highest n, you can predict correctly that when transition metals atoms from positive ions, the outer s electrons are lost first.

Example:
₂₅Mn: [Ar] 4s² 3d⁵ ₂₅Mn²⁺: [Ar] 3d⁵

All the transition metals form cations by a similar process, that is, loss of outer s electrons. Only after those electrons are removed are electrons removed from the inner d sublevel.

Example: Iron forms two different ions Fe²⁺ / Fe³⁺
₂₆Fe: [Ar] 4s² 3d⁶

To form Fe²⁺ the 4s electrons are lost first: ₂₆Fe²⁺: [Ar] 3d⁶

To form Fe³⁺ the 4s electrons are lost first, and then one electron from the 3d subshell is lost: ₂₆Fe³⁺: [Ar] 3d⁵

Multivalence appears to be explained by transition metal atoms:

losing the electrons first in the highest principle energy level, n, (ns2)
losing inner d sublevel electrons after losing the electrons in the highest principle energy level (will lose electrons to obtain a d sublevel that is half filled or empty)
promotion of electrons in the highest principle energy level (ns) to the inner d sublevel ((n-1)d)so that the d sublevel will become half filled or filled

Example: copper forms two different ions Cu²⁺ / Cu⁺
₂₉Cu: [Ar] 4s² 3d⁹

To form Cu²⁺ the 4s electrons are lost: ₂₉Cu²⁺: [Ar] 3d⁹

To form Cu+ one of electrons in the 4s sublevel is promoted to fill the 3d sublevel (₂₉Cu: [Ar] 4s² 3d¹⁰ )

and then electron remaining in the 4s subshell is lost: ₂₉Cu⁺: [Ar] 3d¹⁰