Electron Configurations of Ions:

When electrons are removed from an atom to form a cation, they are always removed first from the occupied orbitals having the highest principal quantum number, n.

Removing one electron from a lithium atom produces:

Li (1s²2s¹) − e⁻ ⇒ Li⁺ (1s²)

Removing two electrons from an iron atom produces:

Fe ([Ar]3d⁶4s²) − 2e⁻ ⇒ Fe⁺² ([Ar]3d⁶)

Removing an electron from Fe⁺² produces:

Fe⁺² ([Ar]3d⁶) − e⁻ ⇒ Fe⁺³ ([Ar]3d⁵)

When electrons are added to an atom to form an anion, they are added to the empty or partially filled orbital having the lowest value of n.

Adding one electron to a fluorine atom produces:

F (1s²2s²2p⁵) + e⁻ ⇒ F⁻ (1s²2s²2p⁶)

 
Electron Configurations and Orbital Diagrams