Ion formation and electron arrangements

Ions and electron arrangements

Metals generally lose electrons when they react to form cations, that is ion with a positive charge. As you might expect the electrons are lost from valance shell e.g.

Na: 1s²2s²2p⁶3s¹ sodium loses its outer 3s¹ electron → Na⁺: 1s²2s²2p⁶

Ca: [Ar]4s² calcium loses its outer 4s² electrons → Ca²⁺: [Ne]3p⁶

A: [Ne]3s²3p¹ aluminium loses its outer 3s² and 3p¹ electrons → Al³⁺: 1s²2s²2p⁶ or simply [Ne]

Similarly for non-metal atoms, these will gain electrons to form ions with a negative charge, that is anions, e.g.

O: 1s²2s²2p⁴ oxygen gains 2 electrons → O^2-: 1s²2s²2p⁶ or [Ne]

Cl: [Ne]3s²3p⁵ chlorine gains 1 electron → Cl^-: 1s²2s²2p⁶3s²3p⁶ or [Ar]

In each of the examples above the ion formed has a noble gas electronic configuration, that is np6.
With transition metals the formation of cations does not quite go as you would expect. When we were working out the electron arrangements for the transition metals using the AUFBAU principle we filled up the sub-shells in the order: .................4s 3d that is the 4s sub-shell was filled before the 3d sub-shell.

However the 4s and the 3d sub-levels are very close in energy. After the 4s sub-level is filled it actually becomes higher in energy than the 3d sub-level. This means that when we remove electrons from a transition metal to form the cation, we remove the 4s electrons before the 3d electrons. For example:
Iron forms two common ions, Fe²⁺and Fe³⁺. The electronic configuration of these ions is shown below:

Formation of Fe²⁺ ion:

Fe: [Ar]4s²3d⁶ iron lose 2 electrons → Fe²⁺:[Ar]3d⁶

The 2 electrons which are lost come from the 4s sub-level and NOT the 3d sub-level.

Formation of Fe³⁺ ion:

Fe: [Ar]4s²3d⁶ iron loses 3 electrons → Fe³⁺:[Ar]3d⁵

This time the iron atoms lose 3 electrons. Two of these electrons come from the 4s sub-level and one electron comes from the 3d sub-level. It is worth noting that the Fe³⁺ ion has a half-filled 3d sub-level, this will make it more stable than the Fe²⁺ion.

Example 2: What is the electronic configuration of the Ti²⁺ ion?

Ti: [Ar]4s²3d² titanium loses 2 electrons from the 4s sub-level → Ti²⁺:[Ar]3d²

Example 3: What is the electronic configuration of the Cr³⁺ ion?

Remember one of the transition metals with a slightly unexpected electronic configuration is chromium; it has a half-filled d sub-shell and 1 electron in the 4s sub-shell.
So the electronic configuration of the Cr³⁺ ion is:

Cr: [Ar]4s¹3d⁵ chromium loses 3 electrons, two from the 3d sub-level and 1 from the 4s sub-level → Cr³⁺:[Ar]3d³

Check your understanding - Questions on ion formation

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