Ion formation

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 electronNa+: 1s22s22p6
Ca: [Ar]4s2 calcium loses its outer 4s2 electronsCa2+: [Ne]3p6
A: [Ne]3s23p1 aluminium loses its outer 3s2 and 3p1 electronsAl3+: 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 electronsO2-: 1s22s22p6 or [Ne]
Cl: [Ne]3s23p5 chlorine gains 1 electronCl-: 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 electronsFe2+:[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 electronsFe3+:[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-levelTi2+:[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-levelCr3+:[Ar]3d3

Check your understanding - Questions on ion formation