Ion formation

Ion formation and electron configurations

Metal Ions and electron configurations

Comic style image to show metal atoms being ionised

Metals lose electrons when they react with non-metals such as chlorine, oxygen or bromine to form metal cations, that is ion with a positive charge. As you might expect the electrons are lost from outer valence electron shell; for example a group 1 alkali metal such as sodium will lose an its 1s electron to form a sodium cation Na+. The three equations below show how the three metals sodium (Na), calcium (Ca) and aluminium can form metal cations by losing electrons.

Na: ( 1s22s22p63s1) sodium loses its outer 3s1 electronNa+(1s22s22p6)
Ca: [Ar]4s2 calcium loses its outer 4s2 electronsCa2+([Ne]3p6)
Al: [Ne]3s23p1 aluminium loses its outer 3s2 and 3p1 electronsAl3+ (1s22s22p6 or simply [Ne])

Non-metal ions and electron configurations

Comic style image to show a non-metal atom gaining an electron

Metals from groups I, II and III in the periodic table may generally lose electrons when they react to positively charged metal cations but non-metals from groups 5, 6 and 7 in the periodic table will gain electrons to form negatively charged non-metal anions; for example the non-metals oxygen and chlorine will gain electrons when they react with metals to form an oxide ion (O2-) and a chloride ion (Cl-) as shown below:

O: (1s22s22p4) oxygen gains 2 electronsO2- ( 1s22s22p6) or [Ne]
Cl ( [Ne]3s23p5) chlorine gains 1 electronCl- ( 1s22s22p63s23p6 or [Ar])

In all of the examples above the ion formed has a noble gas electronic configuration, that is ns2p6; where n is the principal quantum number or shell number.

Transition metals and electron configurations

With the transition metals the formation of metal cations does not quite go as you would expect. When we were working out the electron configurations for the transition metals using the AUFBAU principle the sub-shells or sub-levels filled up 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 and after the 4s sub-level is filled it actually bcomes higher in energy than the 3d sub-level due to electron-electron repulsions between the 3d and the 4s electrons. This means that when a transition metal loses electrons to form a metal cation, the 4s electrons are lost 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?

Recall that one of the transition metals with a slightly unexpected electronic configuration is chromium; now chromium has a half-filled d sub-shell and 1 electron in the 4s sub-level.
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

Practice questions

Check your understanding - Questions on ion formation

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