Electron Configuration - Positive Ions

Now that you have a sense of electron configurations and how electrons are arranged in the orbitals of an atom, it is time to look at ions, specifically positive ions. Some positive ions can be quite simple to understand, while others are a bit messier. We’ll start with the easy ones.

Easy positive ions

Remember that a set of orbitals with the same energy is most stable when it is empty, half-filled or filled. To understand negative ions, we will focus on the last of those three possibilities - when the orbitals are filled and in particular how it applies to the p block of the periodic table.

The p block can hold up to 6 electrons (3 orbitals, 2 electrons in each). The result of that is a column of remarkably stable elements - the noble gases.

Elements that are just beyond those elements often lose electrons to other atoms, and in the process achieve the same electron configuration as a noble gas. For instance, sodium (#11) has an electron configuration of 1s2 2s2 2p6 3s1, which is just one electron more than that of neon (1s2 2s2 2p6). If another atom steals an electron from sodium, the electron configuration becomes the same as that of stable neon. 

Vocabulary alert:

We say that the Na+1 ion is isoelectronic with neon — that is, it has the same electron configuration. That does NOT mean that they are the same. Different numbers of protons in the nucleus and a different overall charge cause them to behave differently.

In the same way, magnesium atoms often lose 2 electrons to become Mg+2 ions which are isoelectronic with neon, and aluminum atoms often lose 3 electrons to become Al+3 ions.

As an aside, it is never technically correct to say that an element gives away an electron (although you will hear this all the time.) No positive nucleus has ever given away negative electrons. However, it turns out that the metals on the left side of the periodic table have fairly low ionization potential. That means that it is relatively easy for another atom to steal an electron or two from them. As a result, they are often found as positive ions.

More complex positive ions

Elements that are further away from the noble gases may also lose electrons to other atoms making positive ions. However, the further an element is from the noble gases, the less likely it is that another atom will take enough electrons to make the ion isoelectronic with the previous noble gas.

At that point, it becomes difficult (or impossible at this point in your chemistry education) to predict how many electrons will be lost, and therefore to predict the charge on the ion. 

We can still write the electron configuration of the ion once we know it’s charge however. It is, unfortunately, not as simple as taking off the right number of electrons. There’s a catch.

Electrons are always removed from the highest numbered orbital first.

Let’s use iron as our working example. Iron (#26) is found as an atom in nature, but more commonly as either a +2 or +3 ion. The +2 ion has lost two electrons so that it now has only 24 electrons and the +3 ion has lost three electrons and now has 23 electrons.

Let’s see how that works. Iron has an electron configuration of [Ar]4s2 3d6. If the atom loses two electrons, it is tempting to think that the configuration would be [Ar]4s2 3d4, but that is WRONG!

We need to follow the rule above, so the two electrons are removed from the 4s orbital, not the 3d orbitals.

The Fe+3 ion loses the 2 electrons in the 4s orbital and then 1 from the 3d (since the 4s orbital is then empty). So the electron configuration of Fe+3 is [Ar]3d5

Things get even more complicated when the element is in the p block. As an example, let’s look at tin (#50).

Tin is commonly found in nature with either a +2 or +4 charge (a loss of 2 or 4 electrons). The atom has an electron configuration of [Kr]5s24d105p2. When tin loses two electrons, they are taken from the 5p orbital, leaving [Kr]5s24d10. When two additional electrons are taken, they are NOT taken from the 4d. Instead, following the rule above, they are taken from the 5s orbital, leaving the Sn+4 ion with a configuration of [Kr]4d10.