If you find this blog too difficult or tedious to read in detail, just notice that some complicated ions consist of several atoms covalently bonded, as in a molecule (see post 16.30), but with one or more extra electrons (negative ions) or missing one or more electrons (positive ions). Then just look at the pictures!
The ions we met in post 16.30 were single atoms that had lost or gained one or more electrons. But some ions consist of several atoms, held together by covalent and/or coordination bonds (see post 16.30), that have gained or lost electrons.
When water reacts with calcium oxide a lot of heat is generated and the product of the reaction is called calcium hydroxide. The reaction occurs because the oxide ion (O2-, post 16.30) reacts with water (H2O, post 16.30) to form the hydroxide ion (OH–):
O2- + H2O → 2OH–.
From this chemical equation (post 16.33) it can be seen that the same atoms are present in both the products and the reactants (see post 16.33). Similarly, the total number of charges on the reactant ions (minus 2) is the same as on the products.
In posts 16.29 and 16.30, we saw that oxygen has 2 electrons in its 2s energy level and 4 electrons in its 2p energy levels and so forms 2 covalent bonds. It is sometimes convenient to simplify this description and to say that oxygen has 2 + 4 = 6 electrons in its incomplete outer shell (the s and p levels preceded by the same number – in this case 2). Its outer shell electron configuration can then be represented by:
It can become more stable by filling its 2s (2 electrons) and 2p (6 electrons) energy levels, that is by having 2 + 6 = 8 electrons in its outer shell. It can achieve this aim by sharing one electron with a hydrogen atom (that is more stable when it has 2 electrons in its 1s energy level (post 16.30) and stealing one electron from another atom to form the hydroxide ion:
In this diagram the electrons from oxygen (blue) and hydrogen (red) and the stolen electron (green) are represented by different colours, to make the picture easier to understand, but really they’re all identical. Another way to represent the structure of the OH– ion is to draw the covalent bond (a shared pair of electrons) and show the positions of the other three pairs of electrons (sometimes called lone pairs):
Where can the hydroxide ion steal its extra electron from? An example is a sodium atom which forms Na+ ions (see post 16.39). Sodium reacts violently with water to form NaOH that is a mixture of Na+ and OH– ions.
The picture of the OH– ion makes it look flat. In reality its four pairs of electrons repel each other and point towards the corners of a tetrahedron – just like the four covalent bonds in a methane (CH4) molecule (see post 16.30).
In post 16.30 we saw that water was made up of H2O molecules. However, a very small proportion of water molecules (1 in 107; see post 16.7 if you need to remind yourself about this way of writing numbers) can break up into hydrogen (H+) and hydroxide (OH–) ions:
H2O → H+ + OH–.
A hydrogen ion is simply a hydrogen atom that has lost its one electron – so it’s just a hydrogen atomic nucleus (see post 16.27). Any substance that can act as a source of H+ ions is called an acid. Because only a very small proportion of water molecules form H+ ions, we say that water is a weak acid. If we mix hydrogen ions with hydroxide ions, most of them combine to form a water molecule:
H+ + OH– → H2O.
We call anything that reacts with H+ ions, like the OH– ion, a base.
Three electrons from the nitrogen atom (blue) are shared the electron from each of the three hydrogen atoms (red). The ammonia molecule has 8 electrons in its outer shell. It can share the two that don’t form covalent bonds with a hydrogen ion (H+) to fill its 1s energy level. But the ammonia molecule contributes both of the shared pair of electrons; so the result is not a conventional covalent bond but a coordination bond (see post 16.30) that we represent by an arrow instead of a line:
Since ammonia reacts with hydrogen ions, it is another example of a base.
A water molecule can also react with a hydrogen ion, by forming a coordination bond, to form the hydronium ion (H3O+):
H2O + H+ → H3O+
So the formation of hydroxide ions by water (see above) should really be represented by
2H2O → H3O+ + OH–
However, the hydronium ion reacts as a hydrogen ion, leaving a water molecule:
H3O+ → H2O + H+.
So that often we don’t need to remember that it exists.
Some ions are even more complicated than the ones we have met so far. Most soap contains sodium stearate that is a mixture of sodium (Na+) and stearate (R.CO2–, where R is C17H35-) ions. We can represent a group like R, but with only 6 carbon atoms as:
Note that each carbon atom forms 4 covalent bonds and each hydrogen atom forms one covalent bond: for the reasons described in post 16.30; the final carbon atom has a bond that it can attach to another atom. Note also that this R group is not really flat; all four bonds from each carbon atom point towards the corners of a tetrahedron (see post 16.30). We can simplify the representation of this R group to:
If we continue this chain, we get
We can now represent the stearate ion as:
All carbon atoms form 4 bonds and all hydrogen atoms form 1 bond; one oxygen atom forms 2 bonds, as usual (see post 16.30). The other oxygen atom forms only 1 bond so needs to steal an electron (coloured red), for example, from a sodium atom, to get 8 outer shell electrons (including the two electrons that form the covalent bond to the carbon atom); as a result of stealing an electron, the stearate ion has an overall negative charge. The mixture of sodium and stearate ions is called sodium stearate.
In stearic acid, the oxygen atom of the stearate ion that makes only one bond forms a bond with hydrogen:
Now each carbon atom makes four bonds, each oxygen atom makes two bonds and each hydrogen atom makes one bond, as described in post 16.30.
An acid with this type of chemical formula is called a carboxylic acid. When R is a long chain of carbon and hydrogen atoms (as in stearic acid), the carboxylic acid is called a fatty acid. (Soaps are made by boiling fat with solutions of sodium hydroxide or potassium hydroxide; potassium hydroxide commonly occurs in wood ash – so perhaps soap was accidentally discovered by someone boiling fatty food over a wood fire.). Stearic acid is a weak acid because it consists mostly of molecules but a few break up to form a hydrogen ion and a negatively charged stearate ion (just as a few water molecules break up into hydrogen and hydroxide ions – see above).
So soap is more complicated than we might expect! And there are ions that are much more complicated than the stearate ion.