shapes of molecules finding the lone pairs

Ionic bonding

Ion formation

Metals are found in the left hand side of the periodic table in group 1 (the alkali metals), group 2 (the alkaline earth metals) and group 3 as well as the middle block of the periodic table called the transition metals or d-block.

Ionic compounds are produced when metals from the left hand side of the periodic table react with non-metal elements on the right hand side of the table (excluding the noble gases). The alkali metal sodium for example undergoes a very violent exothermic reaction with the halogen chlorine to produce the ionic compound sodium chloride (NaCl) . This is shown in the image below where a small piece of sodium metal is placed in a flask containing dry chlorine gas. Initially no reaction takes place, but if a few drops of water are dropped onto the sodium from the pipette a violent reaction is initiated with the chlorine gas. The flask quickly fills with a white powder, sodium chloride.

sodium reacting with dry chlorine 
gas in a flask

An equation for the reaction is shown below.

2Na(s) + Cl2(g) 2NaCl(s)
You learned in gcse chemistry that metals when they react tend to lose electrons to form positive ions (cations) while non-metals tend to gain electrons in their reactions and form negatively charged ions (anions). We also used the Octet rule to decide on the number of electrons lost or gained by an element when it reacts. This simple rule simply stated that elements react to achieve the same electron arrangement as the nearest noble gas to them in the periodic table, that is to achieve full outer electron shells. e.g. Sodium is an alkali metal in group 1 of the periodic table so when it reacts it will lose its outer 3s1 electron and form the cation (Na+), which has the same electron arrangement as the noble gas neon (1s22s22p6), that is a p6 configuration in its outer electron sub-shell. Or from your gcse chemistry we would say that there are 8 electrons in the outer energy level or shell.
Magnesium is an alkaline earth metal in group 2 of the periodic table; when it reacts it will lose its outer 3s2 electrons to form an ion with a charge of 2+, Mg2+. This ion also has the same electron arrangement as the noble gas neon (1s22s22p6). When lithium reacts it will lose its outer 2s1 electron and form a lithium ion, Li+. This lithium ion, Li+, has the same electronic configuration as the noble gas helium, simply 1s2.

The image below shows a simplified way of representing the electron configuration for the atoms of sodium, magnesium and lithium mentioned above. It should help you visualise which electrons are being lost when the metals lose electrons to form positively charged cations.

The octet rule is a remarkable simple but effective aid in helping us to understand the reactions of many elements and to help in understanding the interactions and forces involved in bonding. However it is a rule and not a law and there are exceptions to the octet rule which you should be aware of. However despite these exceptions the octet rule is still a very useful tool in the chemist's toolbox.

Sodium chloride formation

As a simple example of an ionic compound consider the formation of sodium chloride. Sodium is an alkali metal, atomic number 11. Sodium has the electronic configuration 2,8,1 or 1s22s22p63s1. To completely fill its last shell it needs to gain 7 electrons or simply lose the last electron. Obviously it will be easier and require less energy to simply lose 1 electron than gain 7 electrons. So when sodium reacts as mentioned above it will lose its outer shell electron. The sodium atom will now have the electronic arrangement 2,8 or 1s22s22p6 the same as the noble gas neon. It will have 10 electrons. However the nucleus of the sodium atom will still contain 11 protons, each one having a positive charge. This means that the sodium atom has 11 positive charges (protons) but only 10 negatively charged electrons, so overall the atom has more positive charges than negative charges to cancel them out. This leaves the sodium atom with a charge of +1, we call atoms with a charges ions. Atoms are neutral because they have equal numbers of positively charged protons in the nucleus and negatively charged electrons in the electron shells, but in ions the numbers of protons and electrons is NOT equal, so the atom has a charge- it's now an ion. Positively charged ions are often referred to as cations and negatively charged ions are often called anions. This is shown in the diagram below

The chlorine atom has an electron arrangement of 2,8,7 or 1s22s22p63s23p6, this means it only needs to gain 1 electron in order to end up with a noble gas electronic configuration of 3p6. So when sodium and chlorine react the chlorine atom will gain the 3s1 electron from the sodium atom to form a chloride ion, a negatively charged anion. This is shown in the diagram below.

diagram to show electronic 
configuration of chlorine atoms and ion

The electron from the sodium atom is transferred to the chlorine atom which results in the formation of a positively charged sodium ion (Na+) and a negatively charged chloride ion (Cl-).

Example 2- Magnesium chloride

Magnesium is an alkaline earth metal in group 2 of the periodic table, its electron arrangement is 2,8,2 or 1s22s22p63s2, it needs to lose 2 electrons to achieve a full last shell or the 2p6 noble gas electron arrangement. This is exactly the same situation as in the sodium chloride example above. The only difference here is that the magnesium atom needs to lose 2 electrons and so it will form an ion with a 2+ charge. So when magnesium reacts with chlorine, it will require two chlorine atoms to react with, since each chlorine atom is only prepared to accept one electron. The diagram below shows the formation of magnesium chloride, and the transfer of 2 electrons from the magnesium atom to 2 chlorine atoms.

dot and cross diagram for magnesium chloride

In magnesium chloride each chloride ion has 8 electrons in its valency shell or a 3p6 electron configuration, the same as the noble gas argon. The magnesium ion also has 8 outer electrons and full valency shell and a 2p6 electron arrangement in the outer 2p sub-shell, the same as the noble gas neon. In gcse chemistry we drew dot and cross diagrams to show this electron transfer happening, as shown below:

dot and cross diagram for magnesium chloride

Finding the formula of an ionic compound

All ionic compounds have a giant structure composed of positive and negative ions. The formula of an ionic compound simply gives the ratio in which the ions are present within the giant lattice. Sodium chloride for example has the formula NaCl. This simply tells you that the sodium and chloride ions are present in the ratio of 1:1 in the giant ionic lattice.
To calculate the formula for any compound is very straight forward. You only really need a periodic table to be able to check what group an element is in. The group will tell you the number of bonds the element makes. This will be the number of electrons lost or gained to achieve a stable octet of electrons in the last/valency shell. This is shown in the table below:

Group in periodic table 1 2 3 4 5 6 7 8
Charge on ion +1 +2 +3 -3 -2 -1 0
Valency (number of bonds the element makes 1 2 3 4 3 2 1 0

Example: What is the formula for the ionic compound lithium oxide?:

To calculate the formula simply following the rules in the table below:

element lithium oxygen
element symbol Li O
valency 1 2
swap over the valencies 2 1
ratio Li2 O
formula Li2O
The ionic formula is simply the formula with the charges on the ions present. The charges on the ions are simply the number of electrons lost or gained when the element reacts. This is the same as its valency.
ionic formula Li2+O2-

If you have worked out the formula correctly the charges on the ions should always balance or cancel out. In this case there are 2 lithium ions, each with a charge of +1. This gives a total positive charge of 2+. This will be cancelled out by the 2- charge on the single oxide ion present.

So what exactly is an ionic bond?

Most of the discussion above has been about ions and how they are formed, however once you are confident about how ions are formed then it should be obvious that once formed postive and negatively charged particles will attract each other by electrostatic forces. Therefore an ionic bond is simply the electrostatic attraction between oppositely charged ions. Ionic compounds consist of giant structure of ions called a lattice. Click on the link below or here for more information on the structure of ionic compounds.

Key Points

Practice questions

Check your understanding - Questions on ionic bonding