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 formed 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 placed onto the cube of sodium from the pipette a violent reaction is initiated with the chlorine gas. The flask quickly fills with a white powder; this is the ionic compound sodium chloride.
An equation for the reaction is shown below.
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.
As a simple example of an ionic compound consider the formation of sodium chloride. Sodium is an alkali metal with an 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 with each one having a positive charge. This means that the sodium atom has 11 positive charges (protons) but only 10 negatively charged electrons and 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. Recall that we call atoms with 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 sodium atom now 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.
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-).
Magnesium is an alkaline earth metal in group 2 of the periodic table, its electron arrangement is 2,8,2 or 1s22s22p63s2. To achieve a noble gas electron configuration it needs to lose 2 electrons. 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 dot and cross diagram below shows the formation of magnesium chloride and the transfer of 2 electrons from the magnesium atom to 2 chlorine atoms.
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:
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?:
| 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. If you need to revise how to work out the formula for substances then click here.
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 positive 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.