Ions are formed when a metal reacts with a non-metal. Atoms form ions in order to gain an full outer shell. If you're not sure about how to identify metals and non-metals there's a brief explanation here. You may also need to recap the basic structure of an atom/properties of protons neutrons and electrons - this information can be found here.
Metal atoms lose electrons to get/achieve a full outer shell. Look at the diagram of lithium forming an ion to the right, it has one electron in it's outer shell, it could either lose 1 electron or gain 7. Which sounds easier? That's right - losing one electron.
Atoms have a charge of zero so when they lose an electron they are losing negative charge and form positive ions as they now have more protons than electrons (this is shown on the right hand side of the image). Think about it this way, if you're grumpy and someone comes along and cheers you up - you've lost negativity and you've become more positive!
Non-metal atoms gain electrons to get/achieve a full outer shell. If you look at the diagram on the right you can see an oxygen atom reacting to form an oxide ion (non-metals change their name ending to -ide when they bond). An oxygen atom has 6 electrons in its outer shell. There are two ways it can gain a full outer shell, gain 2 electrons or lose 6 electrons. Gaining two is easier.
Overall non-metal ions have a negative charge. This is because non-metal ions have gained electrons so there are now more electrons than protons (i.e. the negative charges outnumber the positive charges). Notice that the gained electrons are drawn as "dots". Can you see why the diagram to the left is called a dot and cross diagram?
An element's group can be used to determine its charge using the table to the right. But how can this be determined from scratch?
First look the element up on the periodic table and find its group no, this gives you the number of electrons in the outer shell. If an element has less than 4 electrons in its outer shell it will lose its outer shell electrons and form positive ions, for example if an element is in group three it will lose three electrons giving it a charge of 3+.
If an element has more than 4 electrons in its outer shell it will gain enough electrons to fill its outer shell and form negative ions, for example if an element is in group 6 it will gain 2 electrons to form an ion with a charge of 2-.
This explains why all elements within a group have similar chemical properties, they have the same number of electrons in their outer shell. So when they bond/undergo chemical reactions they tend to undergo the same change - i.e. they gain/lose/share the same no. of electrons to fill their outer shell.
|Group No.||No. of Electrons in outer shell||Charge|
Oxidation is a loss of electrons. So when metal atoms form metal ions and lose their electrons they have been oxidised.
Reduction is a gain of electrons. So when non-metals gain electrons to form non-metals ions they have been reduced.
Redox reactions are where both oxidation and reduction occur such as when non-metals react with metals to form ions.
Many students find that the phrase "OILRIG" helps them to remember what oxidation and reduction are - this is explained in the diagram to the right.
|L||Loss of electrons|
|G||Gain of electrons|
Class the equations below as oxidation or reduction
1) Na → Na++e-
2) 2O + 4e- → O2
3) Ca → Ca2++2e-
2) 2F + 2e- → F2
1 - Oxidation - the sodium atom has lost electrons.
2 - Reduction - the oxygen atoms have gained electrons.
3 - Oxidation - the calcium atoms have lost electrons.
4 - Reduction - the fluorine atoms have gained electrons.
N.B. This question used to be higher tier only - it now appears in both tiers in the most recent syllabus.