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Ions

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.


By the end of this page you should be able to:

  • Identify when elements will react to form ions
  • Draw ions of the first 20 elements
  • Determine the charge on an ion using its group
  • Explain why elements react to form ions

Metal Ions

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!

Diagram of the formation of a lithium ion
Remember to add brackets and a charge in the top right hand corner!

Check Your Understanding - More Examples of Metal Ions

Try to draw the ions of sodium, beryllium and boron. Then click on the panel below to reveal the answers and check your work.
Diagram of a beryllium ion
Note that because Beryllium has lost two electrons it now has a charge of "2+".
Diagram of a boron ion
This ion is very rare due to the large dense, positive charge on the ion. This makes it really attractive to electrons (remember opposite charges attract) and difficult for them to escape in the first place.


Non-Metal Ions

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?

diagram of an oxygen atom gaining electrons to form an oxide ion

Check Your Understanding - More Example of Non-Metal Ions

Try to draw the ions of fluorine, sulfur and nitrogen. Then click on the panel below to reveal the answers and check your work.
Diagram of a sulfide ion
Note that because the sulfur atom has gained two electrons it now has a charge of 2-".
Diagram of a nitride ion
This ion is very rare due to the large dense, negative charge on the ion. Charges greater than 2 tend to be unstable and short-lived.


Group Number and Ionic Charge

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 shellCharge
111+
222+
333+
44N/A
553-
662-
771-
88N/A

Check Your Understanding - Ionic Charge

Determine the charge possessed by ions of sodium, aluminium and chlorine. Use the tabs below to check your answer.
1+. Sodium has one electron in its outer shell, it's easier to lose one electron than gain 7. Therefore it loses 1 electron to gain a full outer shell, the ion formed has a charge of 1+ (remember that atoms have a charge of zero and electrons have a negative charge - if you lose negativity you beceome positive).
3+. Aluminium has three electrons in its outer shell, it's easier to lsoe three electrons than gain 5. Therefore it loses three electrons to form an ion with a charge of 3+.
1-. Chlorine has 7 electrons in its outer shell, therefore it will gain 1 electron (easier than losing 7!) to fill its outer shell. So it will have a charge of 1- (remember electrons are negative - it's gaining negativity).


Why Do Atoms try to Attain a Full Outer Shell?

When elements gain a full outer shell they become more stable. This is why Noble Gases are unreactive, they already have a full outer shell, more information on Noble Gases can be found here. When elements form ions they have the same electronic configuration as the Noble Gases, this is shown in the diagram to the right.
Comparison of the electronic structure of an oxide ion and a neon atom. A comparison of the electronic configuration of an oxide ion and a neon ion.

Oxidation and Reduction

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.

OOxidation
IIs a
LLoss of electrons
RReduction
IIs a
GGain of electrons

Check Your Understanding - Oxidation/Reduction (Redox)

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.


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