atoms to ions

Higher and foundation tiers

Atoms to ions

Atoms have no charge, they are neutral. Atoms contain positively charged protons in their nucleus and negatively charged electron in the electron shells or rings. The number of positively charged protons and negatively charged electrons is always the same in atom, so atoms are always neutral and have no charge. The atomic number of an element will tell you the number of protons present in the nucleus, it will also tell you the number of electrons present in the electron shells.

The table below gives some examples:

atom atomic number protonnumber of positively charged protons electron number of negatively charged electrons overall charge
Lithium (Li) 3 3 3 0
Sodium (Na) 11 11 11 0
Magnesium (Mg) 12 12 12 0
Oxygen (O) 8 8 8 0
Chlorine (Cl) 17 17 17 0

However when different elements react with each other it may no longer be true that the number of proton and electrons will be the same. For example when metals react with non-metal element they lose electrons. This means that they will have more positively charged protons than negatively charged electron, so the atom will have a positive charge. We call charged atoms ions.

Just as metals lose electrons when they react, non-metals gain electrons when they react with metals. This means that the non-metal will have more negatively charged electrons than positively charged protons. So the non-metal atoms will form ions with a negative charge.

The Octet rule

To work out the number of electrons lost by a metal atom when it reacts or the number of electrons gained by a non-metal atom when it reacts, lets start with elements that do not react, the noble gases in group 0. The noble gases in group 0 are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). These gases are almost totally inert and will only react under the most severe conditions and only with a few very reactive elements such as fluorine. So why are the noble gases so unreactive? Well if you work out the electron arrangement for all the noble gases you will quickly realise that they all consist of atoms with full last electron shells. It is these full electron shells which gives the noble gase their chemical stability and it is these full outer electron shells which makes them almost totally unreactive. This leads to a rule in chemistry which is very helpful in determing how elements react, the octet rule. The octet rule simply states that elements will only react if they can achieve full outer electron shells - similar to those found in the noble gases.

You may recall that it is possible to work out the number of electrons an element has in its last electron shell simply by finding the element in the periodic table. The group in which an element is found in the periodic table tells you the number of electrons in the last shell, so for example the alkali metals in group 1 of the periodic table have 1 electron in their last shell, the alkaline earth metals in group 2 have 2 electrons in their last shell, the halogens in group 7 have 7 electrons in their last shell. This is summarised below:
The metals are found in the left-hand side of the periodic table in groups 1, 2 and 3. We will for the moment ignore the transition metals in the middle block in the periodic table. The table below gives the electron arrangement for several different metals. If you study the table carefully you will notice that the charge on the metal ion formed is the same as the number of electrons in the last shell of the metal or the group in which the metal is found in the periodic table.

metal atomic number Group where the element is found in the periodic table electron arrangement number of electrons in last shell number of electrons lost to obtain a full last shell charge on ion
Sodium (Na) 11 1 2,8,1 1 1 +1
potassium (K) 19 1 2,8,8,1 1 1 +1
magnesium (Mg) 12 2 2,8,2 2 2 +2
aluminium (Al) 13 3 2,8,3 3 3 +3

The pattern found in the table above is fairly obvious:

As an example consider the alkali metal potassium, symbol K. Its atomic number is 19, so it contains 19 protons in its nucleus and 19 electrons in its electron shells. The electron arrangement is 2,8,8,1. When potassium metal reacts with say a non-metal element it can achieve a full last shell in 2 ways:

As you may expect it requires much less energy to simply lose one electron than to gain 7. So it loses its outer shell electron and ends up forming an ion, with only 18 electrons. Its new electron arrangement will be 2,8,8, the same as the noble gas argon. This is outlined in the image below:

potassium atom forms a potassium ion by losing one electron

The non-metals are found in the right-hand side of the periodic table. The non-metals in groups 5, 6 and 7 are the ones which will form ion. The non-metals in group 4 tend to share electrons and form covalent bonds, this means that they do not tend to form compounds containing ions. So we are really only interested in the non-metals in groups 5, 6 and 7. These non-metal elements when they react with metals will gain electrons from a metal and form ions with a negative charge. Like the metals these non-metal elements will only react if they can achieve full last shells.
As an example consider the element chlorine. Chlorine has an atomic number of 17, this means it has 17 protons in its nucleus and 17 electrons in its shells. Chlorine's electron arrangement will be 2,8,7. To fill it last shell it needs to gain 1 electron. It could get this one electron by reacting with an alkali metal such as potassium. When it gains 1 electron the chlorine atom will form a chloride ion. It will have a -1 charge since it has 1 more electron than proton. This is shown below: chloine atoms gain 1 electron to form a chloride ion
So the number of electrons gained by the non-metal atom will depend on the group the non-metal is found in. The number of electrons gained will simply be the number required for the element to end up with full outer shells, this is shown below:

non-metal atomic number Group where the element is found in the periodic table electron arrangement number of electrons in last shell number of electrons gained to obtain a full last shell charge on ion
nitrogen(N) 7 5 2,5 5 3 -3
phosphorus (P) 15 5 2,8,5 5 3 -3
oxygen (O) 8 6 2,6 6 2 2-
fluorine (F) 9 7 2,7 7 1 1-

An outline of the periodic table shown below summarises the charge formed on the ion formed by any element in that particular group. Remember that:

Key Points

Check your understanding - Questions on atoms to ions


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