Higher and foundation tiers
The atomic structure diagram shown opposite should be familiar to you. The model of the atom shown is often called the nuclear atom or the Bohr atom and it is based on the work of Ernest Rutherford who discovered the nucleus in his famous gold foil experiment and on the work of Niels Bohr who suggested that the electrons orbit the nucleus in circular orbits or shells.
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 an 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 of the number of protons and electrons in atoms using elements that you are likely to meet in your school work in chemistry:
| atom | atomic number |  number of positively charged protons |
 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 elements
they lose electrons. This means that they will have more positively charged protons than negatively charged
electrons; so the atom will have a
positive charge. We call charged atoms ions. Positively charged ions are often called cations.
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. Negatively charged ions are often called anions.
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; let's start with elements that do not react; the noble gases in group 0 of the periodic table. 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 outer electron shells. It is these full electron shells which give the noble gases 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 determining 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 of the periodic table have 2 electrons in
their last shell, the halogens in group 7 have 7 electrons in their last shell. This is outlined in the table 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:
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 ions. The non-metal elements in group 4 tend to share electrons when they form compounds and will 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 elements in groups 5, 6 and 7. These non-metal elements when they react with metals will gain electrons from the metal atoms and form ions with a negative charge, we call these negatively charged ions anions. 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 its
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:
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:
Transition metals in the central block of the periodic table do not follow the rules for other metals. They can form ions with variable charges depending on the reaction and the reaction conditions. You will learn more about the chemistry of these metals when you study A-level chemistry.
Test your understanding of how ions are formed by completing the exercise below. The words in the yellow boxes below appear in the drop down menus in the paragraph below, simply select the correct word from the drop down menu.
Atoms are have no charge because they contain __________ numbers of positively charged __________ and negatively charged __________. However when metal atoms react with non-metals they __________ electrons to form positively charged ions called __________ while non-metal atoms gain electrons and form __________ charged ions called __________. Atoms only react if they end up with __________ outer electrons shells, this is often called the __________ rule.