The properties of metals
The general properties of metals can be summarised in the image below:
We can use the general properties of metals
shown in the image above to help us figure out what the structure of a
metal looks
like. The table below tries to offer an explanation for some of the
properties of metals and relate
this property to a possible feature of
the metal structure and the bonding present in the metal
Metal property |
How this relates to its structure |
Metals are good conductors of electricity and heat |
There must be free or delocalised electrons within the structure to conduct the electricity and heat. |
Metals have high melting and boiling points. |
Metals must have a giant structure with lots of strong bonds. |
Metals are malleable (can be hammered into shape) and ductile (can be pulled into wire). |
There are layers of particles that are able to slide over each other. |
Metals are shiny. |
Free or delocalised electrons are able to reflect light at the surface. |
Metals are hard and dense |
The particles within the metal structure are packed tightly together. |
The structure of metals
These basic properties of metals allows us to suggest that
metals consist of a giant structures of ions
which are surrounded by a sea of free moving delocalised electrons.
We also already know that metals are found on the left hand side of the
periodic table in groups 1, 2 and 3 and that
they tend to lose electrons in their reactions to form
positively charged ions.
This leads us to the model shown below. The dark grey balls represent
positively charged metal ions (atoms which have
lost their outer shell electrons). These
electrons are delocalised and free to move through the
giant structure of ions.
This might seem odd since we might expect a giant structure of ions
which all have a positive charge to repel each other and so
the structure would simply break down. However we need to think about
the delocalised electrons within the structure.
The negatively charged electrons are attracted to the
positively charged metal ions and
this prevents the metal ions from
repelling
each other. The electrons are attracted to the metal
ions around them. This attraction of the negatively
charged electrons to the neighbouring positively charged
metal ions is called a metallic
bond. A key feature of
this bond is the fact that metallic bonds are not permanent but are
constantly breaking and reforming
as the electrons move
freely around the structure, this is shown in the diagram below:
The fact that these metallic bonds are not permanent
but are constantly breaking and
reforming allows the layers of ions
to slide over each other. This is shown in the diagram below; which shows
a pushing force being applied to the top two
layers of ions in a metal structure.
The metallic bonds in these layers immediately break
and the layers slide along
but as soon as they stop moving the metallic bonds immediately reform.
This is why metals are malleable (can be hammered into shape) and ductile (can be pulled into wires).
Properties in detail
Metals generally have high melting points, high densities and
are good electrical and thermal conductors, though these
properties
can vary significantly as we cross the periodic table. Let us examine some these typical properties of
metals in a little more detail
Bond strength, conductivity and density of metals
The strength of the metallic bond depends mainly on two factors:
- The charge on the metal cation present in the metallic bond.
The alkali metals in group I will lose their outer ns1 electron
and form
a metal cation with a charge of +1, the group 2 alkaline earth metals
will lose the two electrons from their outer
valency ns subshell. This means that the metal
ion formed will have a 2+ charge. Obviously
the attraction between the delocalised
electrons and the 2+ ion will be stronger than that between the
delocalised electrons and the +1 ion.
There will also be
more metallic bonds present since the group 2 metals
release 2 electrons while the group I metal
only release 1 electron.
- The size of the metal cation. The smaller the
metal cation formed the stronger
the metallic bond will be. The smaller
the metal cation the larger will be its charge to size ratio.
The larger this ratio the stronger will be the attraction
of the delocalised electrons.
The d-block metals not only release their electrons in their s-subshell
but also the d-subshell electrons can be
delocalised. This means that metal ions with larger charges
and a smaller radius will be produced. This means stronger bonds
within the metal structure which will lead to metals
which are hard, dense and have high melting points. We can extend this
argument to trends down a group. Obviously as we descend a group in the periodic table the
size of the metal atoms and ions
will
increase, this means that the attraction
to the delocalised electrons will be weaker;
so the strength of the metallic
bond will be reduced. This means a decrease
in the melting points for metals as we descend a group.
Metals are good electrical conductors
due to the presence of the delocalised electrons. The more
delocalised electrons there are
the better an electrical and thermal conductor
the metal will be. Aluminium for example is a better electrical
conductor than
magnesium because it has 3 valency electrons ([Ne]3s23p1) while magnesium ([Ne]3s2)
only has 2 electrons in its
valence shell
Key Points
- Metals have a giant structure consisting
of positively charged metal cations surrounded by a sea of delocalised electrons.
- The sea of delocalised electrons are attracted to the positively charged metal
cations and this attraction forms temporary metallic bonds.
- Many of the physical properties of metals such as
electrical and thermal conductivity is
due to the presence of the delocalised electrons.
- The strength of the metallic bonds depends on a number of factors
including
- the size of the metal cation
- the charge on the metal cation
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