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PODCAST 12 – 19/9/2015 Metallic Bonding and Properties.
Hello and welcome to this podcast from allerytutors.com. My name is Chris Harris and I will be
running through the shiny topic of metallic bonding and properties. I will go through the structure of
metallic bonding and linking this to the physical properties such as thermal and electrical
conductivity, malleability, ductility, hardness and melting and boiling points. As usual, there will be
opportunities for you to practice too.
This podcast is designed to go through content required for the new A Level Chemistry specifications
and although most content is the same across all exam boards, you should check your specification
to make sure you are learning the right content at the right level of detail. All specifications can be
fou d o the oard s e site a d ill list e a tl hat ou eed to k o .
Just a quick reminder that I have a set of A Level Chemistry Revision Videos on my YouTube channel
just type in Allery Tutors into a search engine or YouTube and you will find them there, please
remember to subscribe and join a growing community of fans. Follow me on Twitter @allerytutors
or like me on Facebook to get all the pre-release info on new videos and podcasts and to ask me
questions on all things Chemistry and I will do my best to answer them. It may be a good idea to
print off the script for this podcast so you can read and listen at the same time. This way you can
scribble all over them with your own notes and thoughts. Visit allerytutors.com to get a hold of
them.
So ith a steel deter i atio let s ake a start! E a tl hat a etalli o d is as t dis o ered
until 1914 when the use of x-ray crystallography was used to determine the crystal structure of
copper by a 24 year old British Scientist William Bragg. Thanks to this technology it became accepted
amongst the scientific community that a metallic bond constituted a regular arrangement of positive
metallic ions and a delocalised sea of electrons. The bond was the strong electrostatic attraction
between positive metal ions and delocalised negative electrons to form a giant metallic structure.
Take a look at the script for this podcast at allerytutors.com to see a diagram showing this-
The delocalised electrons originate from the metal atom itself and the type of metal atom has an
effect on how strong the bond is. A metal atom that can donate 1 electron will have a weaker
metallic bond than an atom of a similar size that can donate 2 electrons. This is because atoms that
donate 1 electron will form a 1+ charge and this is attracted to sparsely dense electron cloud, on the
other hand atoms that can donate 2 electrons will form 2+ charge ion which is attracted to a denser
electron cloud. As you can probably see, there will be a much stronger attraction between a 2+ ion
and a high density electron cloud then a 1+ ion and sparsely dense electron cloud. This could be
likened to Na and Mg. They both form similar sized ions however the bond strength in Mg is higher
than in Na as Mg forms 2+ ions. This leads to Magnesium being harder to cut than Sodium.
© C Harris (AlleryTutors.com)
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This bond strength can also be affected by the size of the ion. So let s say we take two metal ions
with the same charge (so they donate the same number of electrons to the electron cloud) but a
different size (ionic radius). We shall use Lithium (Li) and Potassium (K). Lithium forms smaller ions
than Potassium but both donate 1 electron to the cloud. The electrons are attracted to the positive
ions and if the positive charge is spread over a large area the attraction is reduced leading to a
weaker metallic bond. We can say that the charge density of the ion is lower with Potassium than
with Lithium. This gives rise to a softer metal and is the reason why Potassium can be cut very easily
with a knife whereas Lithium needs a little more force to be cut.
An analogy will help here I think! So imagine you have 2 slices of toast but one is larger than the
other. We will use the toast to represent the positive metal ions, so the larger slice of toast will
represent Potassium ions and the smaller one will represent Lithium ions. We have a small amount
of jam in equal amounts for each slice of toast (this represents the delocalised electrons). You can
cover the smaller slice with a reasonable covering of jam to make it taste nice! With the larger slice
the jam has to be spread thinner making for a less tasty slice of toast. We will use the taste factor to
represent the bond strength! Obviously this would only be good if you like jam and toast in the first
place! Take a look at the script for this podcast at allerytutors.com to see a diagram showing this-
MMMMMM……… Ja
!
So you have a go at this question – Which would be harder between these metals and explain your
answer –
a) Calcium and Barium
b) Sodium and Aluminium
(Visit allerytutors.com view the script for this podcast to find out the answer)
ANSWER
a) Calcium forms smaller ions than Barium but both donate 2 electrons to the cloud. The
electrons are attracted to the positive ions and if the positive charge is spread over a large
area the attraction is reduced leading to a weaker metallic bond. We can say that the charge
density of the ion is lower with Barium than with Calcium. This means Barium is softer than
Calcium.
© C Harris (AlleryTutors.com)
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b) Sodium can donate 1 electron will have a weaker metallic bond than an atom of a similar
size that can donate 3 electrons like Aluminium. This is because atoms that donate 1
electron will form a 1+ charge and this is attracted to sparsely dense electron cloud, on the
other hand Aluminium can donate 3 electrons will form 3+ charge ion which is attracted to a
denser electron cloud. There will be a much stronger attraction between a 3+ ion and a high
density electron cloud then a 1+ ion and sparsely dense electron cloud. So Aluminium will be
harder than Sodium.
Right so now we know about the structure of metals in terms of their bonding we now need to
explore their properties and explain them.
Electrical and Thermal Conductivity
Metals are generally good conductors of heat and electricity. The reason is because there are
delocalised electrons that are able to move freely. This means they can carry a charge in terms of
electricity. They can conduct heat due to the close packing of the electrons. When the electrons are
heated they absorb energy and vibrate more vigorously, they bump into neighbouring electrons and
hence transfer energy.
Melting and Boiling Point
Metals generally have a high melting and boiling point due to the strong attraction between
delocalised negative electrons and positive metal ions. This varies depending on the size of the metal
ion and the density of the electron cloud as described before.
Malleability
Malleability is ability to hammer a material into shape without it cracking or breaking. Metals are
generally malleable due to the fact that even when the organised row structure of the metal ions are
disrupted due to hammering, the electron cloud remains attracted to the ions retaining a solid
structure. Take a look at the script for this podcast at allerytutors.com to see a diagram showing this-
Notice! The layers of ions move
when struck with a hammer but the
electron cloud remains to hold the
structure together.
© C Harris (AlleryTutors.com)
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Ductility
Ductile means the ability for a material to be drawn into a wire. Metals are generally ductile due to
the fact the electron cloud still remains attracted to the positive metal ions even when the wire is
only a few layers thick. This property means metals are ideal for electrical wires and making cables.
Have a go yourself. Duri g the 9 s a d 9 s, Alu i iu
as used i ele tri al iri g due to the
high cost of copper at the time. Explain why Aluminium was used with reference to metallic
structure. (Visit allerytutors.com view the script for this podcast to find out the answer).
ANSWER
Aluminium is ductile which means the ability for a material to be drawn into a wire. Metals are
generally ductile due to the fact the electron cloud still remains attracted to the positive metal ions
even when the wire is only a few layers thick.
Aluminium is a good conductor of electricity. The reason is because there are delocalised electrons
that are able to move freely. This means they can carry a charge which is carried around as an
electrical current.
Aluminium is malleable due to the delocalised electrons being strongly attracted to the positive
metal ions despite the disruption of the ion layers. This is beneficial as wires need to be able to bend
around objects in the building.
If you are still a little stuck then watch my videos on these topi s u der the bonding pla list.
Worked through examples are available on all these mini tutorials.
Well that s the end of this podcast on Metallic Bonding and Properties. I hope this has proved useful
and remember to follow me on Twitter @allerytutors or like me on Facebook for all the pre-release
info on up and coming videos to my YouTube channel and podcasts.
That s all folks! Lo e Che istr !
© C Harris (AlleryTutors.com)