okay this is a tutorial on metallic bonding and we're going to be looking at the electron
see model of metallic bonding and how that can explain such properties as malleability
conductivity and luster so the properties of metals can be understood by looking at
the atomic structure of metallic bonding but first let's get reacquainted with the ionic bond
because if you understand the ionic bond that's going to make it a lot easier to understand what
the electron see model is telling us so here we have a metal reacting with a nonmetal the
transfer of the electron the reaction results in a positive ion and a negative ion so the ionic bond
is simply the electrostatic attraction between the positive and negative charge that the ions
have so you could call this a sodium atom and this a chlorine atom and once the sodium loses
an electron it becomes na plus and the chlorine gains that electron becomes CL minus so the sodium
ion and chloride ion being oppositely charged are electrostatically attracted and so that opposite
charge results in a crystalline structure where we have an ongoing array of alternating positive and
negative positive and negative that are all contracted to each other and that keeps the
crystal together let's look at the arrangement of atoms in a metallic crystal very similar to
the organization of an ionic crystal however they're all positively charged and just looking
at this illustration you would think that the atoms would completely fly apart by repelling
each other due to the positive charge but if we take a closer look the electrons see model
tells us how the positively charged cations in the metallic crystal can stay together so as an
example we'll take a look at aluminum looking at the sub atomic structure we can see each aluminum
atom has 13 protons but only 10 electrons it has given up the three valence electrons to what is
called the electron see this is why it's called the electron see model the space in between each
atom is filled by the valence electrons that have been given up by each metal atom so the
metal atoms become cations positively charged ions and the valence electrons are loose in the
spaces between the cations so what's holding the crystal together is the electrostatic attraction
between the positively charged cations and these free-floating negatively charged electrons and
so the valence electrons are not localized to any one particle they are free to move and occupy the
space between the positive ions and so similar to the electrostatic attraction that holds the
ionic crystal together we have this attraction of opposite charges that are holding the metallic
crystal together so aluminium giving up three electrons means that all the cations have a three
plus charge and each aluminum atom has contributed three electrons to the sea of electrons this
creates quite a strong attraction and we can use this model as a way to explain various properties
for example there is malleability the ability to put pressure on a piece of metal and not have
it break so we can pound metal with a hammer we can pound it into a shape and not have it break
how can this model explain that phenomenon well let's hit it with a hammer and see what happens
through one last blow okay so as you can see the atoms were shifting they didn't
break apart hitting the metal with a hammer does not break it why would that be the case
well the delocalised electrons are always there so any shifting positive ions are going to be
able to be held together by their attraction to the constant presence of those valence
electrons that sea of electrons that the atoms are attracted to and malleability of course is
also intimately related to ductility you can force the metal into the shape of wires so
this is malleability and a nice contrast would be to look and see why does hitting an ionic
crystal not result in this sort of malleability when we hit an ionic crystal it shatters let's
hit this ionic crystal with a hammer and see what happens so no matter how you hit it you're always
going to come to some point where like charges are lined up and so if like charges are lined
up the result is going to be a great deal of repulsion and that's going to split the crystal
and when ionic crystals cleave they often leave a very sheer smooth face if you do it carefully
because of the way the opposite charge is line up all right back to metallic crystals let's so
why would this very strong attraction result in other properties how about their strength and
high melting point well the atoms want to stay together they're very attracted to the sea of
electrons and so therefore the higher the amount of valence electrons given to the sea of electrons
the tougher the metal the harder it is and the higher the melting point transition metals in
particular like titanium and molybdenum each of which contributes a large amount of valence
electrons to the sea they are very tough metals very strong very high melting points how about
electric current can this model explain that well electric current is simply the movement of charge
and electrons are constantly moving through these spaces between the cations they therefore
are very able to carry an electric current and that also explains their heat conductivity the
fact that they can move through the crystal and what about the shininess the luster of metals
if you think about where light comes from the emission of visible light from electrons well
the sea of electrons these valence electrons can very readily absorb and re-emit visible frequency
photons and that makes metals very shiny very lustrous and so here's an example of a giant
structure a sculpture in Chicago called the beam it's extremely shiny it's made out of
stainless steel it is primarily iron chromium usually a little bit of nickel definitely some
carbon and depending on the stainless steel a few other metals thrown in all engaging in
metallic bonding so the shininess comes from the ability of the stainless steel to easily
absorb and re-emit visible spectra light but what's interesting about that is if you think
about what would happen if you put a scratch in the beam well you would see that scratch
because it would scatter light so it would no longer be shiny right where the scratch
is right where there's an irregular surface and an irregular surface will scatter light
in different directions so shining this of any metal does depend on how smooth the surfaces
if it's an if it's a scratchy non smooth surface then you don't get that shininess