in this video we see how different
metals bond together to form alloys
which still retain the metallic
properties of the starting metals but
are usually stronger
metal atoms are typified by having only
a few electrons in their outer shells
this means that even when they bond
there's always room in this valence
shell for more electrons
each metal atom can bond with up to 12
others in the close pact lattice look at
the red atom it is surrounded by six in
its plane and three on top and three
underneath
the less compact crystal structures are
possible too for example this
arrangement where each atom is bonded to
eight others
because there are still not enough
electrons to complete the outer shell of
any of the atoms the electrons can move
easily from one atom to another making
metals good conductors of both
electricity and heat and because the
electrons are not localized in fixed
bonds the atoms can slide past each
other making them ductile allowing the
metal to change shape
it also means that when you try to react
metals together the atoms normally just
mix into the lattice forming metallic
bonds with each other and with no fixed
proportions and randomly distributed
these structures are called alloys
contrast this with compounds between
metals and non-metals or between
non-metallic elements where the
proportions of each element is fixed
the oldest example of an alloy perhaps
is the way bronze took over from copper
in the early human communities of europe
around 6000 years ago during the late
stone age axes began to be made of pure
copper but they were fairly soft when
small amounts of tin were added to make
bronze you got an axe which was twice as
hard and worked well
the bronze age had arrived
the atoms in a metal lattice are held by
non-directional bonds a sort of sea of
loose electrons as we said allowing the
atoms to slide past each other still
touching making metals relatively easy
to melt and bend but hard to vaporize
when metals change shape atoms actually
slip over each other like this
however this process does not happen all
at once but bit by bit rather like
trying to move a carpet by putting a
ruck in it
here is the way it happens in a metal
you see the slip moving easily one atom
at a time where there's a dislocation in
the lattice
it is this easy movement of atoms in the
crystal lattice that makes most pure
metals soft
now put a smaller or bigger atom into
the lattice and this easy movement of
the dislocation is blocked
see the way the bigger atom stabilizes
the dislocation which gets no further
unless you put greater force meaning
that it's harder to bend the alloy
to finish let's look at some well-known
alloys bronze three-quarters copper
quarter-tin for sculptures boat hardware
screws and grill work brass 70 copper 30
zinc musical instruments coins door
knockers
carbon steel 99 iron and up to one
percent carbon for building construction
tools car bodies machinery rails etc
stainless steel iron with about 18
chromium and 8 nickel used for tableware
cookware surgical tools and so on
aluminium alloys for planes contain a
few percent of copper or other metals
amalgam is mercury with silver and other
metals once used for dental fillings
solder lead and tin for joining
electrical wires and components melts
very easily
gold is usually an alloy containing
another metal such as silver for
increasing hardness
the number of carats k defines how many
mass parts of pure gold are found in 24
parts of the alloy
you