structure and bonding of elements and
compounds part two
in part one of this video we developed a
triangular space with the elements
arranged along the top from the most
reactive metal cesium to the left and
the most reactive non-metal fluorine to
the right
all possible compounds formed by the
elements will now fall into the
triangular space the structure triangle
when non-metals bond with each other we
get small self-contained molecules
substances that are already gases or
will vaporize easily these are called
volatile don't confuse this with being
reactive
consider chlorine its seven outer
electrons are held by an effective
nuclear charge from the nucleus of plus
seven as we saw in part one of this
video
as two chlorine atoms approach each
other they attract and their two lone
electrons pair up filling the outer
shells of both atoms we now have a
covalent bond formed by the attraction
of the two electrons and the two nuclei
if a third chlorine atom approaches
there is no room for its electron in the
outer shell of the atoms already there
so it is unable to bond thus chlorine
the element exists as a set of
independent diatomic molecules
the same form of covalent bonding occurs
between any pair of non-metallic
elements because the outer shells
quickly become full
thus most compounds between non-metallic
elements are volatile that means that
they evaporate easily or are gases
here are some drawn on our triangle
molecular covalent bonding
we now consider two metal atoms which
approach each other
as these sodium atoms approach their
electrons overlap and a molecule will
form because there is plenty of room in
their outer shells
such molecules do actually exist for
example in sodium vapor street lights
however as additional atoms approach
they too will be attracted in because
there will always be space in the outer
shell of the atoms so sodium as an
element forms a closely packed
three-dimensional metallic lattice where
the atoms are bonded by a sea of
electrons in these partially filled
outer shells
the loose electrons mean metals conduct
electricity and heat very well
the non-fixed nature of the bonds means
that the atoms can slide past each other
so metals can bend and melt very easily
but they will be difficult to vaporize
this form of bonding will occur between
any metallic elements which have only a
few electrons in their outer shell
here is brass an alloy of copper and
zinc for example retaining the metallic
properties of its elements
thus we can fill in the metallic bonding
part of our structure triangle
with elements of group four carbon and
silicon there are four electrons in the
outer shell and room for four more
electrons so four bonds will form in
this case it's possible to build up
three-dimensional structures with a
bonding going on forever
this is the three-dimensional structure
of diamond and silicon where the atoms
are covalently bonded in this lattice
making these giant covalent rock-like
structures solid at room temperature and
very difficult to melt and vaporize
with carbon we also get more flexible
giant chains called polymers
particularly when it bonds with hydrogen
so we should add life polymers to this
middle triangle
so far these structures can be elements
or compounds with a compound having
similar physical properties to the
elements from which it is made
we now consider where the elements are
very different
take chlorine and a metal such as sodium
the effective nuclear charge holding the
sodium electron is only plus one
compared with plus seven from chlorine
as we saw in part one of this video so
the molecule will be highly polarized
with the electron pulled strongly from
the sodium to the chlorine leaving the
sodium positively charged and the
chlorine negatively charged
when the ions are packed in a
three-dimensional lattice the structure
is stabilized and we have ionic bonding
because the ions have strongly fixed
positions ionic crystals are brittle
with high melting and boiling points
when dissolved in water or melted the
irons can move and the solution or melt
conducts electricity
these ionic properties are completely
different from the contrasting
properties of the metal and the
non-metal that we started with
so to summarize when two atoms approach
they have the possibility to bond if
there is space in their outer electron
shells
non-metallic elements will tend to form
self-contained small molecules giving
rise to volatile solids liquids and all
gases
that's molecular covalent bonding
compounds of carbon and silicon will
give rise to brittle rock-like or
polymerized giant structures
giant covalent bonding
metallic elements will bond together to
form metallic structures with loose
electrons metallic bonding
and if a metal bonds with a non-metal we
get ionic compounds with properties
totally different from their elements
ionic bonding