hello and welcome to the chemistry
solution this tutorial is on electron
affinity the electron affinity is the
energy change associated with an atom
gaining an electron and in most cases
we're talking about a neutral atom
gaining an electron most often electron
affinities are negative values meaning
that energy is released in this process
however there are some exceptions to
this rule where the electron affinity is
a positive value and this occurs in the
case of an atom becoming very unstable
upon gaining an electron the more
negative the electron affinity value the
easier it is to add an electron to a
particular atom let's look at the
electron affinity trend on the periodic
table and as I mentioned before this
trend does contain some exceptions which
we'll cover in just a second the general
trend is that as you go from the bottom
to the top of the periodic table you
don't see much change in electron
affinity but as you go from left to
right across the periodic table you see
the electron affinity values becoming
more negative now let's look at some
exceptions the first exception is the
noble gases
remember that noble gases have
completely filled shells and so the
addition of an electron requires that
electron to reside in a higher energy
shell and this is an unfavorable
reaction and because of this all of the
noble gases have electron affinity
values greater than zero the second
trend is the elements in group 2a and
the electron affinities for the elements
in group 2a are more positive than the
electron affinities for the elements in
group 1a even though we said that
usually we see electron affinity values
become more negative as we go from left
to right across the periodic table and
this is because the addition of an
electron requires it to reside in the
previously unoccupied p subshell
so remember as you're determining
electron configurations for each new
shell the first two electrons go into
the s subshell add a third electron that
electron now has to reside in the P sub
shell and the third exception is the
elements in group 5a and the electron
affinities for the elements in group 5a
are more positive because the addition
of an electron requires it to be put
into an already occupied orbital so
again remember as you're filling in
electrons in each new shell the first
two electrons go into the S sub shell
then you start putting electrons into
the P sub shell with the first three
electrons going into different orbitals
but remember that P sub shells only have
three orbitals so the addition of a
fourth electron into the p orbital
requires it to pair up with an electron
and the pairing of electrons requires
electron-electron repulsions to be
overcome and this makes the electron
affinity values for the elements in
group five a more positive than the
electron affinity values for the
elements in group 4a and this is an
exception to the general trend let's
look at an example problem arrange these
elements in order of increasing electron
affinity
so from least to most negative electron
affinity values germanium potassium
chlorine and sulfur remember that in
general electron affinity values don't
very much in the vertical direction on
the periodic table but they become
increasingly more negative as you move
from left to right across the periodic
table
so first let's locate these elements on
the periodic table we can then see that
following our general trend potassium
should have the least negative electron
affinity value followed by germanium
followed by sulfur and concluding with
chlorine with chlorine having the most
negative electron affinity value in this
series let's look at another question
pertaining to electron affinity why does
phosphorous not follow the general
periodic trend for electron affinity
remember that phosphorous is in the 5 a
group on the periodic table so here I've
drawn out the electron configuration for
phosphorous abbreviating the inner shell
electrons with the neon core you'll see
that the 3 s sub shell is completely
filled and you'll see that the 3p sub
shell is half filled with each orbital
in the 3 P sub shell having one electron
the addition of another electron to
phosphorous would require it to pair up
with another electron in the 3p sub
shell and in order to do that it would
have to overcome electron-electron
repulsions and this makes this process
slightly less favorable then for an
element in group 4a on the periodic
table thanks for watching the chemistry
solution we hope you enjoyed this
tutorial
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