let's practice assessing atomic radii
and ionic radii the first question says place the following elements in order of
increasing atomic radius so we have lead, germanium, Krypton, flerovium, and bromine
the second question says place the following species in order of increasing
ionic radius so we have O 2 minus, Mg2 plus, F minus, Na plus, and neon, a neutral
neon atom so if this seems confusing go back and check out my tutorial on the
periodic table and periodic trends one of those trends is atomic radius and we
also talk about ionic radius so that will give you the understanding you need
to answer these questions
so first let's do the atomic radius trend we need to find these 5 elements
on the periodic table we have gotten lead we've got germanium we've got
Krypton we've got flerovium and we've got
bromine so those are the five elements that we are looking at so now we want to
put them in order of increasing atomic radius so what do we know about atomic
radius we know that atomic radius is going to increase as we go to the left
as we go to the left there are fewer protons in each nucleus and that means
there is less of an electromagnetic attraction between the nucleus and the
electrons. so the nucleus will not pull on the electrons as tightly as we move
to the left, that means that the atoms will get slightly larger moving to the
left on the periodic table. we also know that atomic radius increases going down
the periodic table because each time we go down one period we're adding an
entire shell and so that certainly is going to make the atom quite a bit
larger by adding an entire new shell on the atom. so we know that atomic radius
increases going left and down and that means that if we start at Krypton and go
to the left, Krypton must be smaller than bromine and bromine must be smaller than
germanium. separately going down this group we know that germanium must be
smaller than lead and lead must be smaller than flerovium. so now this is
actually quite easy we can just combine these two trends because germanium is
the biggest one in the first trend and the smallest one in the second trend so
let's just put them together and the correct order in increasing atomic
radius is Krypton bromine germanium lead flerovium. now let's talk about ionic
radius. here we have some ions let's find them, we've got oxygen there we've got
magnesium there we've got fluorine here we've got sodium here and we have neon
here. so with ions it's very important to know how many electrons are in each
species so looking at oxygen first we know that a neutral oxygen atom has
eight electrons for a neutral atom the number of electrons is equal to its
atomic number right same number of protons and electrons so neutral oxygen
would have eight but this is an O 2 minus ion that means there are two
additional electrons two additional negatively charged particles so 8 plus 2
is 10. O 2 minus has 10 electrons. looking at Mg 2 plus, a neutral magnesium atom
has 12 electrons but this is a 2 plus ion so it has two fewer negatively
charged particles it has lost two electrons so Mg 2 plus also has 10
electrons. neutral fluorine has nine electrons but
this is F- so it has gained one electron, F- or the fluoride ion also has 10
electrons for sodium plus neutral sodium normally has 11 electrons but it has
lost one so Na plus also has 10 electrons. and lastly neon is neutral so
that will just have the same number of electrons as its atomic number and that
is 10 so that means these are all isoelectronic species these all have the
same number of electrons and specifically they all have neon electron
configuration because they all have the same number of electrons they are all
going to experience the same amount of electron repulsion so 10 electrons are
going to push against each other and push outwards in the same way for all of
these species so the number of electrons is not a factor in determining this
trend. instead the ionic radii are going to be determined by the number of
protons in the nucleus. so they all have 10 electrons but how many protons are
pulling on those 10 electrons the more protons there are in the nucleus the
more they will pull on the electrons and pull the electrons in towards the
nucleus which means the more the ionic radius will contract so for iso
electronic species the ionic radius will get smaller as we add more protons to
the nucleus that means that fewer protons will pull on the electrons less
and the ionic radius will expand so as we take protons away from the nucleus
the ionic radius will increase. so in order to get this trend we have to start
with the species that has the most protons because that is going to pull
the electrons the most and that is going to be Mg 2 plus because there are 12
protons in that nucleus followed by Na plus which has 11 protons, neon has 10
protons, the fluoride ion has 9 protons and the oxide ion has eight
protons. so fewer protons going to the right that means less and less
attraction between the nucleus and those electrons so the ionic radius will
expand as there are fewer protons in the nucleus and this is the correct order to
place these in increasing ionic radius.