covalent and ionic radii generally increase on descending a group in the periodic table due to the presence of an extra filled shell of electrons . on moving from left to righ across a period , the cobvalent and ionic radii decrease . this is because the extra orbital electrons externally shield the extra nucler charge . thus all the electrons are pulled in closer . the shielding effect of electrons decreases in th order s>p>d>f . the contraction in size from one element to another is fairly small . however , the additive effect over the 14 lanthanide elements from Ce to Lu is about 0.2 angstroms and this is known as lanthanide contractin .

the hardness , melting point , and boiling point of the elements all increase from Ce to Lu . this is because the attraction b/w the atoms increases as the size decreases .
the properties of an ion depend on its size and its charge . the ln3+ lanthanide ions change by only a small amount from one element to thenext. , and thier change is the same , so their chemical properties are fairly similar . since Lu3+ is the smallest ion it is the most heavily hydrated . though the lanthanides do not form complexes very extensively , since Lu3+ and Ce3+ are the largest ions so Ce(OH)3 and Lu(OH)3 are the strongest bases .

the lanthanide contraction reduces the size of the last four elements of the series below that for Y in the preceeding transition series . since the radii of the heavier lanthanide ions , particularly Dy3+ and Ho3+ , are similar to that of Y3+ it follows that thier chemical properties are also very similar . as a result the seperation of these elements is very difficult .