Lanthanoid contraction is a phenomenon in chemistry where there is a steady decrease in the size of the atoms and ions of the rare-earth elements with increasing atomic number from lanthanum (atomic number 57) through lutetium (atomic number 71) . This decrease in size is due to the poor shielding effect of the 4f electrons, which means that the inner-shell electrons do not shield the outer-shell electrons effectively from the increased positive charge of the nucleus, resulting in successive reductions of the atomic and ionic radii/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction).

Some key points about lanthanoid contraction are:

• The lanthanide contraction applies to all 14 elements included in the lanthanide series, including cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction).
• The atomic radius, as according to the lanthanoid contraction, of these elements decreases as the atomic number increases/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction).
• The lanthanoid contraction has consequences such as difficulty in the separation of lanthanides, effect on the basic strength of hydroxides, and complex formation.

In summary, lanthanoid contraction is a phenomenon where there is a steady decrease in the size of the atoms and ions of the rare-earth elements with increasing atomic number due to the poor shielding effect of the 4f electrons.