Actinoids are the series have 15 element in the periodic table.
Chemistry of actinoids are complicated because of the following reasons.
Their outer most orbitals are 6d and 7s but most of the electrons resides in 5f orbitals. These orbitals are most exposed to environment and incoming electron picked up by 5f orbital instead of 6d or 7s orbitals.
They are radioactive elements that couldn't be handle in normal condition to study their properties.
Their f-orbital can accommodate 14 electrons and they have many variable oxidation state.
5d electron donot contribute in the formation of chemical bonds.
They have a very strong tendency to make complexes
Most of the actinoides are artificial and are in very minute in amount. they are in very less quantity that is amounts found in nano-grams so are more expensive too and
As they are radioactive so their half life is very short and in very less time the decay occur so couldn't be study as upon decay their actual properties changed.
Due to all the above reasons it make difficult for a chemist to study about the chemical properties of actinoids.
