Lutetium is the last element in the lanthanide series, which is the series of the rare Earth elements. It is named after lanthanium, the first element in the series. These elements bear many similarities. Given that lutetium is the last element in the series, it is the hardest and the densest of all the elements in the group. It is a member of the d-block of the table unlike all the others in the series, which are located in the f-block. Lutetium is soluble in water and forms a compound with water molecules.