Please explain the lines underlined

Dear Student,
Lanthanoid contraction is the decrease in ionic radius of the lanthanides than expected.
The electrons are filled in 4f orbital which are burried inside and have poor shielding effect due to which there is greater force of attraction between outer electrons and nucleus and hence size decreases.
Therefore as the size of the lanthanides decreases from Ce to Lu, bond strength of the OH with metal ion increases, hence release of OH- ion becomes difficult and therefore basic strength of the hydroxides decreases.
​​​​​​so Ce(OH)3 is most basic while
Lu(OH)3 is least basic.
Regards,

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