The photon of hard gamma radiation knocks a proton out of $${}_{12}^{24}Mg$$ nucleus to form

t_1/4 can be taken as the time taken for the concentration of a reactant to drop to $$3 \over 4$$ of its initial value. If the rate constant for a first order reaction is K, the t_1/4 can be written as

The lanthanide contraction is responsible for the fact that :

Which of the following factors may be regarded as the main cause of lanthanide contraction?