Why do $CrO_3$ and $V_2O_5$ have low melting points?

Fill in the blanks:
$(a)$ The shielding effect of $d$-electrons is ......... effective than $p$-electrons.
$(b)$ $A$ decrease in exchange energy leads to a ......... in stability.
$(c)$ The maximum oxidation state of $Mn$ is ..........
$(d)$ The stability of $Mo(VI)$ and $W(VI)$ is found to be .......... than that of $Cr(VI)$.

Which of the following sequences does not match the property indicated with it?

Choose the element which is not radioactive.

What is the general electronic configuration of the outermost shell of $f$-block elements?

The electronic configuration of copper $({_{29}}Cu)$ is