$d-d$ transition occurs in . . . . . . .

Why are the $d$-block elements known as "Transition" elements? Why are zinc,cadmium,and mercury not considered transition elements?

An amalgam is $.......$

Which element from the following in $+3$ oxidation state forms colourless compounds?

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 $Cr(VI)$.

The number of unpaired electrons in Gadolinium $[Z = 64]$ is