Total number of $d$ electrons present in an element with atomic number $78$ is:

Variable valency is shown by

Identify the element if its observed and expected electronic configuration is considered,specifically focusing on the transition metal series where $d$-orbital filling occurs.

Mercury is a liquid metal because

$A$ first row transition metal in its $+2$ oxidation state has a spin-only magnetic moment value of $3.86 \ BM$. The atomic number of the metal is

All transition elements are $d$-block elements,but all $d$-block elements are not transition elements. Explain.