Fill in the blanks:
$(a)$ The $4f$ and $5f$ series elements are known as lanthanoids and actinoids,respectively.
$(b)$ In transition elements,electrons are filled in the $d$-orbitals of the .......... energy levels.

Which one of the following ions exhibits the highest magnetic moment?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Generally, $3d$ transition metals have high melting points.
Reason $R$: Involvement of $3d$-electrons in addition to $4s$-electrons in the interatomic metallic bonding.
In light of the above statements, choose the most appropriate answer from the options given below:

Platinum, Palladium and Iridium are called noble metals because

The halides of transition elements become more covalent with increasing oxidation state of the metal. Why?

In a transition series,with an increase in atomic number,the paramagnetism: