Which of the following types of metals make the most efficient catalysts?

'Spin only' magnetic moment is same for which of the following ions?
$A$. $Ti^{3+}$ $B$. $Cr^{2+}$ $C$. $Mn^{2+}$ $D$. $Fe^{2+}$ $E$. $Sc^{3+}$
Choose the most appropriate answer from the options given below.

In the $3d$ series of transition elements,the atomic radius of $Zn$ increases instead of decreasing because...

Consider $|x|$ as the difference in oxidation states of Mn in the highest manganese fluoride and the highest manganese oxide. The ions with $|x|$ number of unpaired electrons from the following are: $A$. $Sc^{3+}$ $B$. $Zn^{2+}$ $C$. $V^{2+}$ $D$. $Fe^{2+}$ $E$. $Co^{2+}$ Choose the correct answer from the options given below:

Which ion has the highest magnetic moment (only spin)?

Which of the following statements is $NOT$ correct for $3d$ and $4f$ series electrons?