What should be the difference in atomic radii between two metal elements to form an alloy?

What are interstitial compounds? Why are such compounds well known for transition metals?

The valence shell electronic configuration of $Cr^{2+}$ ion is

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:

Among the following series of transition metal ions,the one in which all the metal ions have $3d^2$ electronic configuration (excluding the $3p^6$ core) is (Atomic number: $Ti=22, V=23, Cr=24, Mn=25$)

Identify the pair of elements in which the number of electrons in the $(n-1)$ shell is the same.