How many elements are there in the third transition series?

The ionisation energies of transition elements are

In which of the following cases,the stability of two oxidation states is correctly represented?

An atom with atomic number $21$ belongs to the category of

Number of paramagnetic ions among the following $d$- and $f$-block metal ions is . . . . . . . $Mn^{2+}$, $Cu^{2+}$, $Zn^{2+}$, $Yb^{2+}$, $Sc^{3+}$, $La^{3+}$, $Gd^{3+}$, $Lu^{3+}$, $Ti^{4+}$, $Ce^{4+}$. (Atomic number of $Mn = 25$, $Cu = 29$, $Zn = 30$, $Yb = 70$, $Sc = 21$, $La = 57$, $Gd = 64$, $Lu = 71$, $Ti = 22$, $Ce = 58$)

What should be the difference in atomic radii of metals to form an alloy?