The electrode potential of M²⁺ / M of 3d-series elements shows positive value of :

1. Zn

2. Fe

3. Co

4. Cu

The correct order of the following elements with respect to their density is:

1. Cr < Zn < Co < Cu < Fe

2. Zn < Cu < Co < Fe < Cr

3. Zn < Cr < Fe < Co < Cu

4. Cr < Fe < Co < Cu < Zn

Which of the following lanthanoids do not form MO₂ [where M is a lanthanoid metal]?

1. Pr

2. Dy

3. Nd

4. Yb

In which of the following order the given complex ions are arranged correctly with respect to their decreasing spin only magnetic moment?

(i) [FeF₆]^3–
(ii) [Co(NH₃)₆]³⁺
(iii) [NiCl₄]^2–
(iv) [Cu(NH₃)₄]²⁺

1. (i) > (iii) > (iv) > (ii)

2. (ii) > (iii) > (i) > (iv)

3. (iii) > (iv) > (ii) > (i)

4. (ii) > (i) > (iii) > (iv)

The correct reactions among the following are:

$\mathrm{a}.$ ${\mathrm{MnO}}_4^{-}+{\mathrm{H}}^{+}+{\mathrm{Fe}}^{2+}\to 2{\mathrm{Mn}}^{2+}+{\mathrm{Fe}}^{3+}+{\mathrm{H}}_2\mathrm{O}$
$\mathrm{b}.$ ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}+{\mathrm{MnO}}_4^{-}+{\mathrm{H}}^{+}\to {\mathrm{Mn}}^{2+}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$
$\mathrm{c}.$ ${\mathrm{SO}}_2+{\mathrm{MnO}}_4^{-}+{\mathrm{H}}^{+}\to {\mathrm{Mn}}^{2+}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{SO}}_3$

Which of the following explains the primary cause of lanthanoid contraction?