Which of the following molecules does NOT obey octet rule?

Calculate van't Hoff factor of $$\mathrm{K}_2 \mathrm{SO}_4$$ if $$0.1 \mathrm{~m}$$ aqueous solution of $$\mathrm{K}_2 \mathrm{SO}_4$$ freezes at $$-0.43{ }^{\circ} \mathrm{C}$$ and cryoscopic constant of water is $$1.86 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$$.

Find the radius of fourth orbit of hydrogen atom if its radius of first orbit is $$\mathrm{R}$$ pm.

Which noble gas element from following exhibits highest number of different oxidation states?