18. Which lanthanoid is most commonly used ? (a) Lanthanum (b) Nobelium (c) Thorium (d) Cerium 19. Ammonium dichromate is use in some fireworks. The green coloured powder blown in air is : (a) CrO3 (b) Cr2O3 (c) Cr (d) CrO(O2)2 20. Which of the following statement is not correct ? (a) La(OH)3 is less basic than Lu(OH)3 (b) La is actually an element of transition series rather than lanthanoids (c) Atomic radii of Zr and Hf are same because of lanthanoid contraction (d) In lanthanid series the ionic radius of Lu3 + is smallest
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