An aqueous solution of a 3d transition metal ion is green in colour. If the spin-only magnetic moment of this ion is approximately 4.90 BM , the ion is:

Step-by-step solution

The spin-only magnetic moment is given by = n(n+2) BM , where n is the number of unpaired electrons. Given 4.90 BM , we have n(n+2) = 4.90 , which gives n = 4 . Let us evaluate the given options: Fe ²⁺ ( 3 d ⁶ ): n = 4 . Colour in aqueous solution is green. Ni ²⁺ ( 3 d ⁸ ): n = 2 . Colour in aqueous solution is green. V ³⁺ ( 3 d ² ): n = 2 . Colour in aqueous solution is green. Cr ²⁺ ( 3 d ⁴ ): n = 4 . Colour in aqueous solution is blue. The only ion that satisfies both the green colour and n = 4 is Fe ²⁺ . Answer: