Chemistry For HSC Part 2 - Chapter No.5 - Scientific Reasons

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Chapter No.5
d- Block Elements
Scientific Reasons

Q.1: Why most of the transition elements and their compounds are paramagnetic?
Ans: Most of the transition elements have their unpaired electron in d-orbital causing paramagnetic property i.e. attracted by the magnetic fields like Sc, V, Ti, Cr, Mn etc.

Q.2: Why H₂O and NH₃ act as ligand but H₃O+ and NH₄+ do not?
Ans: H₂O and NH₃ have lone pair of electrons donating to central atom and act as ligand, but H₃O+ and NH₄+ do not have any lone pair of electrons to form coordinate covalent bond.

Q.3: Why electronic configuration of Cr is 4s¹, 3d⁵ instead of 4s², 3d⁴, while that of Cu is 4s¹, 3d¹⁰ instead of 4s², 3d⁹?
Ans. In Cr and Cu one electron from 4s orbital jumps to 3rd orbital in order to gain the extra stability of lower energy orbital and single spin electron.
Hence Cr has configuration of ls², 2s², 2p⁶,3s²,3p⁶, 4s¹ , 3d⁵
and copper has ls², 2s², 2p⁶,3s²,3p⁶, 4s¹, 3d¹⁰.