SolutionsEighteen Electrons
1. The iron of ferrocyanide
Iron (Z = 26) has the configuration [Ar] 3d6 4s2. In the [Fe(CN)6]4− ion it is present as Fe2+. How many electrons does the iron contribute, and how many the six cyanides?
Fe2+ has lost the two 4s electrons and keeps six 3d electrons. Each cyanide contributes one pair: 6 × 2 = 12. In total, 6 + 12 = 18 electrons.
2. Chromium hexacarbonyl
Chromium contributes six valence electrons and each carbon monoxide molecule, CO, one pair. How many CO molecules does it need to surround itself with 18 electrons?
Six: 6 + 6 × 2 = 18. The compound is chromium hexacarbonyl, Cr(CO)6.