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Answer (1 of 6): CH is the lowest whole number ratio of elements Carbon (C) and Hydrogen (H) in Methane, a chemical compound. Both Lewis acids and bases contain HOMO and LUMOs but only the HOMO is considered for bases and only the LUMO is considered for acids (see figure). A Lewis acid is therefore any substance, that can accept a pair of non-bonding electrons. The pKa value of ammonia is estimated to be approximately 33. The acidity of perchloric acid, chloric acid, chlorous acid, and hypochlorous acid declines from HClO4 to HClO3 to HClO2 to HClO. #"H"_2"S"# is a weak acid. 4.2.25)? New Jersey: Prentice Hall, 2007. [2][16] The BrnstedLowry acidbase theory was published in the same year. Lewis base's HOMO (highest occupied molecular orbital) interacts with the Lewis acid's LUMO (lowest unoccupied molecular orbital) to create bonded molecular orbitals. Another good example to illustrate the effects of solvation enthalpy on solubility is the solubility of the silver halogenides in liquid ammonia (not aqueous ammonia). We can also see that we can determine relative hardness not possible by qualitative inspection. They are all considered soft acids. Rather, it expands the definition of acids to include substances other than the H+ ion. Just as any Arrhenius acid is also a Brnsted acid, any Brnsted acid is also a Lewis acid, so the various acid-base concepts are all "upward compatible". Electron-deficient molecules, which have less than an octet of electrons around one atom, are relatively common. For instance, CN- anions are soft bases even though the donor carbon atom is small because the CN- ligand has -orbitals available for -bonding with Lewis acids. From the standpoint of HSAB, the ammonia molecules are already significantly softer than the water molecules, therefore interactions with the soft anions become significantly stronger. Typical Lewis bases are conventional amines such as ammonia and alkyl amines. Thus, the strongest interactions are expected with the Li+ which is the hardest alkali metal, and the weakest interactions would be expected for the Cs+ which is the softest alkali metal.