Additivity of Atomic Contributions

Clearly, there is one molecular property that can be exactly calculated fi om the contributions of its constituent atoms the molecular weight, or, more correctly, the molecular mass, which is exactly the sum of the masses of its constituent atoms. 

However, there are other molecular properties, such as molar volume, molar refi action [3], diamagnetic susceptibility [4], and parachor [5], that can be obtained to sufficient accuracy fi om contributions, p , of its N atoms (Eq. (5)). 

Investigations to find such additive constituent properties of molecules go back to the 1920s and 1930s with work by Fajans [6] and others. In the 1940s and 1950s lhe focus had shifted to the estimation of thermodynamic properties of molecules such as heat of formation, AHf, entropy S°, and heat capacity, C°. 

As Benson [1] pointed out, the additivity of atomic contributions is an insufficient approximation for estimating enthalpy, AHf, for it would lead to unacceptably large errors. However, the error in the estimation of the molar heat capacity ACp°, by Eq. (5) is not greater than 14.0 J/mol K and is usually closer to 6.0 J/mol K. In a similar fashion the error AS° for Eq. (5) is rarely higher than 20.0 J/mol K and is usually in the region of 8.0 J/mol K. 

For any molecule, additivity of atomic properties requires as many variables as there are different atom types contained in the molecule. For example, for acetic add, C2H+O2, three different atomic increments are needed, one each for a carbon, a hydrogen, and an oxygen atom. 

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