Is it possible to calculate bond enthalpies using the periodic table and the molecule's geometric structure? I am interested in a way of calculating this without getting the answer from measurements or pre-existing bond enthalpies.
Firstly the following basic facts can be helpful :
- Ionic bond is stronger than covalent bond in most cases .
- A metal non-metal bond is more ionic.
- Non-metal non-metal bond is covalent in nature.
By considering Valence Bond Theory , we can draw the fact that overlapping of larger orbitals ( usually of large atoms ) with smaller orbitals is less efficient and will result in a weaker bond .
For example ,
HF>HCl>HBr>HI is the order of their bond strengths .
This is because of the fact that as we go down the group , atomic size increases and so does its p-orbital which takes part in the overlapping with small s-orbital of H , making it a weaker bond .
Electronic configurations of various elements ( which is derived from the atomic number provided in the periodic table ) can be used to make the Lewis structures of most molecules and then the number of covalent bonds can tell comparitive enthalpies. ( more the bonds , more is the enthalpy )
Also , some Periodic tables try to be more informative and provide Pauling EN alongside the atomic number and mass . In that case , bond polarity can be measured by calculating the EN difference . Usually , more the polarity of the covalent bonds, more they act like an ionic bond and are more stable .
Though these tips can be of help in most comparisons , relying on actual values cover anomalous behaviors and exceptions which bump along our ways .