Electronegativity difference

Estimation of the percentage of ion binding as a function of the electronegativity difference

The electronegativity difference ( also called Delta-EN (ΔEN) in laboratory jargon ) is the difference in the electronegativity of the atoms involved in a bond .

The shared electrons are more strongly attracted by the binding partner with the greater electronegativity and lead to a negative partial charge on it. Conversely, a positive partial charge arises at the binding partner with the smaller electronegativity, with which a polar electron bond is formed.

If Delta-EN becomes very large, the bond can often be described more advantageously as an ion bond (electrovalent bond). Substances in which the electrovalent part of the bond dominates are called salts . However, Delta-EN is not the only criterion for the presence of a salt, as can easily be shown using the example of the compounds lithium iodide (LiI) and hydrogen fluoride (HF). Although Delta-EN for HF is 1.9 greater than the value of 1.2 for LiI, hydrogen fluoride is a molecular compound, while lithium iodide is a salt with a typical crystalline structure (table salt structure).

Hydrogen fluoride (HF)	Carbon dioxide (CO ₂ )	Water (H ₂ O)

${\ displaystyle \ Delta E_ {N} = 1 {,} 9}$	${\ displaystyle \ Delta E_ {N} = 1 {,} 0}$	${\ displaystyle \ Delta E_ {N} = 1 {,} 4}$

As a rule of thumb it can be said that molecules with an asymmetrical structure and a difference in electronegativities (ΔEN) according to Pauling smaller than 1.7, but larger than 0.5 appear as dipole molecules , i.e. In other words, they are electrically neutral towards the outside, but have a (measurable) dipole moment. If the ΔEN is greater than 1.7, the character of an ionic bond is assumed. However, the limit ΔEN <1.7 is to be regarded as a guide value, as shown in the examples of hydrogen fluoride and aluminum chloride (ΔEN = 1.5). If ΔEN is less than 0.5, non-polar bonds are assumed.

The compounds that have the H ⁺ (proton) as a theoretical ion cannot be ionic compounds, since the H ⁺ is not a real independent ion due to the completely missing electron shell. With H ⁺ , therefore, the sp-σ- or sq-σ- bond orbital is basically formed , from which the H ⁺ can switch out in a protolysis reaction with sufficient ΔEN.