AS & A-Level Chemistry 03 — Bonding, shape and states of matter

The Pauling electronegativity scale assigns dimensionless numerical values to elements to quantify their electron-attracting power. By examining the absolute difference in Pauling electronegativity values $(\Delta\mathrm{EN})$ between two bonded atoms, chemists can predict the fundamental nature of the bond formed. When two identical atoms bond, such as in $\mathrm{Cl_{2}}$, the electronegativity difference is zero, resulting in a purely non-polar covalent bond where the bonding electrons are shared equally. If the two non-metal atoms have a small to moderate difference in electronegativity, they form a polar covalent bond. In this scenario, the shared electron pair is drawn closer to the more electronegative atom, creating partial electrical charges (dipoles).

However, if the electronegativity difference is very large—typically observed when an s-block metal bonds with a highly electronegative non-metal—the bonding is considered ionic. In such cases, the more electronegative atom effectively pulls the bonding electrons entirely into its own valence shell, forming discrete cations and anions that are subsequently held together by electrostatic forces. While the transition from pure covalent to pure ionic bonding is a continuum, calculating the difference in Pauling values provides a reliable, predictive tool for classifying the primary bonding character in binary compounds.