Is the enthalpy change for the addition of one electron to an atom or ion in the gaseous state. Electron affinity values are given per mole of atoms or ions.

For example: Cl(g) + e^- ® Cl^-(g)     DH⁰ = -349 kJ

The chlorine atom, which is one electron short of a noble gas configuration, adds an electron readily in a exothermic reaction. Unlike ionisation energies, electron affinities can be either positive or negative.

Energy is required to add an electron to an already stable configuration. The electron affinity values for the beryllium family elements, which have ns² configurations, and for the noble gases, with their ns² np⁶ configurations, are positive.

- is the ability of an atom in a covalent bond to attract electrons to itself. In a bond between two atoms of the same element, there is no electronegativity difference between the two atoms and the bond is nonpolar.

Linus Pauling originated the term and derived an electronegativity scale based upon calculations using bond energies. Fluorine is the most electronegative element with a value of 4.

Basically if the electronegativity difference between two atoms > 2 then predominantly ionic bonds occur. If the difference < 2 then covalent.

Summary

 

• Electronegativity increases across the periods
• Electronegativity decreases down the families
• Trends are less regular for transition elements than for representative elements
• Metals have low electronegativities
• Fluorine and Oxygen are the two most electronegative elements
• Ionic bonds form between atoms of electronegativity difference of 2 or greater

Information provided by: http://www.bpx.cov.ac.uk