Electronegativity is the degree to which an atom can attract bonding electrons to itself. The components of this relative measure consist of an atom’s ability to gain electrons and to retain them. The differences between the electronegativities of two atoms can be used to predict the relative strength of the bond. Different scales have been proposed to express electronegativity.
Chemical bonds are the attractive forces between atoms that create molecules. Molecules are the building blocks of all matter. The nature of the chemical bonds determines many of the molecular properties.
Electrons travel in shells around atomic nuclei. Atoms are most stable when their electron shells are filled or half-filled. Covalent bonds occur when one or more electrons are shared between atoms. The electron orbits closer to the more electronegative atom. Although the electron is shared in covalent bonds, the entire molecular structure is stronger.
In ionic bonds, the electronegative difference between two or more atoms is so great that the electron is stripped from the least electronegative atom. These bonds are polar, like tiny magnets. They can disassociate in water or other solvents into two or more separate ions.
In 1934, the American scientist Robert S. Muliken suggested that electronegativity be measured as half the difference between the ionization energy (IE) and the electron affinity (EA). IE is the energy necessary to remove an electron from an atom, and EA is the energy released when an atom gains an electron. His scale was not adopted because electron affinity was difficult to measure at the time.
Another American scientist, Linus Pauling, had developed an earlier scale based on the relative strength of chemical bonds. Fluorine, the most electronegative atom, was assigned an electronegativity of 4.0. Lithium, on the opposite side of the periodic table, was assigned a value of 1.0. Cesium, with a value of 0.7, is below lithium. In general, electronegativity increases from left to right across the periodic table. It decreases from top to bottom.
The Pauling scale gives a good measure of the type of bond atoms will form. The electronegative difference between two non-metal atoms is small; thus, covalent bonds are formed. The carbon-nitrogen (C-N bond) in pyridine (C5H5N) is an example. Carbon has an electronegativity of 2.5; nitrogen is 3.0; and the difference is 0.5.
A non-metal and a metal atom form an ionic bond due to the large electronegative difference. Potassium chloride is an example (KCl). Potassium has a value of 0.8; chloride has a value of 3.0; and the difference is 2.2.
