effective nuclear charge
My grandmother had a Boston Terrier that was obsessed with playing tug of war. You couldn't sit down in her house without having him push a nasty damp sock into your hand. If you were bored enough to accept it, he'd growl (ferociously, he thought) and pull. He was a little dog, though, and you could easily pick up the sock with dog still attached. He'd dangle like a little Christmas tree ornament until he got tired and let go.
The contest was futile because my mass was about ten times his. If I'd been a Boston Terrier, the match would have been different.
A tug-of-war also goes on between atoms involved in a chemical bond. The bonding electrons are the sock. The atom that can pull on the bonding electrons more strongly will get them. The winner is expected to be the atom with the higher effective nuclear charge.
Different atoms pull on bonding electrons to differing degrees, and this difference is the pivotal property in determining whether a covalent or ionic bond is formed. When one atom has a much higher pull on bonding electrons than its partner, electron transfer occurs, and an ionic compound is formed. When both atoms have similar attractions for the bonding electrons, they'll share them more equitably and a covalent compound is formed.
How can you predict whether a bond will be ionic or covalent? Elements can be ranked by their relative attraction for bonding electrons. The more different their ranks, the more likely the bond is to be ionic.
How can you rank elements by their ability to attract bonding electrons? Linus Pauling and others considered several elemental properties to develop a consistent ranking scheme. Pauling used the element's ionization energy and electron affinity to predict how it will behave in a bond. The more energy it takes to pull off the outer electron of an atom, he reasoned, the less likely it is to allow another atom to take those electrons. The more energy the atom releases when it gains an electron, the more likely it is to take electrons from another atom in bonding. These two energies were used to compute a numerical score called an electronegativity. Electronegativity ranks the element's tendency to attract electrons and acquire a more negative charge in a bonding situation.
Here is the electronegativity scale for the elements of the second period:
Notice the definite trend of electronegativity to increase going left to right across the periodic table. Fluorine's high nuclear charge coupled with its small size make it hold onto bonding electrons more tightly than any other element. Lithium has a lower nuclear charge and is actually larger than fluorine. Its valence electron is not tightly held and it tends to surrender it in chemical bonds.
How are electronegativity values used?
Consider coupling two fluorine atoms together. Even though each atom has a high attraction for bonding electrons, both attract them equally. (We've got two well-matched Boston Terriers pulling on this sock.) The electronegativity difference between the atoms is zero, and the bond is pure covalent. The electron distribution around the fluorine atoms is shown at right.
Coupling fluorine to oxygen (in F2O, for example) results in a bond that is polarized. Bonding electrons spend more time around the fluorine than the oxygen because fluorine has the higher electronegativity. The fluorine end of the bond has a partial negative charge, and the oxygen end has a partial positive charge. The bond has some ionic character. But the electronegativity difference is only 0.5, so the bonding electrons are shared, too. The bond is best described as polar covalent. (Think of a Boston Terrier pulling against a German Shepherd).
Coupling fluorine to lithium results in a bond with a much larger electronegativity difference (4.0 for F minus 1.0 for Li is 3.0). The bonding electrons spend almost all of their time around the fluorine, and this bond is best described as an ionic bond.
The line between covalent and ionic bonds isn't sharp. The bond between a pair of identical atoms is purely covalent, because neither has a stronger attraction than the other for the bonding electrons. But when the atoms are different, even slightly, the bonding electrons are shared unequally and the bond has some ionic character. Pure ionic bonding (corresponding to complete electron transfer) probably never occurs. The bonding electrons on the negative ion tend to spend more time on the side facing the positive ion. They spend extra time in the space between the ions, and so are always 'shared' to some small degree.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/bonds/faq/print-what-is-electronegativity.shtml