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Name:  Mike
Status: educator
Grade: 9-12
Location:MI
Country: N/A
Date: 1/9/2005


Question:
Nitrogen has a high electronegativity, meaning that it strongly attracts electrons. This makes sense, since it has a significant nuclear charge, very little screening, and a small radius. Nitrogen also has electron affinity > 0, meaning that it does not attract electrons. This makes sense, since an electron added to a nitrogen atom will be paired with another electron in a p-orbital, resulting in more repulsion in a small atom and therefore, a less stable atom after the addition of the electron. So, how is it that nitrogen both attracts electrons (high electronegativity) and does not attract electrons (E.A. > 0)?


Replies:
Mike,

Electronegativity, or EN (the relative tendency of an element's atoms to attract valence electrons) is related both to the size of the element's atomic electron affinity, or EA (energy liberated when an extra electron attaches to a single neutral atom) and its first ionization energy, or IE (energy required to remove an existing valence electrons from the neutral atom). After Pauling's work, Robert Millikan proposed the following formula for EN:

EN(atom) = constant*(|EA| + |IE|)/2

When the right value for (constant) is chosen, this produces electronegativities in reasonable (but not perfect) agreement with Pauling's original scale (which is derived from bond energies). So in this way of thinking, an atoms EN has a contribution due to its tendency to hold onto its own valence electrons (manifested in IE) and also its tendency to accept other atom's valence electrons (manifested in EA).

In this case, although N has low EA, it has high enough IE for it to have reasonably high EN. F has large EA and IE. C has a larger EA than N but its IE is so much smaller that C ends up having lower EN than N.

EN is a somewhat controversial concept and there are a number of EN scales that have been developed since Pauling's original work. EN is not as fundamental a quantity as EA or IE, both of which be can directly measured by experiment. However, EN is a very useful concept as long as you use it simply as a helpful organizing principle for certain chemical phenomena.

hope this helps!
Dr. Topper


There are a number of issues here that you need to distinguish:

1. Electron affinity is the ENERGY RELEASED when an atom in the gas phase adds an electron to form a negative ion: E + e(-1) ---> E(-1). In some cases this can be measured experimentally; in other cases it is calculated.

2. Unfortunately, even though most electron affinities are EXOTHERMIC, historically these are tabulated as positive quantities, which is the opposite the normal sign convention that exothermic reactions have a negative value. Electron affinity is a property of ATOMS IN THE GAS PHASE.

3. In contrast to electron affinity, ELECTRONEGATIVITY is an empirical scale of the ability of an atom IN A COVALENTLY BONDED MOLECULE to attract electrons. This is quite a different matter than electron affinity. Linus Pauling first introduced this concept (See: "General Chemistry" by Linus Pauling) but other empirical scales have been suggested by other authors. (See: "Inorganic Chemistry" by James E. Huheey, for example). Any text on inorganic chemistry will discuss electronegativity in detail as well.

One needs to be cautious in applying any of these empirical relations too strictly. They are empirical and show trends.

Vince Calder



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