Chemical Bond Energy Considerations

A chemical bond forms when it is energetically favorable, i.e., when the energy of the bonded atoms is less than the energies of the separated atoms. Some of the types of tabulated data associated with chemical bonds are:

Ionization energy: the energy required to remove an electron from a neutral atom.

Electron affinity: the energy change when a neutral atom attracts an electron to become a negative ion.

Electronegativity: the ability of an atom in a molecule to draw bonding electrons to itself.

Ionization Energy and Electron Affinity

The ionization energy or ionization potential is the energy necessary to remove an electron from the neutral atom. It is a minimum for the alkali metals which have a single electron outside a closed shell. It generally increases across a row on the periodic maximum for the noble gases which have closed shells. For example, sodium requires only 496 kJ/mol or 5.14 eV/atom to ionize it while neon, the noble gas immediately preceding it in the periodic table, requires 2081 kJ/mol or 21.56 eV/atom. The ionization energy can be thought of as a kind of counter property to electronegativity in the sense that a low ionization energy implies that an element readily gives electrons to a reaction, while a high electronegativity implies that an element strongly seeks to take electrons in a reaction.

The electron affinity is a measure of the energy change when an electron is added to a neutral atom to form a negative ion. For example, when a neutral chlorine atom in the gaseous form picks up an electron to form a Cl- ion, it releases an energy of 349 kJ/mol or 3.6 eV/atom. It is said to have an electron affinity of -349 kJ/mol and this large number indicates that it forms a stable negative ion. Small numbers indicate that a less stable negative ion is formed. Groups VIA and VIIA in the periodic table have the largest electron affinities.

Electronegativity

Electronegativity is a measure of the ability of an atom in a molecule to draw bonding electrons to itself. The most commonly used scale of electronegativity is that developed by Linus Pauling in which the value 4.0 is assigned to fluorine, the most electronegative element. Lithium, at the other end of the same period on the periodic table, is assigned a value of 1. Electronegativity generally increases from left to right on the periodic table and decreases from top to bottom. Metals are the least electronegative of the elements. The Pauling electronegativities for the elements are often included as a part of the chart of the elements.

An important application of electronegativity is in the prediction of the polarity of a chemical bond. Because hydrogen has an electrognegativity of 2.1 and chlorine has an electronegativity of 3.0, they would be expected to form a polar molecule with the chlorine being the negative side of the dipole. The difference between the electronegativities of Na(0.9) and Cl(3.0) are so great that they form an ionic bond. The hydrogen molecule on the other hand, with zero electronegativty difference, becomes the classic example of a covalent bond.

After fluorine, oxygen is the next highest in electronegativity at 3.44, and this has enormous consequences in practice. Since oxygen is the most abundant element on the Earth, its high chemical activity makes it a part of most common substances. It's electronegativity leads to the polar nature of the water molecule and contributes to the remarkable properties of water.