Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 54

The ability of an atom to lose, gain, or share electrons is crucial to the type of bond that it forms with other atoms. This has been quantified in terms of a measureable property called the electronegativity, defined as the ability of an atom in a particular molecule to attract electrons to itself. Linus Pauling originally developed an electronegativity scale for atoms in the 1930s, referred to as the Pauling electronegativity scale, based on measurements of the strengths of covalent bonds between different elements. In this scale, atoms have an electronegativity value between 0 and 4, a range selected arbitrarily by Pauling. A limitation of Pauling’s method is that many elements do not form stable covalent compounds with other elements and, thus, their electronegativity cannot be measured. Other methods have since been developed which address this problem but the Pauling electronegativity scale is still widely used. For the main group elements, that is, elements not in the transition metal blocks, the electronegativity values increase as the atomic number decreases in a particular column of the periodic table and increase with atomic number along a particular row.

Empirical relations based on electronegativity values have been proposed to predict or compare the type of bond but they provide only an approximation and involve some degree of arbitrariness. One simple relation, based on the Pauling electronegativity scale, considers the difference in electronegativity, ΔEN, as follows:

 ΔEN > 2.0: ionic bond

 ΔEN < 0.4: covalent bond

 0.4 < ΔEN < 2.0: combination of ionic and covalent character

While these ΔEN values are useful for predicting whether a bond is predominantly ionic or covalent, there is no sharp distinction between an ionic bond and a bond that has a combination of ionic and covalent character, and between a covalent bond and a bond that has a combination of ionic and covalent character. Empirical equations have been developed to predict the percentage of ionic character I of a bond between two atoms with a difference in electronegativity ΔEN. One such equation is

(2.5)

However, the exact nature of a bond that has a certain percentage of ionic and covalent character is unclear.