Chemical Bonds

We usually think of the different types of bonds as discrete entities. This is not how they should be viewed. Ionic and covalent bonds are the opposite extremes of a continuum. Bonds that are 100% ionic or 100% covalent are not the usual. Most elements that combine do so in some sort of blend of ionic and covalent characteristics. Since they are the extremes of a single process, the dividing point is at 50%. When a bond is more than 50% ionic it is referenced as an ionic bond, even though it may have substantial covalent character. Likewise, when a bond is more than 50% covalent, it is referenced as a covalent bond, even though it may have substantial ionic character.

These measurements are, of course, done by physical chemists.

In practice, histological application is that most dyes attach to tissue components by ionic bonds. Covalent bonds are much less important, except in mordant dyeing in conjunction with coordinate bonds.


Ionic bond

Ionic bonds depend on the transfer of an electron from one atom to another. The atom that loses the electron then becomes a positively charged ion. The atom that receives the electron becomes a negatively charged ion. Ions with opposite charges attract each other, and the charges are what keep the atoms together.

The ability of an element to attract electrons is called electronegativity. The electronegativity of elements is expressed as a number up to 4.0. The greater the difference between the electronegativities of two elements, the more likely compounds formed from them are ionic in nature.

The fundamental reaction between dyes and tissue components is the ionic attraction between the charged carboxyl groups of one with the charged amino groups of the other. Both dyes and tissues can have carboxyl and/or amino groups, so it is applicable from both perspectives.

Although the ionic attraction between negatively charged carboxyl groups and positively charged amino groups is the one of most importance, ionic attractions between amino and hydroxyl, between amino and phosphate, and between amino and sulphonic are also frequently present. Some other ionic reactions are also possible (and likely) but are of low overall importance, except in very specific circumstances.

Chemical group Non ionised Ionised Type
Amino -NH2 -NH3+ Cationic
Carboxyl -COOH -COO- Anionic
Hydroxyl -OH -O- Anionic
Sulphonic -SO3H -SO3- Anionic


Covalent bond
When the disparity between the electronegativities of the two atoms is not great enough to result in transfer of an electron, the effect is that both atoms share the electrons involved. The electrons spend time associated with both of the atoms. Depending on the specific electronegativity difference, the electrons may be associated with one of the atoms more than with the other. This is called polarity, and compounds that exhibit it are called polar compounds. Only in the case of exactly equal electronegativities do compounds become completely non polar. This is seen when the atoms are of the same element, as in elemental gases such as hydrogen (H2) and oxygen (O2).


Coordinate bond

Coordinate bonds are similar to covalent bonds. The difference is in the source of the electrons that make up the bond pair. In covalent bonds each atom participating in the bond donates an electron, both of which are then shared by the two atoms. With coordinate bonds, only one of the atoms donates electrons (two of them) and these are then shared by both of the atoms participating in the bond. A compound formed by coordinate bonds is called a coordination compound.

Except for mordants, covalent and coordinate bonds are of little importance in staining. The vast majority of staining is ionic in nature.



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