What is a Bond?
A chemical bond occurs when the sharing or transfer of electrons between two atoms results in them being attracted to one another. Since there is a great variation in the strength of attraction between the two atoms, and the causes for that attraction, bonds can be categorized accordingly.
When discussing bonding, the most frequently discussed types are ionic and covalent. The first type, referred to as a covalent bond, is frequently described as a situation where two atoms are "sharing" bonded electrons. An ionic bond is defined as a situation where one atom gives away an electron (making it a cation) to another atom that accepts the electron (making it an anion). The resultant charges of the ions keep them attracted to each other, hence the name "ionic" bond.
Truly, the only situation where two atoms are sharing electrons 100% is when the two atoms are of the same element, like the diatomic element O2. Otherwise, there will always be one element that wants electrons more than another. Electronegativity values are used to determine which element wants electrons more, and if the difference between the values determine whether the bond is ionic or covalent. A small difference in electronegativity values indicates that the two atoms involved in the bond have a similar attraction for bonded electrons, therefore they are apt to "share" the electrons in a covalent bond. A large difference in electronegativity values indicates that the two atoms do not have a similar attraction for bonded electrons. In fact, one atom is willing to give away its electron easily, and the other atom does not need to share, it can accept the electron outright. Typically, the difference between ionic and covalent can be summarized by the chart below:
|Difference in Electronegativity||Bond Type|
|Less than 1.65||Covalent|
|Greater than 1.65||Ionic|
The number 1.65 may seem like an arbitrary place to divide the difference between ionic and covalent. However, inspection of an electronegativity table reveals that 1.65 is exactly 50% of the maximum possible difference between electronegativity values. Consider the following situation as an example. Francium, the most electropositive element (willing to give electrons away) has an electronegativity value of 0.7, lowest of all the elements. Fluorine is the opposite, it wants to accept electrons and thus has an electronegativity value of 4.0, the maximum value of all the elements on the table. A bond formed between these elements means the difference in electronegativity value is 3.3 (obtained by: 4.0 - 0.7). Since these two elements represent the extremes of the elements, 3.3 is the maximum difference in electronegativity, and 50% of that value is defined as the separation between ionic and covalent.
There are Two Types of Covalent
This may beg the question, "Why do elements go from sharing to a give/take situation suddenly at the 50% mark?" The answer lies partly in the definition of covalent. Although covalent is "sharing" it is only 100% sharing when the difference in electronegativity is 0. Two elements may have electronegativity values that are quite different, and the result is that the "sharing" taking place is not exactly fair. Consider a water molecule as an example. Hydrogen has an electronegativity value of 2.1 while oxygen's value is a more robust 3.5 (recall fluorine has the maximum value at 4.0). The difference between these values is 1.4, clearly less than the 50% watermark discussed earlier. However, with a difference this large, and considering the values given for hydrogen and oxygen, it is evident that oxygen wants the bonded electrons more than hydrogen. Those electrons are held more closely to oxygen than hydrogen, but not strongly enough to fully remove them from the attraction of hydrogen's nucleus. This is still defined as "sharing", but it's a similar type of sharing that takes place when an older sibling shares a cookie with a younger sibling and then proceeds to unfairly breaks a cookie in two pieces of a vastly different size. The situation between oxygen and hydrogen is typical of polar compounds. Polarity is the phenomenon where a molecule has a positive and negative pole as a consequence of the concentration of electrons around certain elements (creating a negative pole), and a lack of electron density (positive pole) around another element. The diagram below shows the polarity of a water molecule.
Covalent compounds are broken down into two categories, nonpolar covalent and polar covalent. In nonpolar covalent compounds, the difference in electronegativity is small enough that no discernible positive and negative region arises from an uneven distribution of electrons. Polar covalent compounds, like water, have a distinct positive and negative pole. Here, the negative region is concentrated around oxygen (the more electronegative element) and the positive region is located near the hydrogen atoms.