The greater the electronegativity difference between two bonded atoms, the

1. Electronegativity and Bonding
   1. Strong bonds form between atoms when they share or transfer electrons.
   2. Depending on how even or uneven the sharing is between the atoms several deferent kinds of strong bonds can form.
   3. The way to determine if the atoms will share their electrons evenly or unevenly is to examine the electronegativity of each atom.
   4. Electronegativity is how strongly an atom attracts electrons to itself when bonded with another atom.
   5. The illustration below shows that atoms in the upper right corner of the periodic table tend to attract electrons very strongly when bonded, while the atoms in the lower left corner don't attract electrons to themselves very well. (Except under unusual conditions, the noble gasses don't usually form bonds, so electronegativity has no meaning for atoms which are not bonded to other atoms.)

      or if you prefer sphere sizes to represent the electronegativity:
      

   6. When two atoms are bonded together there are three basic ways to pair them up:
      1. Two atoms with the same electronegativity, either both high or both low.
         1. This will cause the electrons that are shared in the bond to be evenly shared between the atoms.
         2. When atoms share electrons evenly between each other the bond formed is called a non-polar covalent bond.
            

      2. One atom with a somewhat higher electronegativity than the other.
         1. This will cause the electrons to be shared unevenly, such that the shared electrons will spend more time on average closer to the atom that has the higher electronegativity.
         2. When atoms share electrons unevenly but not very unevenly the bond formed is called a polar covalent bond.
            

      3. One atom's electronegativity is much higher than the other atom.
         1. In extreme cases the electrons in the bond spend so much time closer to the atom with high electronegativity that the shared electrons are considered to be transferred to that atom. The "sharing" is so uneven that one atom basically "takes" one or more electrons from the other atom.
         2. When the electrons being "shared" are so unevenly distributed between the atoms the bond that is formed is called an ionic bond.
            

2. Covalent Bonds
   1. Non-polar covalent bonds
      1. If the difference in the electronegativity between the two bonded atoms is less than 0.5 then the bond formed is considered to be non-polar covalent.
      2. Each atom attracts the other atom's electrons about equally so that the electrons spend equal amounts of time near each atom.
      3. Overall, both atoms will be neutral, having the same charge.
   2. Polar covalent bonds
      1. If the difference in the electronegativity between the two bonded atoms is between 0.5 and 2.1, then the bond formed is considered to be polar covalent.
      2. One atom attracts the other atom's electrons better, so the electrons stay closer (on average) to that atom. This causes an imbalance of electric charge within the bond between the two atoms.
      3. The atom that pulls the negative electrons better toward itself will be slightly negative and the other atom will be slightly positive.

         Non-Polar	Polar
         (H – H) Electronegativity Difference = 0.0	(C – O) Electronegativity Difference = 0.89	(H – F) Electronegativity Difference = 1.88

3. Ionic Bonds
   1. If the difference in the electronegativity between the two bonded atoms is greater than 2.1, then the bond is considered to be ionic.
   2. Because one atom pulls the other atom's electrons so well toward itself, there is a great imbalance of electric charge. If for some reason the bond between the atoms is broken, the atom with the higher electronegativity will actually keep the electron for itself.
   3. In this case the atoms with the higher electronegativity will be fully negative (due to the "gaining" of an electron) while the other atom is fully positive (due to its virtual loss of an electron).
4. Summary of Electronegativity and Bond formation
   1. Only the extreme cases are very clear. Very small differences in electronegativity result in non-polar covalent bonds, and very large differences in electronegativity result in ionic bonds. All other bonds are somewhere in-between.

      Type of Bond	Difference in Electronegativity
      Non-Polar Covalent	less than 0.5
      Polar Covalent	between 0.5 and 2.1
      Ionic	greater than 2.1

   2. What kind of bond will form between the following atom pairs:

The absolute value of the difference in electronegativity (ΔEN) of two bonded atoms provides a rough measure of the polarity to be expected in the bond and, thus, the bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is large, the bond is polar covalent or ionic. The absolute values of the electronegativity differences between the atoms in the bonds H–H, H–Cl, and Na–Cl are 0 (nonpolar), 0.9 (polar covalent), and 2.1 (ionic), respectively. The degree to which electrons are shared between atoms varies from completely equal (pure covalent bonding) to not at all (ionic bonding). Figure \(\PageIndex{2}\) shows the relationship between electronegativity difference and bond type.

Figure \(\PageIndex{2}\): As the electronegativity difference increases between two atoms, the bond becomes more ionic.

A rough approximation of the electronegativity differences associated with covalent, polar covalent, and ionic bonds is shown in Figure \(\PageIndex{4}\). This table is just a general guide, however, with many exceptions. For example, the H and F atoms in HF have an electronegativity difference of 1.9, and the N and H atoms in NH3 a difference of 0.9, yet both of these compounds form bonds that are considered polar covalent. Likewise, the Na and Cl atoms in NaCl have an electronegativity difference of 2.1, and the Mn and I atoms in MnI2 have a difference of 1.0, yet both of these substances form ionic compounds.

The best guide to the covalent or ionic character of a bond is to consider the types of atoms involved and their relative positions in the periodic table. Bonds between two nonmetals are generally covalent; bonding between a metal and a nonmetal is often ionic.

Some compounds contain both covalent and ionic bonds. The atoms in polyatomic ions, such as OH–, \(\ce{NO3-}\), and \(\ce{NH4+}\), are held together by polar covalent bonds. However, these polyatomic ions form ionic compounds by combining with ions of opposite charge. For example, potassium nitrate, KNO3, contains the K+ cation and the polyatomic \(\ce{NO3-}\) anion. Thus, bonding in potassium nitrate is ionic, resulting from the electrostatic attraction between the ions K+ and \(\ce{NO3-}\), as well as covalent between the nitrogen and oxygen atoms in \(\ce{NO3-}\).

Exercise \(\PageIndex{1}\)

Silicones are polymeric compounds containing, among others, the following types of covalent bonds: Si–O, Si–C, C–H, and C–C. Using the electronegativity values in Figure \(\PageIndex{3}\), arrange the bonds in order of increasing polarity and designate the positive and negative atoms using the symbols δ+ and δ–.

Answer