Covalent Bonds

1. As mentioned earlier, when you pair two atoms with similar electronegativities and ionization energies they will share electrons.
2. There are two possibilities when the above situation is true:
   1. When metal atoms are bonded together:
      1. If metal atoms are close enough to bond they don't form true covalent bonds. Although electrons are shared between atoms, the valence electrons become delocalized, jumping from one atom to another. A common analogy is to say that the nuclei of atoms in a metal exist in a "sea of mobile electrons".
      2. This is due to the low ionization energy of these electrons, and is what gives metals the property of conductivity. A typical electric current can be described as electrons moving from one place to another. This can easily happen in metallic substances as depicted below.
   2. When non-metal atoms are close together:
      1. If two non-metals come close enough to bond, then covalent bonds will form between specific atoms. Whenever, this kind of bonding occurs polar or non-polar covalent bonds form, creating a molecule.
      2. If two atoms have the same electronegativity then they will share electrons equally forming a covalent bond in which the electrons being shared are distributed evenly between the two atoms.
      3. What would happen if you bonded two atoms which have different electronegativities but not different enough to cause ionization (transfer of an electron)?
      4. If two atoms of differing electronegativities bond together to share electrons, they will form a covalent bond, but they won't share the electrons evenly. The two molecules displayed below show the two types of covalent bonding: non-polar covalent bonding in which electrons are shared evenly, and polar covalent bonding in which electrons are shared unevenly. Notice their electronegativity values by referring to your electronegativity tables.

         Hydrogen - H2 (non-polar)	Hydrogen Fluoride - HF (polar)
         Create molecular surface. Show electrostatic potential. Make surface transparent.	Create molecular surface. Show electrostatic potential. Make surface transparent.

         

      5. Depending on the difference in electronegativities between two atoms the bond formed could be -
         • Ionic: very polar; full positive and negative charge difference between atoms; no sharing of electrons; large difference in electronegativity between the bonded atoms.
         • Polar covalent: slightly polar; regions of partial positive and negative charge formed; uneven sharing of electrons; moderate difference in electronegativities between the bonded atoms.
         • Non-polar covalent: neutral bond; no regions or very slight regions of positive and negative charge formed; equal or almost equal sharing of electrons; little or no difference in electronegativities between the bonded atoms.
3. When electrons are shared between specific atoms, polar and non-polar covalent bonds link atoms together to form a molecule. See the molecule of Vitamin C below:

   Vitamin C

4. Grab the molecule above and drag it around.
5. Notice that some atoms above are bonded to one, two, three, or four other atoms and that some are double bonded and some are single bonded.
6. To understand why we need to look at the valence electrons once again.
   1. The red atoms above are oxygen atoms with 6 valence electrons.
      O = 1s²2s²2p⁴
   2. In order for the oxygen to have the stable 8 valence electron structure it must share two electrons with other atoms. In other words it must form two covalent bonds. Look at the red atoms above. You should notice that they always form two covalent bonds (either two single covalent bonds or one double covalent bonds).
   3. The gray atoms above are carbon atoms with 4 valence electrons.
      C = 1s²2s²2p²
   4. In order for carbon to have the stable 8 valence electron structure it must share four electrons with other atoms. Look at the gray atoms above to see if they always form four bonds.
   5. The white atoms are hydrogen atoms with one valence electron. Hydrogen is closest to the Nobel Gas, Helium which only has 2 valence electrons. Therefore, hydrogen will be stable if it can share enough electrons to get two.
      H = 1s¹
   6. Notice that all the hydrogen atoms only form one bond above.