Chemical Bonding: Covalent Bonding

·      Covalent bonding occurs when two (or more) elements share electrons.

·      Covalent bonding occurs because the atoms in the compound have a similar tendency for electrons (generally to gain electrons).

·      This most commonly occurs when two nonmetals bond together. Because both of the nonmetals will want to gain electrons.

·      Covalent Compounds:

1. Gases, liquids, or solids (made of molecules)
2. Low melting and boiling points
3. Poor electrical conductors in all phases
4. Many soluble in nonpolar liquids but not in water.

·      A good example of a covalent bond is that which occurs between two hydrogen atoms. Atoms of hydrogen (H) have one valence electron in their first electron shell. Since the capacity of this shell is two electrons, each hydrogen atom will "want" to pick up a second electron. Hydrogen atoms will react with nearby hydrogen (H) atoms to form the compound H₂. Because the hydrogen compound is a combination of equally matched atoms, the atoms will share each other's single electron, forming one covalent bond.

·      Unlike ionic compounds, covalent molecules exist as true molecules. Because electrons are shared in covalent molecules, no full ionic charges are formed.  Thus covalent molecules are not strongly attracted to one another.  As a result, covalent molecules move about freely and tend to exist as liquids or gases at room temperature.  

·      For every pair of electrons shared between two atoms, a single covalent bond is formed. When two oxygen atoms form the compound O₂, they share two pairs of electrons, (4 electrons) forming two covalent bonds.   

·      Lewis structures can also be used to show bonding between atoms. Each dash represents one pair ( 2 electrons) of electrons, or one bond

·         Elements which are close together in electron negativity tend to form covalent bonds and can exist as stable free molecules. Carbon dioxide is a common example.

·          Polar and non-polar covalent bonding: two subtypes of covalent bonds.

·      Non-polar Covalent bonding:

1.      The H₂ molecule is a good example of the first type of covalent bond, the non-polar bond; because both atoms in the H₂ molecule have an equal attraction (or affinity) for electrons.

2.       The bonding electrons are equally shared by the two atoms, and a non-polar covalent bond is formed.

3.      Whenever two atoms of the same element bond together, a non-polar bond is formed.

·     Polar Covalent bonding:

1.      A polar bond is formed when electrons are unequally shared between two atoms.

2.      Polar covalent bonding occurs because one atom has a stronger affinity for electrons than the other (yet not enough to pull the electrons away completely and form an ion). In a polar covalent bond, the bonding electrons will spend a greater amount of time around the atom that has the stronger affinity for electrons.

3.       A good example of a polar covalent bond is the hydrogen-oxygen bond in the water molecule.