Covalent Bond

Covalent bond is another type of bond that involves the sharing of valence electrons between nonmetal atoms forming a molecule.  Covalent compounds are the compounds that contains only covalent bonds. Examples of this are the diatomic molecules:  H₂, O₂, N₂.

To show the sharing of electrons between atoms in a molecule lewis structure must be written.  A Lewis structure is a representation of covalent bonding in which shared electron pairs are shown either as line or pair of dots, and lone pairs are shown as pair of dots on the individual atoms.  A line is used when electron pair is shared between atoms and dots are the unshared pair electrons.

In writing the Lewis structure, octet rule must also be followed and with a few exemptions.

Lewis Structure

There are rules to be followed when writing Lewis structure:
1.  Add the total valence electrons from all atoms.  For polyatomic anions, add the negative charges to the total number of valence electrons and for the polyatomic cations, subtract the positive charges from the total number of valence electrons.

2.  Write the symbols of all the elements that comprise the molecule, arranging them in such a way that the central atom is the least electronegative atom or the atom with the least number of atoms.  Example for the molecule CF₄, C is the central atom having only one atom and F  are the bonded atoms having 4 F atoms, F also is the most electronegative atom.

3.  Draw a single covalent bond between the central atom and the surrounding atoms.  Complete the 8 valence electrons of the bonded atoms and the excess valence electrons will be placed to the central atom and will represent the non-bonding electrons.

4.  After completing step 1 - 3, if the central atom did not complete 8 number of valence electrons, try adding double bond or triple bond between the surrounding atoms and the central atoms.


Examples 1

Write the Lewis structure of the NF₃
Step 1.  Add the total number of valence electrons.  Nitrogen is located in group VA therefore has 5 valence electrons and Flourine is in group VIIA , 7 valence electrons.

Total valence electrons:   5 + 3(7) = 5  +  21  = 26 valence electrons

Step 2.  Arranging the atoms. the least electronegative atom is the central atom (N)  and  F are the bonded atoms.

Step 3. Draw a single covalent bond between the central atom and the surrounding atoms. Complete the 8 valence electrons of the bonded atoms and the excess valence electrons will be placed to the central atom and will represent the non-bonding electrons.

Step 4.  No more step 4 since all the atoms in the compound achieved stability by having eight valence electrons.

Example 2. 

 Write the Lewis structure of the carbonate ion, CO₃^-2

Step 1.  Calculate the total valence electrons.  C has 4 valence electrons (IVA) and O has 6 valence electrons (VIA).

Total valence electrons:   4 +  3 (6)  +  2 = 4  +  18  +  2  =  24 valence electrons
                                       

Step 2.  Arranging the atoms

Step 3.  Draw a single covalent bond between the central atom and the surrounding atoms. Complete the 8 valence electrons of the bonded atoms and the excess valence electrons will be placed to the central atom and will represent the non-bonding electrons.

Step 4.  Since the central atom carbon is not yet stable, the two electrons from either of the oxygen can be made into double bond so the eight valence electrons in carbon will be meet. As shown below:

Example 3.

Draw the lewis structure of  F₂ gas.

Step 1.  Calculate the total valence electrons of F_2.   2(7) =  14 valence electrons

Step 2.  Arranging the atoms.  Since there are only 2 electrons, you can arrange them in linear form.

Step 3.  Distribution of valence electrons, first a single bond will be placed in between the two fluorine atoms, the rest of electrons will be placed surrounding the F atom.

Step 4.  Since each F atom, already achieved 8 valence electrons the structure above is already the lewis structure.  If in case each atom will not achieve 8 valence electrons you can make double bond or triple bond between atoms.

TRY THIS:

Write the Lewis structure of the following:

1.  SF₆

2.  PO₃^-3

3. CCl₄

4.  CO₂

5.  H₂S