Lewis Dot Structures

Tutorial and Videos on how to determine Bond Type
Tutorial on Chemical Bonds

Music video: What Kind of Bonds Are These?

How do we determine the type of bonds (single, double, or triple) that hold a covalent molecule together.  We use the Lewis dot method of constructing the molecule.  The Lewis method is only used to determine structures of covalently bonded molecules.  Ionic cpds create large crystalline structure referred to as lattice structures and do not involve the sharing of the valence electrons. 

Lewis Dot Method of Determining  Covalent  Bonds
Remember covalent bonds involve atoms sharing their outer electrons.  This usually occurs when the atoms difference in electronegativity is small (less than 1.9).  The Lewis method is based on the Octet rule which assumes that all elements are seeking a noble gas core configuration which typically involves having 8 electrons in their outer valence shell. 

For Covalently bonded atoms you must assume that there is at least  1 covalent bond holding each of atoms together.  So therefore you must……

1.  Attach all atoms with at least l  single (sigma) bond. 

Try and make the molecule as symmetrical as possible.

2. Next draw 8 electrons around each atom.

Except H which does not need 8 to be stable,  only 2 e-

3. Count the electrons drawn in your diagram.

4. Count the number of valence electrons actually possessed by the atoms (Use periodic table and only count the S/P electrons in the outer shell.)

         a)  add  1 for each additional negative charge

         b)  subtract 1 for each additional positive charge

5.       For every 2 e- extra, cross of 4 adjacent electrons and add a 2nd bond (pi bond).

First bond is the sigma, each additional bond is called a pi bond

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