How do I practice drawing resonance structures?

Several different methods exist for constructing Lewis dot structures. These instructions outline the Kelter strategy to draw Lewis structures for molecules.

1. Calculate the number of valence electrons

   Add up the total number of valence electrons from all the atoms in the molecule.

2. Determine the number of octet electrons needed to fill the atom's shell

   All elements up to period four on the periodic table need eight electrons to fill their outer electron shell. This property is often known as the "octet rule." The exceptions to this rule are hydrogen, which needs only two electrons in its outer shell, and boron, which requires six.

3. Determine the number of covalent bonds in the molecule

   Subtract the number of valence electrons in the molecule from the number of octet electrons (usually eight). The answer is the total number of electrons needed to fill the outer electron shells of all the atoms in the molecule. Divide the answer by two, and the result is the number of covalent bonds in the molecule. Each bond uses two electrons, so the number of bonds equals half the number of required electrons.

4. Choose a central atom

   The central atom of a molecule is usually the least electronegative atom or the atom with the highest valence. Hydrogen and halogen atoms tend to appear on the outside of the molecule and are rarely the central atom.

5. Draw a skeletal structure

   Connect each surrounding atom to the central atom using a straight line. The line represents a bond between the two atoms. The central atom can connect to a total of four other atoms.

6. Place electrons around outside atoms

   Draw electrons to complete the octets around each of the outer atoms. If there are not enough electrons to complete the octets, the skeletal structure is incorrect. Try a different arrangement.

7. Place remaining electrons around the central atom

   Complete the octet for the central atom by drawing in the remaining electrons. If there are unused covalent bonds, create double bonds with lone pairs on the outside atoms. If there are more than eight electrons on the central atom and the atom is not one of the exceptions to the octet rule, the number of valence atoms may have been calculated incorrectly. This completes the Lewis dot structure.

8. Practice Resonance

   Move shared bonds onto other atoms, and turn lone pairs into shared bonds. Always keep consistency with each atom's individual charge. This exercise provides practice with the concept of Lewis dot structures and their resonance structures.