General Assumptions while writing Lewis Structures
Step 1: Decide which atoms are bonded.
Step 2: Count all valence electrons
Step 3: Place two electrons in each bond.
Step 4: Complete the octets of the atoms attached to the central atom by adding e-‘s in pairs.
Step 5: Place any remaining electrons on the central atom in pairs.
Step 6: If the central atom does not have an octet, form double bonds. If necessary form triple bonds.
Some examples that may be interesting to solve.
1. What is the probable skeletal structure of sulphuric acid, H2SO4?
2. Predict reasonable skeletal structures for NO3-, HClO3 and H3PO4.
3. How many dots, representing valence electrons, must appear in the Lewis structures of SO3, NO3-, and NH4+?
4. How many dots should appear in the Lewis structures of SO2, PO43-, and NO+?
5. Write the Lewis structure for SO42-
6. What is the Lewis structure for the ClO2- ion?
7. What is the Lewis structure for the carbon monoxide molecule, CO?
8. What is the Lewis structure for SF4?
9. Draw Lewis structures for OF2, ClF3 and HClO4
When the Octet Rule Fails
Sometimes it is just impossible to write a Lewis structure in which all of the atoms in a molecule obey the octet rule. This happens most often when an atom forms more than four bonds. Examples are PCl5 and SF6, in which there are five P-Cl bonds and six S-F bonds, respectively. Since each covalent bond requires the sharing of a pair of electrons, P and S must exceed eight electrons in their outer shells. The Lewis formula of these two molecules are shown below.
Elements in period 2 such as carbon or nitrogen, never exceed an octet simply because their valence shell, having n=2, can hold a maximum of only 8 electrons. Elements in periods below period 2, however, sometimes do exceed an octet, because their valence shells can hold more than 8 electrons. For example, the valence shell for elements in period 3, for which n=3, can hold a maximum of 18 electrons, and the valence shell for period 4 elements can hold as many as 32 electrons.
In some molecules (but not many), an atom has less than an octet. The most common examples are compounds of beryllium and boron.