trong>40. Although many simple molecules fulfill the octet rule, some common molecules are exceptions to this rule. Give three examples of molecules whose Lewis structures are exceptions to the octet rule.

Interpretation:

Three examples of molecules whose Lewis structures are exceptions to the octet rule should be given.

Concept Introduction:

Most of the elements having atomic number below 20 follow the octet rule but there are several exceptions for compounds including:

• Lithium, beryllium, boron and aluminum.
• Compounds formed from elements from main group in the third period and below deviate from the octet rule.
• Molecules with an odd number of electrons disobey the octet rule.

In writing the Lewis structure for BCl₃ ; sum of valence electrons is 24. Boron has 3 valence electrons and Chlorine has 7 valence electrons, hence 21 electrons from the three chlorine atoms. Using a pair of electrons per bond, we draw 3 B-Cl bonds, using a line to indicate each pair of bonding electrons. So 6 electrons are used for the bonds. Remaining 18 electrons are arranged around the atoms to achieve a noble gas electron configuration for each atom. Chlorine needs eight electrons to have a noble gas electron configuration. Each chlorine atom already has 2 electrons from the three bonds, so chlorine atom needs only 6 other electrons to complete the octet. So we draw three nonbonding electron pairs on each chlorine atom. Boron also should have eight electrons to have a noble gas configuration. But around boron octet is incomplete. This is an exception for octet rule. So BCl₃ is electron deficient compound and tend to make compounds with electrons rich compounds.

Lewis structure for BCl₃ :

In writing the Lewis structure for NO; sum of valence electrons is 11. Nitrogen has 5 valence electrons and oxygen has 6 valence electrons. Using a pair of electrons per bond, we draw a N-O bond, using a line to indicate each pair of bonding electrons. So 2 electrons are used in bonding. Remaining 9 electrons are arranged around the atoms to achieve a noble gas electron configuration for each atom. Oxygen needs eight electrons to have a noble gas electron configuration. Oxygen atom already has 2 electrons from the bond, so oxygen atom needs only 6 other electrons to complete the octet