Answer:
The octet rule is a guideline that suggests that atoms tend to combine in a way that allows each atom to have eight electrons in its outermost energy level (except for hydrogen, which is stable with two electrons). However, there are some molecules that do not obey the octet rule. Here are two common types of exceptions and examples of molecules that fall into each category:
Incomplete Octet: In this type of exception, the atoms in the molecule do not have a complete octet of valence electrons. Examples of molecules that have incomplete octets include beryllium chloride (BeCl2) and boron trifluoride (BF3).
In beryllium chloride, beryllium has only four valence electrons, while chlorine has seven. When the two atoms combine, beryllium shares its electrons with two chlorine atoms, but it still has only four electrons around it, which is fewer than the octet rule suggests. In boron trifluoride, boron has only three valence electrons, while fluorine has seven. When the two atoms combine, boron shares its electrons with three fluorine atoms, but it still has only six electrons around it, which is also fewer than the octet rule suggests.
Expanded Octet: In this type of exception, the atoms in the molecule have more than eight valence electrons. Examples of molecules that have expanded octets include sulfur hexafluoride (SF6) and phosphorus pentachloride (PCl5).
In sulfur hexafluoride, sulfur has six valence electrons, while each of the six fluorine atoms has seven valence electrons. When the atoms combine, sulfur shares its electrons with all six fluorine atoms, resulting in a total of 12 electrons around the sulfur atom, which is more than the octet rule suggests. In phosphorus pentachloride, phosphorus has five valence electrons, while each of the five chlorine atoms has seven valence electrons. When the atoms combine, phosphorus shares its electrons with all five chlorine atoms, resulting in a total of 10 electrons around the phosphorus atom, which is also more than the octet rule suggests.
In both cases, the atoms in these molecules are able to deviate from the octet rule due to the availability of empty d orbitals in the central atom that can accommodate additional electrons beyond the octet. Additionally, the size and electronegativity of the atoms involved in the bonding also play a role in determining whether the molecule will obey the octet rule or not.