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C. Exception of the octet rule
Based on the octet rule, it is possible to predict the molecular formulas of covalently
bonded compounds. In this case, the number of paired electrons must be equal.
However, it should be remembered that the octet rule is not always obeyed, there
are some covalent compounds that violate the octet rule.
a. Compounds that do not reach the octet rule
Simple binary covalent compounds from beryllium (Be), Boron (B) and
Aluminum, namely elements with less than four valence electrons do not reach
the octet rule. For examples, BeCl2 and BeCl3.
b. Compounds with an odd number of valence electrons
Compounds with an odd number of valence electrons cannot obey the octet
rule. For example, NO2 which has a total of valence electrons (5 + 6 + 6) = 17.
c. Compounds with expanded octets
Elements from period three or more can form compounds that go beyond the
octet rule (more than 8 electrons in the outermost shell). This can happen
because the outer shell can have 18 electrons. Some examples are PCl5, SF6,
ClF3, IF7.
If you are still having trouble of understanding, here is a video visualization of
the formation process of covalent bonds.
Covalent Compound| 11
