Answer – Some molecules that are exceptions to the octet rule are nitric oxide (NO), nitrogen dioxide (NO2), boron trifluoride (BF3), titanium tetrachloride (TiCl4), phosphorus pentafluoride (PF5), and sulfur hexafluoride (SF6).
Explanation:
The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight electrons in their outermost shell. However, there are three exception categories to this rule:
Odd-electron molecules include molecules having an odd number of valence electrons. For example, in a nitric oxide (NO) molecule, nitrogen has 5 valence electrons and oxygen has 6. Nitrogen forms a bond with oxygen by sharing one of its valence electrons with oxygen. Now, the total number of valence electrons is 11 (an odd number) and cannot result in a stable octet. NO2 is also another example (nitrogen has seven electrons in its outer shell, deviating from the octet rule) and it has 17 valence electrons (5 from nitrogen and 12 from 2 oxygen molecules). Each oxygen atom shares one electron with the nitrogen atom, resulting in two covalent bonds.
Electron-deficient molecules represent the second exception to the octet rule. These stable compounds have less than eight electrons around an atom in the molecule. The most common examples are covalent compounds of beryllium and boron. In a molecule of boron trifluoride (BF3), boron commonly forms three covalent bonds, resulting in six valence electrons around the B atom, as seen in the diagram below.
The phosphorus pentafluoride (PF5) molecule is an exception to the octet rule, as it accommodates 10 electrons in its empty d orbitals. Image credit: OpenStax / Wikimedia Commons (licensed under CC BY-SA 4.0)
Molecules that can have more than eight valence electrons (expanded octets) are the third exception. These molecules are present in the third row and beyond the periodic table; they also have empty d orbitals in their valence shell. Therefore, they can have more than eight electrons in their ultimate shell. In phosphorus pentafluoride (PF5), phosphorus can have ten electrons in its valence shell. This is because phosphorus (P) has 5 valence electrons, and each fluorine atom contributes one electron to a bond, thus resulting in a total of 10 valence electrons. P can accommodate 10 electrons because it uses its empty 3d orbitals, forming five covalent bonds with the fluorine atom.
The phosphorus pentafluoride (PF5) molecule is an exception to the octet rule, as it accommodates 10 electrons in its empty d orbitals. Image credit: OpenStax / Wikimedia Commons (licensed under CC BY-SA 4.0)
Similarly, sulfur in sulfur hexafluoride (SF6) forms six covalent bonds with fluorine atoms, resulting in a total of 12 electrons around the sulfur atom, which exceeds the octet rule. In titanium tetrachloride (TiCl4), titanium has four valence electrons, and it forms four single bonds with chlorine atoms. This results in a total of 10 electrons around the titanium atom, exceeding the octet rule.
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