Question

After thoroughly reviewing the octet rule, respond to the following: Discuss the octet rule and why there are exceptions to the rule. Give examples of the rule and of exceptions written in 250 words minimum

Answer 1

The octet rule is a chemical rule of thumb that reflects observation that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.

The valence electrons can be counted using a Lewis electron dot diagram as shown at the right for carbon dioxide. The electrons shared by the two atoms in a covalent bond are counted twice, once for each atom. In carbon dioxide each oxygen shares four electrons with the central carbon, two (shown in red) from the oxygen itself and two (shown in black) from the carbon. All these four electrons are counted in both the carbon octet and the oxygen octet.

Exceptions

• Incomplete valence shell:
  • Seven electron species are free radicals with unpaired electrons. For example, the methyl radical (CH₃) has an unpaired electron in a non-bonding orbital on the carbon atom, and no electron of opposite spin in the same orbital. Another example is the chlorine radical produced by CFCs, known to be harmful to the ozone layer.
  • Six electron species are highly reactive and short lived. An example is the carbenes, which have two unshared valence electrons on the same carbon atom in a triplet or singlet state. Another example is BH₃, which dimerizes into diborane (B₂H₆) to achieve stability.
• Other rules:
  • The duet rule of the first shell—the noble gas helium has two electrons in its outer shell, which is very stable. (Since there is no 1p subshell, 1s is followed immediately by 2s, and thus shell 1 can only have at most 2 valence electrons). Hydrogen only needs one additional electron to attain this stable configuration, while lithium needs to lose one.
  • For transition metals, some classes of molecule tend to obey the 18-electron rule which corresponds to the utilization of valence-shell s, p and d orbitals to form bonding and non-bonding orbitals. However newer theoretical treatments support a model with the duodectet rule (12 electrons) using mainly the d and s valence orbitals.^[4] The exclusion of p orbitals for transition metal atoms is analogous to the exclusion of d orbitals for hypervalent molecules of main group atoms.

Hypervalent molecules

Main-group elements in the third and later rows of the periodic table can form hypercoordinate or hypervalent molecules in which the central main-group atom is bonded to more than four other atoms, such as phosphorus pentachloride, PCl₅, and sulfur hexafluoride, SF₆. For example in PCl₅, if it is supposed that there are five true covalent bonds in which five distinct electron pairs are shared, then the phosphorus would be surrounded by 10 valence electrons in violation of the octet rule. In the early days of quantum mechanics, Pauling proposed that third-row atoms can form five bonds by using one s, three p and one d orbitals, or six bonds by using one s, three p and two d orbitals. To form five bonds, the one s, three p and one d orbitals combine to form five sp³d hybrid orbitals which each share an electron pair with a halogen atom, for a total of 10 shared electrons, two more than the octet rule predicts. Similarly to form six bonds, the six sp³d² hybrid orbitals form six bonds with 12 shared electrons. In this model the availability of empty d orbitals is used to explain the fact that third-row atoms such as phosphorus and sulfur can form more than four covalent bonds, whereas second-row atoms such as nitrogen and oxygen are strictly limited by the octet rule.