exceptions to the octet rule

Examples of exceptions to the octet rule in the following topics:

• Formal Charge and Lewis Structure

  • Generally, most Lewis structures follow the octet rule; they will share electrons until they achieve 8 electrons in their outermost valence shell.
  • However, there are exceptions to the octet rule, such as boron, which is stable with only 6 electrons in its valence shell.
  • The elements hydrogen (H) and helium (He) follow the duet rule, which says their outermost valence shell is full with 2 electrons in it.
  • However, atoms can share electrons with each other to fulfill this octet requirement.
  • If the octet rule is still not satisfied, atoms may form a double (4 shared electrons) or triple bond (6 shared electrons).

• Introduction to Lewis Structures for Covalent Molecules

  • We refer to this chemical tendency of atoms as 'the octet rule,' and it guides us in predicting how atoms combine to form molecules and compounds.
  • These are exceptions to the octet rule because they only require 2 electrons to have a full valence level.
  • It therefore has 7 valence electrons and only needs 1 more in order to have an octet.
  • In order to achieve an octet for all three atoms in CO2, two pairs of electrons must be shared between the carbon and each oxygen.
  • You can see that this is how the octet rule is satisfied for all atoms in this case.

• The Incomplete Octet

  • While most elements below atomic number 20 follow the octet rule, several exceptions exist, including compounds of boron and aluminum.
  • The rule is applicable to the main-group elements, especially carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium and magnesium.
  • These atoms each have three valence electrons, so we would predict that these atoms want to bond covalently in order to gain 5 electrons (through sharing) to fulfill the octet rule.
  • Although the octet rule can still be of some utility in understanding the chemistry of boron and aluminum, the compounds of these elements are harder to predict than for other elements.
  • Describe the ways that B, Al, Li, and H deviate from the octet rule

• The Expanded Octet

  • Main group elements in the third period and below form compounds that deviate from the octet rule by having more than 8 valence electrons.
  • As a result, the second period elements (more specifically, the nonmetals C, N, O, F) obey the octet rule without exceptions.
  • However, some of the third-period elements (Si, P, S, and Cl) have been observed to bond to more than four other atoms, and thus need to involve more than the four pairs of electrons available in an s2p6 octet.
  • For atoms in the fourth period and beyond, higher d orbitals can be used to accommodate additional shared pairs beyond the octet.
  • In the PCl5 molecule, the central phosphorus atom is bonded to five Cl atoms, thus having 10 bonding electrons and violating the octet rule.

• Odd-Electron Molecules

  • Molecules with an odd number of electrons disobey the octet rule.
  • As the octet rule requires eight electrons around each atom, a molecule with an odd number of electrons must disobey the octet rule.
  • The two oxygen atoms in this molecule follow the octet rule.
  • Nitrogen dioxide is another stable molecule that disobeys the octet rule.
  • It does not obey the octet rule on the nitrogen atom.

• Physical Properties of Covalent Molecules

  • These cases of electron sharing can be predicted by the octet rule.
  • The octet rule is a chemical rule that generalizes that atoms of low atomic number (< 20) will combine in a way that results in their having 8 electrons in their valence shells.
  • In a covalent bond, the shared electrons contribute to each atom's octet and thus enhance the stability of the compound.
  • A H atom needs one additional electron to fill its valence level, and the halogens need one more electron to fill the octet in their valence levels.
  • Lewis bonding theory states that these atoms will share their valence electrons, effectively allowing each atom to create its own octet.

• Ionic Bonds

  • By satisfying the duet rule or the octet rule, ions are more stable.
  • Both ions form because the ion is more stable than the atom due to the octet rule.
  • This is because Mg has two valence electrons and it would like to get rid of those two ions to obey the octet rule.
  • Fluorine has seven valence electrons and usually forms the F - ion because it gains one electron to satisfy the octet rule.
  • This is because Mg has two valence electrons and it would like to get rid of those two ions to obey the octet rule.

• Formulas of Ionic Compounds

  • An ionic formula must satisfy the octet rule for the constituent atoms and electric neutrality for the whole compound.
  • The transfer of electrons allows the atoms to effectively achieve the much more stable electron configuration of having eight electrons in the outermost valence shell (octet rule).
  • First, the charge on the constituent ions can be determined based on the transfer of valence electrons necessary in order to satisfy the octet rule.
  • To create a neutral compound, CaCl2, two 1- chloride ions were needed to balance out the 2+ charge from calcium.
  • Two hydroxides were needed to balance off the +2 charge of calcium.

• Covalent Bonds

  • If it shares one electron with a carbon atom (which has four valence electrons), the fluorine will have a full octet (its seven electrons plus the one it is sharing with carbon).
  • Carbon will have to form four single bonds with four different fluorine atoms to fill its octet.
  • The formation of a covalent bond allows the nonmetals to obey the octet rule and thus become more stable.
  • Carbon will have to form four single bonds with four different fluorine atoms to fill its octet.
  • The overlapped orbitals allow the shared electrons to move freely between atoms.

• Electron Shells and the Bohr Model

  • For example, 1n represents the first energy level located closest to the nucleus.
  • This is known as the octet rule which states that, with the exception of the innermost shell, atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell.
  • Theoretically, they would be more energetically stable if they followed the octet rule and had eight.
  • An electron normally exists in the lowest energy shell available, which is the one closest to the nucleus.
  • Energy from a photon of light can bump it up to a higher energy shell, but this situation is unstable and the electron quickly decays back to the ground state.