expanded octet

Examples of expanded octet in the following topics:

• 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.
  • Therefore, the d orbitals participate in bonding with other atoms and an expanded octet is produced.
  • Examples of molecules in which a third period central atom contains an expanded octet are the phosphorus pentahalides and sulfur hexafluoride.
  • For atoms in the fourth period and beyond, higher d orbitals can be used to accommodate additional shared pairs beyond the octet.

• Table of Geometries

  • If the central atom possesses partially occupied d-orbitals, it may be able to accommodate five or six electron pairs, forming what is sometimes called an "expanded octet."

• Oxidation States of Sulfur Compounds

  • If you restrict your formulas to valence shell electron octets, most of the higher oxidation states will have formal charge separation, as in equation 2 above.
  • The formulas written here neutralize this charge separation by double bonding that expands the valence octet of sulfur.
  • In this way sulfur may expand an argon-like valence shell octet by two (e.g. sulfoxides) or four (e.g. sulfones) electrons.

• Oxidation States of Phosphorus Compounds

  • In this way phosphorus may expand an argon-like valence shell octet by two electrons (e.g. phosphine oxides).

• Chemical Bonding & Valence

  • In the other examples carbon, oxygen and fluorine achieve neon-like valence octets by a similar sharing of electron pairs.
  • Boron compounds such as BH3 and BF3 are exceptional in that conventional covalent bonding does not expand the valence shell occupancy of boron to an octet.
  • The number of valence shell electrons an atom must gain or lose to achieve a valence octet is called valence.

• The Incomplete Octet

  • These four electrons are counted in both the carbon octet and the oxygen octet because they are shared.
  • However, many atoms below atomic number 20 often form compounds that do not follow the octet rule.
  • There are also a variety of molecules in which there are too few electrons to provide an octet for every atom.
  • This covalent compound (NH3BF3) shows that boron can have an octet of electrons in its valence level.
  • Describe the ways that B, Al, Li, and H deviate from 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

  • The octet rule states that an atom is most stable when there are eight electrons in its valence shell.
  • 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.
  • This is because Mg has two valence electrons and it would like to get rid of those two ions to obey the octet rule.

• 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.
  • 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.