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Octet Rule


Compound formation is governed by the octet rule. The rule stems from the observations that most atoms tend to end up with an octet in their outermost shell during compound formation. whenever an atom gains, loses or shares electrons to achieve an octet in its outermost shell during compound formation, the arrangement of electrons in the resulting atom matches the configuration of a nonreactive noble gas atom.

Octet rule was introduced by Lewis and Kossel. The chemical compounds exist in nature had to follow the octet rule. A too active element is one whose drive to satisfy the octet rule by having a "full" outer ring is so strong that the pure element cannot just exist in nature or float about in the atmosphere without attempting to satisfy that drive and comply with the octet rule.

What is the Octet Rule?

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The tendency of atoms to react in ways that achieve an outer shell of eight valence electrons is particularly common among Group IA-7A elements and is given the special name of the octet rule. This rule explains how atoms form ions. The octet rule is a useful guide in understanding the chemical bonds but there are many elements to which it does not apply.

In 1916, Gilbert N.Lewis devised a beautifully simple model that unified many of the observations about chemical bonding and chemical reactions. He pointed out the lack of chemical reactivity of the noble gases indicates a high degree of stability of their electronic configurations.

Octet Rule Definition

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Definition of Octet Rule

The octet rule states that "atoms tend to gain, lose or share electrons so that their outermost shell holds eight electrons as in the noble gases." The octet rule is a general rule and we can explain the formation of most ionic and covalent compounds with the help of octet rule.

The octet rule explains that all atoms must strive to reach an ultimate state of atomic stability that is they must attempt to reach a state where the atoms are totally satisfied and need not enter into any chemical reactions to become stable. Elements having eight electrons in the outermost shell are said to have octet configuration, which is a stable configuration.

Explain the Octet Rule

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The arrangement of chemical elements in the periodic table is explained by the way in which electrons fill the available energy levels or shells in atoms. In particular the noble gases such as neon, xenon and argon have eight electrons in their outermost shell which is why they do not easily engage in chemical reactions. In energy terms a filled outermost electron shell is the lowest state most atoms can assume. This is the basis of octet rule.

Octet rule explains the formation of ions from atoms. Consider for example, sodium. It has 11 electrons, 2 in the innermost shell, 8 in the next and 1 in the outermost shell. The outermost electron is easy to remove, so that when energy is added to the sodium atom, a sodium ion with one positive charge is easily formed. It takes ten times as much energy to remove an electron from an inner shell.

The octet rule is one of those rules that were originally discovered as seemingly arbitrary regularities by chemists through experiment and observation, but which are easy to understand in terms of the atomic theory.

Octet Rule Examples

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The tendency of molecules an polyatomic ions to have structures in which eight electrons surround each atom is known as the octet rule. As an example, a triple bond is necessary in dinitrogen in order to have an octet around each nitrogen atom.

Boron Octet Rule

Consider the molecule boron trifluoride. Boron atom has six electrons around it. The octet rule for boron could be satisfied by drawing a structure with a double bond between born and one of the fluorine.

Boron Octet Rule

Sulfur Octet Rule

Sulfur another period 3 element, forms compounds in which it has 8, 10 and even 12 electrons in its valence shell. The sulfur atom in H2S has 8 electrons in its valence shell and obeys the octet rule. The sulfur atoms in SO2 and H2SO4 have 10 and 12 electrons respectively in their valence shells and are exceptions to the octet rule.

Sulfur Octet Rule

Nitrogen Octet Rule

Nitrogen obtains an octet when two nitrogen atoms are united by a triple bond. Nitrogen has five valence electrons and requires three bonds in order to achieve an octet of electrons. Nitrogen contains one pair of unshared or non bonding electrons called a lone pair.

Nitrogen Octet Rule

Bromine Octet Rule

A bromine atom contains seven electrons in its valence shell. A bromine molecule contains two bromine atoms bonded by a single covalent bond and with three lone pairs of electrons on each bromine atom. A bromide ion is a bromine atom that has gained one electron in its valence shell and has a complete octet and a charge of -1.

Bromine Octet Rule

Beryllium Octet Rule

Beryllium is an unusual element in that it does not obey the octet rule. Beryllium like hydrogen, boron and aluminum can have fewer than eight outer electrons.

Expanded Octet Rule

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Phosphorus trichloride is a volatile liquid which does not conduct electricity. This substance has the properties of a typical covalent compound and the octet rule predicts correctly that phosphorus and chlorine atoms bond together to form molecules with the formula PCl3.

PCl3 again reacts with chlorine to form PCl5. This substance has the mole ratio 1:5 and its properties suggests that they are covalent compounds. But in vapor state these molecules are completely separate.

The octet does not hold for the molecule PCl5. Five P-Cl single covalent bonds involve five shared pairs of electrons with the result that the valence shell of phosphorus contains a total of ten electrons. In this case phosphorus is said to have "expanded its octet" that is the valence shell contains more than eight electrons. Lewis structure can be drawn for PCl5 but the electronic structure does not correspond to that of a noble gas and that it does not follow the octet rule.

Exceptions to the Octet Rule

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Many molecules composed of atoms of the main group elements have electronic structures that satisfy the octet rule, but a number of them do not. A few molecules such as NO have an odd number of electrons and so cannot satisfy the octet rule. Other exceptions to the octet rule fall into two group
  • One a group of molecules with an atom having fewer than eight valence electrons around it and
  • Other a group of molecules with an atom having more than eight valence electrons around it.

Note too the following Octet Rule Exceptions.

  1. Hydrogen needs only two electrons to complete its valence shell, so it normally forms only one bond and does not have additional electrons associated with it.
  2. Boron and aluminum have only six electrons surrounding them when they are bonded to three atoms.
  3. Elements in the third period and higher can accommodate 10 or 12 electrons in their valence shell. Phosphorus and sulfur are the most common elements that may have more than 8 electrons surrounding them.

Octet Rule Violations

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Although octet rule is successful in explaining the valencies of large number of elements yet it fails to explain many of the structure. Some of the violations are
  1. Formation of compounds with electron deficient atoms
  2. Formation of super octet molecules like PCl5, SF6 and IF7
  3. Formation of compounds of xenon
  4. Inability to predict energy changes