Which elements defy the octet rule

However, the stability of aluminum hydride ions AlH 4 — indicates that Al can also support an octet of valence shell electrons. 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. Some elements, most notably nitrogen, can form compounds that do not obey the octet rule.

One class of such compounds are those that have an odd number of electrons. As the octet rule requires eight electrons around each atom, a molecule with an odd number of electrons must disobey the octet rule. Recall that the Lewis structure of a molecule must depict the total number of valence electrons from all the atoms which are bonded together. Nitric oxide has the formula NO. Therefore, no matter how electrons are shared between the nitrogen and oxygen atoms, there is no way for nitrogen to have an octet.

It will have seven electrons, assuming that the oxygen atom does satisfy the octet. Nitric oxide : Nitric oxide NO is an example of a stable free radical. It does not obey the octet rule on the nitrogen atom.

Each line around the atoms represents a pair of electrons. Nitric oxide is a by-product of combustion reactions that occur in engines, like those in automobile engines and fossil fuel power plants. It is also produced naturally during the electrical discharge of lightning during thunderstorms. Nitrogen dioxide is the chemical compound with the formula NO 2. Again, nitrogen dioxide does not follow the octet rule for one of its atoms, namely nitrogen.

There is persistent radical character on nitrogen because it has an unpaired electron. The two oxygen atoms in this molecule follow the octet rule. Nitrogen dioxide : Nitrogen dioxide is another stable molecule that disobeys the octet rule. Note the seven electrons around nitrogen. Nitrogen dioxide is an intermediate in the industrial synthesis of nitric acid, millions of tons of which is produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent air pollutant.

Main group elements in the third period and below form compounds that deviate from the octet rule by having more than 8 valence electrons. A hypervalent molecule is a molecule that contains one or more main group elements that bear more than eight electrons in their valence levels as a result of bonding.

As a result, the second period elements more specifically, the nonmetals C, N, O, F obey the octet rule without exceptions. Phosphorus pentachloride : In the PCl 5 molecule, the central phosphorus atom is bonded to five Cl atoms, thus having 10 bonding electrons and violating the octet rule.

The overall geometry of the molecule is depicted trigonal bipyramidal , and bond angles and lengths are highlighted. 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 s 2 p 6 octet.

Although the energy of empty 3d-orbitals is ordinarily higher than that of the 4s orbital, that difference is small and the additional d orbitals can accommodate more electrons. An electron that contains a complete outermost shell or half-filled is said to be more stable whereas electrons less than half-filled are unstable and can easily lose their electrons.

Noble gases are said to be highly stable elements. Those elements which follow octet rule are said to be more stable. Octet Rule and Valence Electron. The octet rule states that the elements which can lose, gain, or share electrons from its outermost shell to complete the valence shell with a set of eight electrons. Valence electrons mean the total number of electrons present in the outermost shell of an element that can participate in the bond formation. Such electrons are considered as the valence electron of that particular element.

For Example:. Let us take an example of NaCl:. Sodium has one electron in its outermost shell. So we can say that the valence electron of Na is 1. Whereas Cl has seven electrons in its outermost shell. So we can say the Valence electron for Cl is 7. When both sodium and chlorine combine and share their electron and have eight electrons in their outermost shell then it is said that the compound formed follows the octet rule.

The two molecules depicted in this picture show the central beryllium and boron atoms with fewer than eight valence electrons. Molecules, where some atoms have fewer than eight electrons, are called electron deficient.

Elements in periods greater than period 3 on the periodic table have a d orbital available with the same energy quantum number. Atoms in these periods may follow the octet rule , but there are conditions where they can expand their valence shells to accommodate more than eight electrons.

Sulfur and phosphorus are common examples of this behavior. Sulfur can follow the octet rule as in the molecule SF 2. Each atom is surrounded by eight electrons.

It is possible to excite the sulfur atom sufficiently to push valence atoms into the d orbital to allow molecules such as SF 4 and SF 6. The sulfur atom in SF 4 has 10 valence electrons and 12 valence electrons in SF 6. Most stable molecules and complex ions contain pairs of electrons. There is a class of compounds where the valence electrons contain an odd number of electrons in the valence shell. These molecules are known as free radicals. Free radicals contain at least one unpaired electron in their valence shell.

In general, molecules with an odd number of electrons tend to be free radicals. Nitrogen IV oxide NO 2 is a well-known example. Note the lone electron on the nitrogen atom in the Lewis structure. Oxygen is another interesting example. Molecular oxygen molecules can have two single unpaired electrons. Compounds like these are known as biradicals.