Why does boron violate octet rule

Hydrogen , beryllium, and boron have too few electrons to form an octet. Hydrogen has only one valence electron and only one place to form a bond with another atom. Beryllium has only two valence atoms , and can form only electron pair bonds in two locations. Boron has three valence electrons. 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. Odd-electron molecules A molecule with an odd number of electrons in the valence shell of an atom. Although they are few, some stable compounds have an odd number of electrons in their valence shells. With an odd number of electrons, at least one atom in the molecule will have to violate the octet rule.

The Lewis electron dot diagram for NO is as follows:. Although the O atom has an octet of electrons, the N atom has only seven electrons in its valence shell.

Although NO is a stable compound, it is very chemically reactive, as are most other odd-electron compounds. Electron-deficient molecules A molecule with less than eight electrons in the valence shell of an atom. These stable compounds have less than eight electrons around an atom in the molecule. The most common examples are the covalent compounds of beryllium and boron. For example, beryllium can form two covalent bonds, resulting in only four electrons in its valence shell:.

Consider aluminum chloride, which has 24 valence electrons. While all chlorine atoms reach the octet, aluminum gets only 6 valence electrons — an incomplete octet.

Although aluminum chloride is stable, it reacts with molecules like ammonia that have an unshared pair of electrons. The nitrogen in ammonia donates its lone pair to aluminum, forming a special bond called a coordinate covalent or dative bond. The third exception is elements that can accommodate more than 8 valence electrons or an expanded octet. These elements are located in the third row of the periodic table and below.

Elements, such as phosphorus, sulfur, or iodine, have access to d orbitals, allowing them to accommodate more than 8 valence electrons — often up to 12 or Consider the tetrachloroiodide anion, which has 36 valence electrons. Even after assigning the bonding electron pairs and satisfying the octet for all atoms, 4 valence electrons remain unassigned.

These electrons are placed on the central iodine atom, yielding an expanded octet with 12 electrons. Molecules with more than 8 valence electrons around the central atom are called hypervalent. Remember, elements from the second row of the periodic table, such as carbon or oxygen, have only s and p orbitals and never form hypervalent compounds because collectively they can only hold up to 8 valence electrons. Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures.

These molecules fall into three categories:. Molecules that contain an odd number of electrons are called radicals. Nitric oxide, NO, is an example of an odd-electron molecule; it is produced in internal combustion engines when oxygen and nitrogen react at high temperatures.

To draw the Lewis structure for an odd-electron molecule like NO, the following steps are considered:. Some molecules, however, contain central atoms that do not have a filled valence shell. Generally, these are molecules with central atoms from groups 2 and 13, outer atoms that are hydrogen, or other atoms that do not form multiple bonds.

For example, in the Lewis structures of beryllium dihydride, BeH 2 , and boron trifluoride, BF 3 , the beryllium and boron atoms each have only four and six electrons, respectively.

It is possible to draw a structure with a double bond between a boron atom and a fluorine atom in BF 3 , satisfying the octet rule, but experimental evidence indicates the bond lengths are closer to that expected for B—F single bonds. This suggests the best Lewis structure has three B—F single bonds and electron-deficient boron. The reactivity of the compound is also consistent with electron-deficient boron. However, the B—F bonds are slightly shorter than what is actually expected for B—F single bonds, indicating that some double-bond character is found in the actual molecule.

An atom like the boron atom in BF 3 , which does not have eight electrons, is very reactive. It readily combines with a molecule containing an atom with a lone pair of electrons. Thanks a lot!

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