Boron, octet rule exceptions

Arrange the remaining electrons as lone pairs or create double or triple bonds to satisfy the octet rule. Exceptions Hydrogen satisfies the duet (two) rule, and boron and aluminum satisfy the six-electron rule. 

The same principle applies for BeCl2, BeH2, BCl3 etc. Beryllium and boron compounds are exceptions to the octet rule. 

The Lewis dot structure for BF3 shows the central boron atom being surrounded by only six electrons, which violates the octet rule. This illustrates that the octet rule, while providing general guidelines, has exceptions. 

Exceptions Sometimes atoms break the octet rule. Molecules with such atoms include molecules with an odd number of electrons, molecules with an atom having less than an octet, and molecules with an atom having more than an octet. Compounds containing boron and beryllium may contain less than an octet. Molecules with an atom containing more than an octet must contain an atom from the third period or greater in the periodic table because only these a tours have vacant d orbitals available for hybridization. 

One last exception to the octet rule lies in the bonding of the atom boron. Boron prefers six electrons in its outermost principal energy level. This allows compounds containing boron to make three bonds in a trigonal planar arrangement. Two examples are BH3 and BF3 as shown in Figure 3.13. 

Most of the common elements in organic compounds—C, N, O, and the halogens— follow the octet rule. Hydrogen is a notable exception, because it accommodates only two electrons in bonding. Additional exceptions include boron and beryllium (second-row elements in groups 3A and 2A, respectively), and elements in the third row (particularly phosphorus and sulfur). 

Distribute the electron dots so that each atom, except for hydrogen, beryllium, and boron, satisfies the octet rule. 

Count the number of electrons surrounding each atom. Except for hydrogen, beryllium, and boron, all atoms must satisfy the octet rule. Check that the number of valence electrons is still the same number you determined in step 1. 

Suboctets and coordinate covalent bonds Another exception to the octet rule is due to a few compounds that form suboctets—stable configurations with fewer than eight electrons present around an atom. This group is relatively rare, and BH3 is an example. Boron, a group 3 nonmetal, forms three covalent bonds with other nonmetallic atoms. 

The other exceptions to this rule involve compounds of boron and beryllium, which form compounds like BeHj and BF3, in which there are four and six electrons, respectively, in their completed valence shells. However, the octet rule applies to all of the other elements in the first and second periods of the Periodic Table, on whose compounds we will now focus our attention. 

Numerous molecules contain only boron and hydrogen, a family of compounds called boranes. The simplest borane is BHg. This molecule contains only six valence electrons and is tiierefore an exception to tile octet rule. - (Section 8.7) As a result, BHg reacts witii itself to form dtboraiie (B2H6). This reaction can be viewed as a Lewis acid-base reaction (Section 16.11), in which one B—H bonding pair of electrons in each BHg molecule is donated to the other. As a result, diborane is an unusual molecule in which hydrogen atoms appear to form two bonds (Figure 22.55 ). 

There are some apparent exceptions to the octet rule neutral compounds of boron and aluminum can have only six valence electrons. 

The nitrogen atom does not have an octet, so this is not a great Lewis structure. However, NO exists in nature. Why As with any simple theory, Lewis theory is not sophisticated enough to be correct every time. It is impossible to write good Lewis structures for molecules with odd numbers of electrons, yet some of these molecules exist in nature. In such cases, we simply write the best Lewis structure that we can. Another significant exception to the octet rule is boron, which tends to form compounds with only six electrons around B, rather than eight. For example, BF3 and BH3—both of which exist in nature— lack an octet for B. 

The other group of exceptions to the octet rule consists mostly of molecules containing Group IIA or IIIA atoms. Consider boron trifluoride, BF3. The molecule consists of boron surrounded by the much more electronegative fluorine atoms. The total number of valence electrons is 3 + (3 X 7) = 24. If you connect boron and fluorine atoms by electron pairs and fill out the fluorine atoms with octets of electrons, you obtain ... 

Acceptable. Boron is an exception to the octet rule and requires only three bonds. 

This molecule is an exception to the octet rule because in this case B forms only three bonds. Boron trichloride is an AB3 type of molecule. Therefore, according to VSEPR theory, it should have trigonal-planar geometry. 

While the octet rule is useful for bonding in many compounds, there are exceptions. We have already seen that a hydrogen (H2) molecule requires just two electrons or a single bond. Usually the nomnetals form octets. However, in BCI3, the B atom has only three valence electrons to share. Boron compounds typically have six valence electrons on the central B atoms and form just three bonds. While we will generally see compounds of P, S, Cl, Br, and I with octets, they can form molecules in which they share more of then-valence electrons. This expands their valence electrons to 10, 12, or even 14 electrons. 

Another significant exception to the octet rule involves those elements that tend to form incomplete octets. The most important of these is boron, which forms compounds with only six electrons around B, rather than eight. For example, BF3 and BFI3 lack an octet for B. 

Other exceptions to the octet rule include molecules with incomplete octets—usually totaling 6 electrons (especially important in compounds containing boron)—and molecules with expanded octets—usually 10 or 12 electrons (which can occur in compounds containing elements from the third row of the periodic table and below). Expanded octets never occur in second-period elements. 

The idea that covalent bonding can be predicted by achieving noble gas electron configurations for all atoms is a simple and very successful idea. The rules we have used for Lewis structures describe correctly the bonding in most molecules. However, with such a simple model, we should expect some exceptions. Boron, for example, tends to form compounds in which the boron atom has fewer than eight electrons around it—that is, it does not have a complete octet. Boron trifluoride, BF3, a gas at normal temperatures and pressures, reacts very energetically with molecules such as water and ammonia that have unshared electron pairs (lone pairs). 

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