Boron Exception To Octet Rule
Exceptions to the Octet Dominion
Exceptions to the Octet Rule
Contents
- Exceptions to the Octet Rule
- Odd-electron Molecules
- Electron-deficient Molecules
- Hypervalent Molecules
- Instance: Writing Lewis Structures: Octet Rule Violations
- Solution
Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules autumn into three categories:
- Odd-electron molecules have an odd number of valence electrons, and therefore have an unpaired electron.
- Electron-deficient molecules accept a cardinal cantlet that has fewer electrons than needed for a noble gas configuration.
- Hypervalent molecules have a primal atom that has more than electrons than needed for a noble gas configuration.
Odd-electron Molecules
We call molecules that contain an odd number of electrons free radicals. Nitric oxide, NO, is an example of an odd-electron molecule; information technology is produced in internal combustion engines when oxygen and nitrogen react at high temperatures.
To draw the Lewis structure for an odd-electron molecule similar NO, nosotros follow the same 5 steps nosotros would for other molecules, only with a few modest changes:
- Determine the total number of valence (outer shell) electrons. The sum of the valence electrons is 5 (from Northward) + half-dozen (from O) = 11. The odd number immediately tells us that nosotros accept a gratuitous radical, so we know that not every atom can have viii electrons in its valence shell.
- Draw a skeleton structure of the molecule. We can hands draw a skeleton with an N–O single bond:
Northward–O
- Distribute the remaining electrons as lonely pairs on the terminal atoms. In this case, there is no central atom, so we distribute the electrons around both atoms. We give eight electrons to the more electronegative cantlet in these situations; thus oxygen has the filled valence shell:
- Place all remaining electrons on the central atom. Since there are no remaining electrons, this pace does non apply.
- Rearrange the electrons to make multiple bonds with the central atom in club to obtain octets wherever possible. We know that an odd-electron molecule cannot have an octet for every atom, simply we want to become each cantlet as close to an octet as possible. In this instance, nitrogen has merely v electrons around information technology. To move closer to an octet for nitrogen, nosotros have one of the solitary pairs from oxygen and utilise information technology to form a NO double bail. (Nosotros cannot accept another solitary pair of electrons on oxygen and form a triple bond because nitrogen would then accept 9 electrons:)
Electron-deficient Molecules
We will also see a few molecules that incorporate fundamental atoms that practise non have a filled valence shell. More often than not, these are molecules with central atoms from groups 2 and 13, outer atoms that are hydrogen, or other atoms that do not class multiple bonds. For case, in the Lewis structures of glucinium dihydride, BeH2, and boron trifluoride, BF3, the beryllium and boron atoms each have only four and six electrons, respectively. It is possible to draw a structure with a double bail between a boron atom and a fluorine cantlet in BF3, satisfying the octet dominion, but experimental evidence indicates the bail lengths are closer to that expected for B–F single bonds. This suggests the all-time Lewis structure has three B–F single bonds and an electron deficient boron. The reactivity of the compound is also consistent with an electron deficient boron. Notwithstanding, the B–F bonds are slightly shorter than what is actually expected for B–F single bonds, indicating that some double bond character is plant in the actual molecule.
An cantlet like the boron cantlet in BFthree, which does not have 8 electrons, is very reactive. Information technology readily combines with a molecule containing an atom with a lone pair of electrons. For example, NH3 reacts with BF3 because the lone pair on nitrogen tin be shared with the boron atom:
Hypervalent Molecules
Elements in the second period of the periodic tabular array (n = ii) can accommodate simply eight electrons in their valence shell orbitals because they have only 4 valence orbitals (one 2s and three twop orbitals). Elements in the third and higher periods (n ≥ 3) have more than four valence orbitals and tin can share more than four pairs of electrons with other atoms because they have empty d orbitals in the aforementioned crush. Molecules formed from these elements are sometimes chosen hypervalent molecules. The figure below shows the Lewis structures for two hypervalent molecules, PCl5 and SF6.
In PCl5, the cardinal cantlet phosphorus shares five pairs of electrons. In SF6, sulfur shares six pairs of electrons.
In some hypervalent molecules, such as IF5 and XeF4, some of the electrons in the outer shell of the key atom are lone pairs:
When we write the Lewis structures for these molecules, we find that we have electrons left over after filling the valence shells of the outer atoms with 8 electrons. These boosted electrons must exist assigned to the primal atom.
Example: Writing Lewis Structures: Octet Rule Violations
Xenon is a noble gas, but it forms a number of stable compounds. We examined XeF4 earlier. What are the Lewis structures of XeFii and XeFsix?
Solution
We tin can draw the Lewis construction of any covalent molecule past post-obit the half dozen steps discussed earlier. In this instance, nosotros tin condense the final few steps, since not all of them use.
- Calculate the number of valence electrons:
XeFii: 8 + (2 \(×\) seven) = 22
XeFsix: 8 + (6 \(×\) vii) = 50
- Describe a skeleton joining the atoms past single bonds. Xenon will be the central atom because fluorine cannot exist a central cantlet:
- Distribute the remaining electrons.
XeF2: We place three lone pairs of electrons around each F atom, accounting for 12 electrons and giving each F cantlet 8 electrons. Thus, six electrons (three lone pairs) remain. These lone pairs must be placed on the Xe cantlet. This is acceptable considering Xe atoms accept empty valence beat out d orbitals and tin suit more eight electrons. The Lewis structure of XeFtwo shows two bonding pairs and 3 lone pairs of electrons effectually the Xe atom:
XeFhalf dozen: We place three lone pairs of electrons around each F atom, accounting for 36 electrons. Two electrons remain, and this lonely pair is placed on the Xe atom:
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Boron Exception To Octet Rule,
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