How can expanded octets be possible

Boron and aluminum, from Group III or 13 , display different bonding behavior than previously discussed. These atoms each have three valence electrons, so we would predict that these atoms want to bond covalently in order to gain 5 electrons through sharing to fulfill the octet rule. However, compounds in which boron or aluminum atoms form five bonds are never observed, so we must conclude that simple predictions based on the octet rule are not reliable for Group III.

Consider boron trifluoride BF 3. The bonding is relatively simple to model with a Lewis structure if we allow each valence level electron in the boron atom to be shared in a covalent bond with each fluorine atom.

In this compound, the boron atom only has six valence shell electrons, but the octet rule is satisfied by the fluorine atoms. Lewis structure of boron trifluoride : Each pair of dots represents a pair of electrons. When placed between two atoms, the electrons are in a bond. A bond can be drawn as a line between two atoms, which also indicates two electrons.

We might conclude from this one example that boron atoms obey a sextet rule. However, boron will form a stable ion with hydrogen, BH 4 — , in which the boron atom does have a complete octet. In addition, BF 3 will react with ammonia NH 3 , to form a stable compound, NH 3 BF 3 , for which a Lewis structure can be drawn that shows boron with a complete octet. Boron trifluoride-ammonia complex : This covalent compound NH 3 BF 3 shows that boron can have an octet of electrons in its valence level.

Compounds of aluminum follow similar trends. Aluminum trichloride AlCl 3 , aluminum hydride AlH 3 , and aluminum hydroxide Al OH 3 indicate a valence of three for aluminum, with six valence electrons in the bonded molecule.

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. This matter is still under hot debate, however and there is even debate as to what makes an expanded octet more favorable than a configuration that follows the octet rule.

One of the situations where expanded octet structures are treated as more favorable than Lewis structures that follow the octet rule is when the formal charges in the expanded octet structure are smaller than in a structure that adheres to the octet rule, or when there are less formal charges in the expanded octet than in the structure a structure that adheres to the octet rule.

The sulfate ion, SO 4 A strict adherence to the octet rule forms the following Lewis structure:. If we look at the formal charges on this molecule, we can see that all of the oxygen atoms have seven electrons around them six from the three lone pairs and one from the bond with sulfur.

This is one more electron than the number of valence electrons then they would have normally, and as such each of the oxygen atoms in this structure has a formal charge of If instead we made a structure for the sulfate ion with an expanded octet, it would look like this:.

Looking at the formal charges for this structure, the sulfur ion has six electrons around it one from each of its bonds. This is the same amount as the number of valence electrons it would have naturally. This leaves sulfur with a formal charge of zero. The two oxygens that have double bonds to sulfur have six electrons each around them four from the two lone pairs and one each from the two bonds with sulfur.

This is the same amount of electrons as the number of valence electrons that oxygen atoms have on their own, and as such both of these oxygen atoms have a formal charge of zero.

The two oxygens with the single bonds to sulfur have seven electrons around them in this structure six from the three lone pairs and one from the bond to sulfur.

That is one electron more than the number of valence electrons that oxygen would have on its own, and as such those two oxygens carry a formal charge of Remember that with formal charges, the goal is to keep the formal charges or the difference between the formal charges of each atom as small as possible.

The ICl 4 - ion thus has 12 valence electrons around the central Iodine in the 5 d orbitals. Expanded Lewis structures are also plausible depictions of molecules when experimentally determined bond lengths suggest partial double bond characters even when single bonds would already fully fill the octet of the central atom.

Despite the cases for expanded octets, as mentioned for incomplete octets, it is important to keep in mind that, in general, the octet rule applies. Mike Blaber Florida State University. Exception 1: Species with Odd Numbers of Electrons The first exception to the Octet Rule is when there are an odd number of valence electrons.

That is exactly what is done to get the correct Lewis structure for nitrogen monoxide: Figure 2. Exception 2: Incomplete Octets The second exception to the Octet Rule is when there are too few valence electrons that results in an incomplete Octet. Figure 4 If you look Figure 4, you can see that the fluorine atoms possess extra lone pairs that they can use to make additional bonds with boron, and you might think that all you have to do is make one lone pair into a bond and the structure will be correct.

If we add one double bond between boron and one of the fluorines we get the following Lewis Structure Figure 5 : Figure 5 Each fluorine has eight electrons, and the boron atom has eight as well! This suggests the possibility of a semi-ionic structure such as seen in Figure 6: Figure 6 None of these three structures is the "correct" structure in this instance.

Solution 1. Draw connectivities: 3. Add octets to outer atoms: 4. Does central electron have octet? It has 6 electrons Add a multiple bond double bond to see if central atom can achieve an octet: 6. The central Boron now has an octet there would be three resonance Lewis structures However In this structure with a double bond the fluorine atom is sharing extra electrons with the boron.

Thus, the structure of BF 3 , with single bonds, and 6 valence electrons around the central boron is the most likely structure BF 3 reacts strongly with compounds which have an unshared pair of electrons which can be used to form a bond with the boron:. Exception 3: Expanded Valence Shells More common than incomplete octets are expanded octets where the central atom in a Lewis structure has more than eight electrons in its valence shell.

A strict adherence to the octet rule forms the following Lewis structure: Figure 12 If we look at the formal charges on this molecule, we can see that all of the oxygen atoms have seven electrons around them six from the three lone pairs and one from the bond with sulfur. If instead we made a structure for the sulfate ion with an expanded octet, it would look like this: Figure 13 Looking at the formal charges for this structure, the sulfur ion has six electrons around it one from each of its bonds.

Draw the connectivities: 3. Add octet of electrons to outer atoms: 4. Practice Problems Draw the Lewis structure for the molecule I 3 -.

Draw the molecule ClF 3. The central atom for an expanded octet must have an atomic number larger than what? However, there are a few stable molecules which contain an odd number of electrons. These molecules, called "free radicals" , contain at least one unpaired electron, a clear violation of the octet rule. Free radicals play many important roles a wide range of applied chemistry fields, including biology, medicine, and astrochemistry.

Three well-known examples of such molecules are nitrogen II oxide, nitrogen IV oxide, and chlorine dioxide. The most plausible Lewis structures for these molecules are. Free radicals are usually more reactive than the average molecule in which all electrons are paired. In particular they tend to combine with other molecules so that their unpaired electron finds a partner of opposite spin.

Since most molecules have all electrons paired, such reactions usually produce a new free radical. This is one reason why automobile emissions which cause even small concentrations of NO and NO 2 to be present in the air can be a serious pollution problem.

When one of these free radicals reacts with other automobile emissions, the problem does not go away. Instead a different free radical is produced which is just as reactive as the one which was consumed. To make matters worse, when sunlight interacts with NO 2 , it produces two free radicals for each one destroyed:. A fourth very interesting example of a free radical is oxygen gas. The Lewis structure for Oxygen usually hides the fact that it is a "diradical" , containing two unpaired electrons.

This is often cited as a serious flaw in Lewis bond theory , and was a major impetus for development of molecular orbital theory. We know oxygen is a diradical because of its paramagnetic character, which is easily demonstrated by attraction of oxygen to an external magnet. The structure is Instead of an octet the valence shell of Be contains only two electron pairs.