por The lewis structure It is all that representation of the covalent bonds within a molecule or an ion. In it, these bonds and electrons are represented with dots or long dashes, although most of the time the dots correspond to unshared electrons and the dashes to covalent bonds.
But what is a covalent bond? It is the sharing of a pair of electrons (or points) between any two atoms of the periodic table. With these diagrams, many skeletons for a given compound can be sketched. Which of them is correct will depend on the formal charges and the chemical nature of the atoms themselves.
The image above shows an example of what a Lewis structure is. In this case the compound represented is 2-bromopropane. The black dots corresponding to the electrons can be seen, both those that participate in the bonds and those that are not shared (the only pair just above Br).
If the pairs of dots “:” were replaced by an em dash “–“, then the carbon skeleton of 2-bromopropane would be represented as: C–C–C. Why couldn’t it be C–H–H–C instead of the “molecular framework” drawn? The answer lies in the electronic characteristics of each atom.
Thus, because hydrogen has only one electron and only one orbital available to fill, it forms only one covalent bond. Therefore, it can never form two bonds (not to be confused with hydrogen bonds). On the other hand, the electronic configuration of the carbon atom allows (and requires) the formation of four covalent bonds.
For this reason, the Lewis structures where C and H intervene must be coherent and respect what is governed by their electronic configurations. In this way, if carbon has more than four bonds, or hydrogen more than one, then the sketch can be discarded and a new one more realistic can be started.
It is here where some of the main motifs or supports for these structures appear, introduced by Gilbert Newton Lewis in his search for molecular representations faithful to experimental data: molecular structure and formal charges.
All existing compounds can be represented by Lewis structures, giving a first approximation to how the molecule or ions could be.
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What is the Lewis structure?
It is a representative structure of the valence electrons and covalent bonds in a molecule or ion that serves to give an idea of its molecular structure.
However, this structure fails to predict some important details such as the molecular geometry with respect to an atom and its surroundings (if it is square, trigonal planar, bipyramidal, etc.).
Likewise, it does not say anything about the chemical hybridization of its atoms, but it does say where the double or triple bonds are located and if there is resonance in the structure.
With this information, arguments can be made about the reactivity of a compound, its stability, how and what mechanism the molecule will follow when it reacts.
For this reason, Lewis structures never cease to be considered and are very useful, since new chemical learning can be condensed in them.
How is it done?
To draw or sketch a structure, formula or Lewis diagram, the chemical formula of the compound is essential. Without it, you cannot even know which are the atoms that make it up. Once with it, the periodic table is used to locate which groups they belong to.
For example, if you have the compound C14O2N3 then you would have to look for the groups where carbon, oxygen and nitrogen are. Once this is done, no matter what the compound is, the number of valence electrons remains the same, so sooner or later they are memorized.
Thus, carbon belongs to the IVA group, oxygen to the VIA group and nitrogen to the VA. The group number is equal to the number of valence electrons (dots). They all have in common the tendency to complete the octet of the valence shell.
What is the octet rule?
This says that there is a tendency for atoms to complete their energy level with eight electrons to reach stability. This applies to all non-metallic elements or those found in the sop blocks of the periodic table.
However, not all elements obey the octet rule. Particular cases are the transition metals, whose structures are based more on formal charges and their group number.
Applying the mathematical formula
Knowing to which group the elements belong, and therefore, the number of valence electrons available to form bonds, we proceed with the following formula, which is useful for drawing Lewis structures:
C=N–D
where C stands for shared electrons, that is, those that participate in covalent bonds. Since each bond is made up of two electrons, then C/2 is equal to the number of bonds (or dashes) that must be drawn.
They are not the needed electrons, which the atom must have in its valence shell to be isoelectronic to the noble gas that follows it in the same period. For all elements other than H (since it requires two electrons to compare to He) they need eight electrons.
D are the available electrons, which are determined by the group or numbers of valence electrons. Thus, since Cl belongs to group VIIA, it must be surrounded by seven black dots or electrons, and keep in mind that a pair is needed to form a bond.
Given the atoms, their points, and the number of C/2 bonds, a Lewis structure can then be improvised. But additionally, it is necessary to have a notion of other “rules”.
Where to place the least electronegative atoms
The least electronegative atoms in the vast majority of structures occupy the centers. For this reason, if you have a compound with P, O and F atoms, P must therefore be located in the center of the hypothetical structure.
Also, it is important to note that hydrogens normally bond to highly electronegative atoms. If you have Zn, H and O in a compound, the H will go together with the O and not with the Zn (Zn–O–H and not H–Zn–O). There are exceptions to this rule, but it usually occurs with non-metal atoms.
Symmetry and formal charges
Nature has a high preference for creating molecular structures that are as symmetrical as possible. This helps to avoid creating disordered structures, with the atoms arranged in such a way that they do not obey any apparent pattern.
For example, for the compound C2A3, where A is a fictional atom, the most likely structure would be A–C–A–C–A. Note the symmetry of its sides, both reflections of the other.
Formal charges also play an important role when drawing Lewis structures, especially for ions. Thus, bonds can be added or removed so that the formal charge of an atom corresponds to the total charge exhibited. This criterion is very helpful for transition metal compounds.
Limitations on the octet rule
Not all the rules hold, which does not necessarily mean that the structure is wrong. Typical examples of this are seen in many compounds involving group IIIA elements (B, Al, Ga, In, Tl). Aluminum trifluoride (AlF3) is specifically considered here.
Then applying the formula described above, we have:
D= 1×3 (one aluminum atom) + 7×3 (three fluorine atoms) = 24 electrons
Here the 3 and 7 are the respective groups or numbers of valence electrons available for aluminum and fluorine. Then, considering the necessary electrons N:
N= 8×1 (one aluminum atom) + 8×3 (three fluorine atoms) = 32 electrons
And therefore the shared electrons are:
C= N–D
C= 32 – 24 = 8 electrons
C/2 = 4 links
Since aluminum is the least electronegative atom, it must be placed in the center, and fluorine only forms one bond. Considering this, we have the Lewis structure of AlF3 (upper image). Shared electrons are highlighted with green dots to distinguish them from unshared ones.
Although the calculations predict that there are 4 bonds that must be formed, aluminum lacks sufficient electrons and also there is no fourth fluorine atom. As a result aluminum does not comply with the octet rule and this fact is not reflected in the calculations.
Examples of Lewis structures
Iodine
Iodine is a halogen and therefore belongs to group VIIA. It then has seven valence electrons, and this simple diatomic molecule can be represented improvising or applying the formula:
D= 2×7 (two iodine atoms) = 14 electrons
N= 2×8 = 16 electrons
C = 16 – 14 = 2 electrons
C/2 = 1 link
Since 2 of the 14 electrons participate in the covalent bond (green dots and dash), 12 remain unshared; and because they are two iodine atoms, 6 must be shared for one of them (their valence electrons). In this molecule, only this structure is possible, whose geometry is linear.
Ammonia
What is the Lewis structure for the ammonia molecule? Since nitrogen is from the VA group, it has five valence electrons, and then:
D = 1×5 (one nitrogen atom) + 1×3 (three hydrogen atoms) = 8 electrons
N = 8×1 + 2×3 = 14 electrons
C = 14 – 8 = 6 electrons
C/2= 3 links
This time the formula is correct with the number of links (three green links). Since 6 of the 8 available electrons participate in the bonds, there is an unshared pair that is located on top of the nitrogen atom.
This structure says everything that needs to be known about the ammonia base. Applying the knowledge of TEV and TRPEV, it can be deduced that the geometry is tetrahedral distorted by the lone pair of nitrogen and that its hybridization is therefore sp3.
C2H6O
The formula corresponds to an organic compound. Before applying the formula, it must be remembered that hydrogens form a single bond, oxygen two, carbon four, and that the structure must be as symmetrical as possible. Proceeding as in the previous examples, we have:
D= 6×1 (six hydrogen atoms) + 6×1 (one oxygen atom) + 4×2 (two carbon atoms) = 20 electrons
N= 6×2 (six hydrogen atoms) + 8×1 (one oxygen atom) + 8×2 (two carbon atoms) = 36 electrons
C = 36 – 20 = 16 electrons
C/2 = 8 links
The number of green dashes correspond to the 8 calculated links. The proposed Lewis structure is that of ethanol CH3CH2OH. However, it would also have been correct to propose the structure of dimethyl ether CH3OCH3, which is even more symmetrical.
Which of the two is «more» correct? Both are equally, since the structures arose as structural isomers of the same molecular formula C2H6O.
permanganate ion
The situation is complicated when it is desired to make Lewis structures for transition metal compounds. Manganese belongs to group VIIB, likewise, the electron of the negative charge must be added among the available electrons. Applying the formula we have:
D = 7×1 (an atom of…