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Lewis structures are representations of molecules and ionic compounds that show the distribution of valence electrons in these substances as well as the way in which atoms share these electrons to form chemical bonds. They are based on the Lewis dot symbols, as well as the electronic configuration of each element that is part of the compound.
In Lewis structures, covalent bonds are represented by lines, each of which represents a pair of bonding electrons, while unshared electrons are represented as dots located around each atom, in the same way as is done with Lewis dot symbols.
These structures allow us to give a very simplified first description of the chemical bond between two atoms. From the Lewis structure of a compound, conclusions can be drawn about the shape and molecular geometry, as well as explain properties of substances such as polarity, melting and boiling points, and even chemical reactivity.
These types of representations are particularly useful in organic chemistry, where they allow us to clearly observe the changes that occur in the distribution of electrons when a chemical reaction occurs, which, in turn, allows us to elucidate the mechanisms by which chemical reactions occur.
Elements that make up a Lewis structure
As mentioned above, Lewis structures are based on Lewis dot symbols. These are written starting with the chemical symbol of the atom in question, and then drawing the valence electrons distributed around it in the form of dots.
In a Lewis structure, the electrons not shared between two atoms are represented as points located above or below the chemical symbol, or to either of its two sides, as the case may be.
Each pair of electrons that is part of a covalent bond, on the other hand, is represented in a Lewis structure by a solid line connecting the center of the two bonded atoms.
But how do you draw a Lewis structure? This turns out to be much simpler than it seems, and it only involves following a series of ordered steps, and applying a little common sense when necessary.
Rules for drawing the Lewis structure
To make it easier to write Lewis structures, you should have some background information before you begin:
- The molecular formula of the compound whose structure you want to draw, including the electric charge, in case it is an ion. For example, if we want to write the Lewis structure of the nitrate ion, then it is necessary to know that its formula is NO 3 – .
- The number of valence electrons of each of the atoms present in the molecular formula must be known . For example, nitrogen is an element that has 5 valence electrons, while oxygen has 6. For representative elements, knowing this number is very easy. It is only necessary to know which group it belongs to, since all elements within a group have the same number of valence electrons.
- It is often helpful (although not strictly necessary) to have an idea of the relative electronegativities of each atom in the formula. The important thing here is not to know how much electronegativity is worth, but to know which element is more or less electronegative than the others.
Once this basic information is gathered, we proceed to describe the steps necessary to write the Lewis structure.
Lewis structures step by step
The steps below can be applied to any chemical species, including covalent or neutral molecular compounds, monatomic or polyatomic ions, or by extension ionic compounds such as ionic salts or oxides.
Step 1: Count the total number of valence electrons.
The Lewis structure must include all the valence electrons of all the elements present in the molecule, and must ensure that the balance of electrical charges is fulfilled. To find the total number of electrons, simply add the product of the number of atoms of each element in the formula and its number of valence electrons, and, at the end, subtract the electrical charge, if any. The formula is:
Example:
If we are writing the Lewis structure of the nitrate ion (NO 3 – ) which has 1 N, 3 O and a charge of –1, then the total number of valence electrons is:
Step 2: Write the fundamental structure of the molecule.
This consists of indicating which atom will be connected with which other atom (what is called the connectivity of the molecule). In other words, the basic skeleton of the molecule will be established.
Some general rules to keep in mind when carrying out this step are:
- The central atom is almost always the least electronegative of all.
- Hydrogen atoms always go at the ends, never in the center. The same is true of the halogens in most compounds where they are not bound to oxygen.
- More than one possible structure can be proposed. Later it is determined which is more probable.
Example – The nitrate ion
An illustrative example of all the steps presented above is represented by the nitrate ion, which is a polyatomic ion formed by three oxygens and one nitrogen joined together by covalent bonds. In this case, the least electronegative element is nitrogen, so it is placed in the center, and the three oxygens are distributed to the sides:
Step 3: Draw a single covalent bond between all the atoms that are linked together.
After this step, the compound begins to acquire the shape of a molecule. End bonds can become double or triple, but they all start out as single bonds.
nitrate ion continued
Step 4: Fill in the octets with the remaining valence electrons, starting with the most electronegative.
After discounting the electrons that are part of the bonds, the remaining electrons are added in pairs around the most electronegative elements to complete their octets (with the exception of hydrogen).
nitrate ion continued
Step 5: Form multiple links if necessary.
If it happens that at the end of the valence electrons some atom is left with its incomplete octet, use an unshared pair of electrons from a neighboring atom to form a double bond or two pairs to form a triple bond if necessary.
nitrate ion continued
Step 6: Calculate formal charges (optional).
Once step 5 is completed, the structure of the molecule is completely drawn. It only remains to add the electrical charges if any. At this point you can proceed in two different ways. The first is that if there is a net charge (in the case of an ion), you enclose the entire structure in square brackets and add the charge as a superscript. The second (which is preferable) consists in determining the formal charge (CF) on each atom of the structure.
This is done by means of the following formula:
nitrate ion continued
In the case of the nitrate ion, the formal charge of the nitrogen atom is:
The formal charges of the two types of oxygen present are:
After calculating the formal charges, these are placed next to each atom that is not neutral, it is verified that the sum of all the charges results in the net charge of the ion (or zero, if the molecule is neutral). As can be seen in the following image, the sum of all charges is +1-2=-1
Example of Lewis structures
As mentioned above, Lewis structures of any chemical species can be written. Below are some examples of structures of different types of compounds:
Neutral molecular compounds – Ethylene
Simple Ionic Compounds – Sodium Chloride
More Complex Ionic Compounds – Sodium Nitrate and Ammonium Nitrate
References
Flowers, P., Theopold, K., Langely, R., Robinson, WR, & O. (2019). Chemistry 2e (2nd ed .). :OpenStax .
The International Union of Pure and Applied Chemistry (IUPAC). (2014). IUPAC – Lewis formula (electron dot or Lewis structure) (L03513). Retrieved from https://goldbook.iupac.org/terms/view/L03513
