How to Draw Lewis Structure

Drawing Lewis Structures

Lewis structures can help us understand the bonding in many compounds and are frequently used when discussing the properties of molecules. For this reason, drawing Lewis structures is an important skill that you should practice. To do so, you should use the following procedure:

1. Sum the valence electrons from all atoms. Use the periodic table to help you determine the number of valence electrons in each atom.
   • For an anion, add one electron to the total for each negative charge.
   • For a cation, subtract one electron from the total for each positive charge.
   Do not worry about keeping track of which electrons come from which atoms. Only the total number is important.
2. Write the symbols for the atoms, show which atoms are attached to which, and connect them with a single bond. A single bond is represented by a dash and accounts for two electrons.

   Chemical formulas are often written in the order in which the atoms are connected in the molecule or ion. The formula HCN, for example, tells you that the carbon atom is bonded to the H and to the N. In many polyatomic molecules and ions, the central atom is usually written first, as in CO₃²⁻ and SF₄. Remember that the central atom is generally less electronegative than the atoms surrounding it. In other cases, you may need more information before you can draw the Lewis structure.

3. Complete the octets around all the atoms bonded to the central atom. Remember, however, that a hydrogen atom has only a single pair of electrons around it.
4. Place any leftover electrons on the central atom, even if doing so results in more than an octet of electrons around the atom.
5. If there are not enough electrons to give the central atom an octet, try multiple bonds. Use one or more of the unshared pairs of electrons on the atoms bonded to the central atom to form double or triple bonds.

Draw the Lewis structure for phosphorus trichloride, PCl₃.

SOLUTION

We are asked to draw a Lewis structure from a molecular formula. Our plan is to follow the five-step procedure just described.

First, we sum the valence electrons. Phosphorus (group 5A) has five valence electrons, and each chlorine (group 7A) has seven. The total number of valence electrons is therefore,

5 + (3 × 7) = 26

Second, we arrange the atoms to show which atom is connected to which, and we draw a single bond between them. There are various ways the atoms might be arranged. In binary compounds, however, the first element in the chemical formula is generally surrounded by the remaining atoms. Thus, we begin with a skeleton structure that shows a single bond between the P atom and each Cl atom:

(It is not crucial that the Cl atoms be left of, right of, and below the P atom—any structure that shows each of the three Cl atoms bonded to P will work.)

Third, we complete the octets on the atoms bonded to the central atom. Placing octets around each Cl atom accounts for 24 electrons (remember, each line in our structure represents two electrons):

Fourth, recalling that our total number of electrons is 26, we place the remaining two electrons on the central atom, completing the octet around it:

This structure gives each atom an octet, so we stop at this point. (In checking for octets, remember to count both electrons in a single bond twice, once for each atom in the bond.)

🧠 Draw the Lewis structure for the BrO₃⁻ ion.

References: 12th Edition Chemistry The Central Science