D7.4 General Guidance for Drawing Lewis Structures

Often you can draw a Lewis structure based on the number of bonds formed by each kind of atom. In more complicated cases, here is a step-by-step procedure for drawing Lewis structures of molecules:

1. Determine the total number of valence electrons by summing the number of valence electrons on all atoms.
   • For a polyatomic cation, subtract one electron for each positive charge.
   • For a polyatomic anion, add one electron for each negative charge.
2. Choose one or more central atoms; a central atom bonds to several other atoms and is usually the atom that forms the greatest number of bonds.
   • Usually the central atom is written first in a chemical formula, such as P in PCl₃.
   • If there are two or more central atoms, connect them using single bond lines.
3. Draw a skeleton structure of the molecule by arranging the other atoms (which are called terminal atoms) around the central atom or atoms.
   • Connect terminal atoms to the central atom(s) by single bond lines.
4. Distribute the remaining electrons as lone pairs on the terminal atoms (except hydrogen), completing an octet around each atom.
5. If there are still valence electrons available, place them on the central atom(s).
6. If the number of electrons around a central atom is less than an octet, rearrange the electrons to make multiple bonds with the central atom(s) until each atom has an octet.

Let’s apply these rules to a simple molecule, ammonia, NH₃.

Here is a more complicated case: ethene (ethylene), C₂H₄.