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:
- 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.
- 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.
- 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.
- Distribute the remaining electrons as lone pairs on the terminal atoms (except hydrogen), completing an octet around each atom.
- If there are still valence electrons available, place them on the central atom(s).
- 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.