# How do I draw a Lewis structure for a molecule?

I'm having trouble understanding electron dot structures and how to set the problem up. (The only thing I get is hydrogen 2H = H:H) Everything else is complicated to me. Can you help me?
B. D. Ray

Start by learning this basic step-by-step procedure for drawing Lewis structures.
1. Draw a skeleton structure. A skeleton structure is a rough map showing the arrangement of atoms within the molecule. In general, you need to determine the skeleton experimentally, but here are a few guidelines for predicting skeleton structures from molecular formulas.
• Central atoms are usually
• the atoms with highest valence, or
• the largest atoms, or
• the least electronegative atom.
• H and the halogens are usually outside atoms.
• Don't put more than four atoms around a central atom unless the central atom is third period or lower.
2. Count total valence electrons.
• Add the number of electrons in the valence shells of all atoms in the molecule.
• If the molecule is charged, add an electron for each negative charge and subtract an electron for each positive charge.
• Noble gas compounds are very uncommon (except on general chemistry tests!) Should you encounter one, each noble gas atom has 8 valence electrons.
3. Connect the structure.
• Draw a bond between the central atom and each outside atom.
• Each bond uses 2 valence electrons.
4. Place electrons on outside atoms.
• Use remaining electrons to satisfy the octets for each of the outside atoms.
• If you run out of electrons at this point, the skeleton structure was wrong. Go back to step I.
5. Place all remaining electrons on the central atom.
• If there are more than 8 electrons on the central atom, and the central atom is not third period or lower, you counted the number of valence electrons incorrectly. Go back to step II.
• If the octet on the central atom is not complete, try sharing lone pairs of outside atoms to form double or triple bonds. Write one multiply bound structure for each outside atom with a lone pair to share; these are resonance structures.
• If you can't get an octet on the central atom, at this point, check to see whether the total number of valence electrons for this molecule is odd. It's impossible to give octets to all atoms in an odd electron molecules. Get as close to an octet as possible by forming multiple bonds.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How do I draw a Lewis structure for a molecule?