CHM 123
Review Electron-dot formulas (Lewis structure)
Rules for Wring Dot Lewis formula
Step 1: Calculate the total number of valence electrons of all atoms in the molecule
Step 2: Create a skeletal structure using the following rules:
 Hydrogen atoms (if present) are always on the “outside” of the structure. They form
only one bond
 The central atom is usually least electronegative. It is also often unique (i.e,. the only
one atom of the element in the molecule). Remember, there might be no “central”
atom.
 Connect bonded atoms by line (2 shared electrons)
Step 3: Place lone pairs around outer atoms (except hydrogen) so that each atom has
an octet
Step 4: Calculate the number of electrons you haven’t used. Subtract the number of
electrons used so far, including electrons in lone pair and bonding pairs, from the total in Step 1.
Assign any remaining electrons to the central atom as lone pair
Step 5: If the central atom is B (boron) or Be (beryllium), skip this step
If the central atom has an octet after step 4, skip this step
 If the central atom has only 6 electrons, move a lone pair from an outer atom to form
a double bond between outer atom and the central atom
 If the central atom has only 4 electrons, do Step 5a to two different outer atoms (i.e,
form two double bonds) or twice to one outer atom (i.e., form one triple bond)
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Exceptions to the Octet Rule
There are three situations where the octet rule.
 1. Molecules with an odd number of electrons (i.e. NO2).
 If you have an odd number of electrons there is no way to satisfy the octet rule.
Try it if you don’t believe me.
 2. Molecules where an atom has less than an octet (i.e. BF3, BeH2, AlCl3).
This only happens to atoms near the boundary between metals and non-metals, such as Be,
B, Al and Ga. The electronegativity of these atoms is not high enough to force more
electronegative non-metals into forming double and triple bonds. When working with such
atoms never draw multiple bonds and you will be able to get the correct Lewis dot
structures.
3. Molecules where an atom has more than an octet of electrons (i.e. ClF3, PCl5, XeF2).
This is fairly common for elements in the 3rd period (row) and below. However, elements in
the first two periods, H – Ne, cannot violate the octet rule in this way.