Electron sharing in bonds

Count the number of bonds used in the structure and multiply by two electrons per bond to get the number of electrons shared in bonds. Subtract the number of shared electrons from the number of valence electrons to get the number of unshared... 

Putz, M. V. (2011b). On relationship between electronic sharing in bonding and electronegativity equalization of atoms in molecules. Int J. Chem. Model. 3(4), 371-384. 

Step 4. We are not done yet The species is charged, and we must assign this charge a formal location in order to finish with a complete, correct Lewis structure (Rule 4) Because each hydrogen has a valence electron count of one, namely that of the neutral atom, the -1 formal charge must reside on boron. This conclusion can be verified by counting the valence electrons around boron it is four, half of the total of 8 electrons shared in bonds, and one more than the number associated with the free neutral atom. The correct Lewis structure is therefore... 

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