Electron Dot Formulas

• try to arrange atoms to give the most symmetric structure
• put the most electropositive element in as the central atom
• H and F will always have 1 single bond that connects them
• remember to adjust your electron total when you have an ion
• any extra electrons after you have satisfied the octet rule should go on the central atom as lone pairs
• check formal charge to see if better electron arrangements exist (the more negative f.c. goes on the more electronegative atom)

You might find this helpful...

S = N - A

S = shared electrons : total number of electrons that will be shared and therefore are the bonding electrons. Divide "S" by 2 and you'll have the number of bonds (lines) in the structure.

N = needed electrons : needed is based on the octet rule which is 8 electrons for all atoms except hydrogen which is 2.

A = available electrons : actual number of valence electrons that each atom brings to the structure. Remember to adjust your total up or down here if you have an anion (up) or a cation (down).

Formal Charges

The kernel charge is the same as the regular number of valence electrons. It's the number of the group on the American periodic table. The kernel charge is positive due to the nucleus being positive.

formal charge = kernel charge - assigned electrons

• calculate by assigning electrons to each atom
• lone pairs belong completely to their parent atom
• bonding electrons are perfectly split 50/50 between the two atoms
• the sum of all formal charge values must equal the charge on the species
• best (most favored) structure will have the most zeroes in it
• +1 and -1 are needed sometimes - especially for charged species
• better to have multiple -1's than one -2 or -3
• when chosing the best structure when faced with only 0, +1, and -1 for formal charges, the best structure will have the -1 formal charge on the more electronegative atom
• +2's and -2's are only possible in rare instances and must be rationalized in some other clever way