[lykzomg]

I feel like starting from the scratch (valence dots on each atom, making single bonds, making double bonds if any valence are left, moving electrons to complete octets, etc etc) is redundant and screams for a shortcut. I am aware that practice will undoubtedly speed my abilities but still, anyone have general tips and skills on drawing LDS efficiently?  Thanks

[jp92celtic]

think about individual atoms as having bonding 4 bonding sites [octet rule] (can have either bonds or lone pairs). hydrogen can have only one bonding site, but all others should have 4 sites. now think of each bond or lone pair as taking up ONE bonding site. double bonds take up two bonding sites, triple = 3 etc.

now think about formal charge - the act of assigning arbitrary numbers to atoms in lewis structures to see which are generally the most stable. the group number of the atom in the periodic table determines this. ie, carbon is group 4A and wants 4 electrons around it (obviously following the octet rule). the only way to achieve this is to have 4 covalent bonds around carbon since they share 1 electron at each site. the formal charge is thus, (desired electrons - actual #electrons around atom).   

remember, in formal charge, a value of zero is best, and the formal charge matches the charge on the molecule (zero in neutral molecules). negative formal charges sometimes are unavoidable, and if there is an option should be placed on the more electronegative atom (check PT). ie, a formal charge of negative would rather be on O than on N.

hope this helps somewhat.

[dzoys]

i can tell you that phosphorus can form 5 bonds. im finishing up my general chem and the revisiting is a good idea for orgo.

would be nice to have a list of "exceptions"

[dzoys]

think about individual atoms as having bonding 4 bonding sites [octet rule] (can have either bonds or lone pairs). hydrogen can have only one bonding site, but all others should have 4 sites. now think of each bond or lone pair as taking up ONE bonding site. double bonds take up two bonding sites, triple = 3 etc.

now think about formal charge - the act of assigning arbitrary numbers to atoms in lewis structures to see which are generally the most stable. the group number of the atom in the periodic table determines this. ie, carbon is group 4A and wants 4 electrons around it (obviously following the octet rule). the only way to achieve this is to have 4 covalent bonds around carbon since they share 1 electron at each site. the formal charge is thus, (desired electrons - actual #electrons around atom).   

remember, in formal charge, a value of zero is best, and the formal charge matches the charge on the molecule (zero in neutral molecules). negative formal charges sometimes are unavoidable, and if there is an option should be placed on the more electronegative atom (check PT). ie, a formal charge of negative would rather be on O than on N.

hope this helps somewhat.

ok so (desired # electrons-actual # electrons around atom) not to be redundant but this includes the bonded pairs right? how are they factored in? desired electron # involves the neutral atomic charge, so that # can be positive or negative?