the formal charges are the positive and negative charges on atoms in the Louis structures the formal charges typically we can calculate the formal charges as uh formal charge equals to the group number of the atom minus the electron count on the atom or the group number of the atom minus the number of unshared electrons on the atom minus 1/2 of the number of shared electrons so the last two items here will be considered as electron count the number of shared electrons sorry the number of unshared electrons and half of the number of shared electrons so here I want you to distinguish between two things first is the count counting electrons for the purpose of counting the formal charges second is the counting electrons to see if the octed rule is satisfied you have all learned that when we count electrons uh for octet we say the electrons that connect the two atoms by a coent bond will count toward filling the ve share of both atoms so we will count electrons around the atom for octet all the electrons for octet but when counting electrons for the purpose of computing the formal charges only half of the number of electrons in calent bond can be considered to be owned by an atom so that's why here we have one half of shared electrons plus unshared electrons as the electron count on the atom so now let me give you a few examples so let's see this um molecule nitrogen in the Middle with four hydrogens attached to the central nitrogen atom let's calculate the form charge on both hydrogen atom and the nitrogen atom first let's see the hydrogen so the formal charge on the hydrogen equals to the group number of hydrogen which is one minus the electron count on the nitrogen on the hydrogen so the number of unshared electrons on hydrogen is zero the number of shared electrons on hydrogen hydrogen has only one Bond so shared electrons that's two electrons half of two that's one right so 1 - 1 equal to 0 so there is no formal charge on the hydrogen next let's take a look at nitrogen atom the group number for nitrogen is number five unshared electrons on nitrogen in this case we did not show any lone pair of electrons on the nitrogen so there is no unshared electrons zero for shared electrons this nitrogen has four bonds so the nitrogen shares eight electrons with the surrounding atoms half of 8 is 4 and 5 - 4 is one plus one so there will be a positive charge on the central nitrogen atom and the structure shown here is the structure for ammonium ion so from now on when you are asked to write Lou structures for the molecule or an ion you will have to include the formal charges on the atoms otherwise it will not be a complete structure okay so for this case ammonium ion you have to include the positive charge on the nitrogen well next let's see three more examples on carbon containing species the first one has a carbon attached to three hydrogens second one has a carbon attached to three hydrogens and one L pair the third has a carbon attached to two hydrogens and a lone pair now let's calculate the formal charges on the carbon atoms okay now the first species let's see carbon group number four unshared electrons because there's no l pair on the carbon there is no unshared electrons so zero for shared electrons three bonds six electrons are shared so half of six is three 4 - 3 that's 1 positive 1 there is a positive charge on the carbon so this is an example that we call it as a carbo cine there is a positive charge on the carbon it's a cine on the carbon carbo you will hear these terms a lot later on in the semester the second example the only difference is this long pair of electrons okay so let's see the group number for carbon four unshared electrons two because we have a pair of long pair of electrons so two and the share the electrons three bonds so six half of six is three now we have 4 minus 5 that will give you -1 as a result there will be one negative charge on this carbon atom and this is an example of a carbine it's an anine on the carbon carbon last example on this page okay so let's see the group number of carbon four we have one L pair on the carbon so two unshared electrons we have only two bonds around the carbon so we have four shared electrons half of four that's 2 and 4 - 4 equals to zero there is no charge on this last carbon atom and this is an example of carbine and we probably will mention carbine a little bit later in the semester all right so I hope from these examples you get a sense of how to calculate the formal charges so now I want you to do some practice problems so here is a list of three oxygen containing species with different numbers of bonds around the oxygen and different number of L pair electrons on oxygen so calculate the formal charges on the oxygen atom and on the right hand side we have different species of the nitrogen containing compound we've seen one just now ammonium Ion with four nitrogens attached to oh sorry four hydrogens attached to the central nitrogen atom now we have two more okay different number of hydrogens different number of long pairs of electrons so calculate the formal charges on the central nitrogen atoms and we will verify our answer in class