Using formal charges to determine how many bonds to make, a different perspective. You’ll only see the first four of them in chemical compounds the last two are extremely radioactive. Note: these six elements from group 7A are called halogens: F, Cl, Br, I At, Tn. If the central atom has fewer than 8 electrons around it, but all of the outer atoms are in group 7A, you’re done Step 5) If the central atom has 8 or more electrons around it, you’re done. If the counts do not match, add the remaining electrons to the central atom. PF 3: should have 26 electrons (from step 1)īrF 3: should have 28 electrons (from step 1) Step 4) Count the electrons in each structure.Įach of these structures has 24 electrons.ĭo these match the counts you got in step 1?īF 3: should have 24 electrons (from step 1) Step 3) Add electrons to the outer atoms, to complete their octets.Įach outer atom needs three electron pairs. Step 2) Attach the atoms to each other using single bonds (“draw the skeleton structure”). The final answers MUST have these numbers of electrons‼! Step 1) Figure out how many electrons each molecule must have. Next, we’ll look at three molecules side-by-side. The molecules are BF 3, PF 3, and BrF 3, all of which have a central atom bonded to three fluorine atoms. (If you’re wondering “what about resonance?”… we’ll get to that later on.)Įxample: Drawing Lewis structures for BF 3, PF 3 and BrF 3 Note: we could have put the double bond in two other locations. Any of the three options is fine you only need to draw one of them. If the central atom still has fewer than 8 electrons around it, do this as many more times as you need. But do not go beyond 8 electrons on the central atom. When you reach 8 electrons, you’re done.Ĭhemists often draw square brackets around the structure of a polyatomic ion and write the charge outside the brackets, like this: It just moves 2 electrons to a different location. Notice that this step does not change the total number of electrons in our structure. (Exception: don’t do this if all of the outer atoms are from group 7A.) Otherwise, create an extra bond by changing one of the nonbonding pairs into a bonding pair. The outer atoms are oxygen atoms, and oxygen is in group 6A, so we aren’t finished yet… If the central atom has fewer than 8 electrons, but all of the outer atoms are in group 7A, you’re done. The carbon atom has only 6 electrons around it, so we aren’t finished yet… If the central atom has 8 or more electrons, you’re done. Our structure has 24 electrons – check! It MATCHES! Step 4) Count the electrons in the structure.ĭoes this match the count you got in step 1?ĬO 3 2 –: should have 24 electrons (from step 1) Each outer atom needs three electron pairs, since it already has one bonding pair. The outer atoms are the oxygen atoms here. Step 3) Add electrons to all outer atoms (except H) to complete their octets. Step 2) Attach the atoms to each other using single bonds (“draw the skeleton structure”) The final answer MUST have this number of electrons‼! The -2 charge means that there are 2 extra electrons Step 1) Figure out how many electrons the molecule must have. Otherwise, repeat this process until the central atom has 8 electrons.Įxample: drawing the Lewis structure of CO 3 2 – If the central atom now has 8 electrons around it, you’re finished.
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