- WELCOME TO THE VIDEO TUTORIAL
FOR LEWIS STRUCTURES AND COVALENT COMPOUNDS
ALSO CALLED MOLECULAR COMPOUNDS.
ALL RIGHTY. SO LET'S START WITH THE OCTET RULE.
THE OCTET RULE FOR COVALENT COMPOUNDS TELLS US
THAT ADAMS CREATE COMPOUNDS BY SHARING VALENCE ELECTRONS,
RIGHT?
SO THESE SHARED VALENCE ELECTRONS ARE THE COVALENT BOND.
ALL RIGHT.
SO THIS APPROACH IS VERY DIFFERENT FROM IONIC COMPOUNDS
WHERE THE ELECTRONS ARE GAINED AND LOST, HERE THEY'RE SHARED.
AND SO IT CAN BE VERY HELPFUL A DRAW LEWIS STRUCTURES.
AND SO A LEWIS STRUCTURE IS A DIAGRAM SHOWING
HOW THE VALENCE ELECTRONS ARE ARRANGED AMONG ATOMS
IN A COMPOUND.
AND SO WE CAN REMEMBER THAT THE GROUP NUMBER FOR AN ELEMENT
TELLS US THE NUMBER OF VALENCE ELECTRONS.
SO IF THE--IF THE STATEMENT-- IF WHAT I DESCRIBE TO YOU SO FAR
DOES NOT MAKE ANY SENSE AT ALL
THEN IT WOULD BE A GOOD IDEA FOR YOU
TO WATCH THE VALENCE ELECTRONS AND OCTET RULE VIDEO FIRST.
BUT I'M GOING TO KEEP GOING
BECAUSE THIS ALL IS GREAT FOR YOU.
SO HERE WE HAVE AN EXCERPT OF THE PERIODIC TABLE.
SO HERE WE SEE HYDROGEN AS GROUP 1A, WE KNOW IT HAS ONE ELECTRON,
ONE VALENCE ELECTRON.
SO HYDROGEN ONLY LIKES TO FORM ONE BOND.
SO THE LITTLE STRUCTURE HERE
WOULD BE THAT WE SHOW THE ELEMENT WITH ITS ONE
AND IN THIS CASE, IT'S A VALENCE ELECTRON WHICH HYDROGEN HAS ONE.
SO THERE IS THE GROUP NUMBER ONE
INDICATING THE NUMBER OF VALENCE ELECTRONS.
AND SO HYDROGEN ONLY LIKES TO FORM ONE BOND.
NOW BORON, WE DON'T INTERACT WITH BORON A LOT
AND IN CHEM 309,
BUT BORON IS AN EXCEPTION TO THE OCTET RULE
AND THAT BORON ONLY LIKES TO HAVE THREE BONDS.
BORON HAS THREE VALENCE ELECTRONS.
SO--BUT WE'RE GOING TO FOCUS MORE ON THE FUNDAMENTAL RULES
THAT APPLY BROADLY.
WE'RE NOT GOING TO WORRY A LOT ABOUT EXCEPTIONS.
WHEN WE GET TO CARBON, CARBON IS GROUP 4,
CARBON WILL HAVE FOUR VALENCE ELECTRONS.
SO WE WILL SEE THAT CARBON LIKES TO FORM FOUR BONDS.
NOW, NITROGEN ON THE OTHER HAND BEING GROUP 5
HAS FIVE VALENCE ELECTRONS,
SO BECAUSE TWO OF THE ELECTRON OF NITROGEN HAVE BECOME PAIRED,
NITROGEN REALLY ONLY WANTS TO FORM THREE BONDS.
NITROGEN WILL HAVE ONE LONE PAIR
AND THEN IT WILL HAVE THREE ELECTRONS AVAILABLE
TO FORM BONDS.
OXYGEN IS GROUP SIX, SO IT HAS SIX VALENCE ELECTRONS.
SO OXYGEN WILL HAVE ITS SIX VALENCE ELECTRONS,
WE WILL HAVE FOUR OF THEM CREATE PAIRS
AND THEN WE WILL HAVE TWO UNPAIRED ELECTRONS.
SO THE BONDING PATTERN FOR OXYGEN
IS OXYGEN LIKES TO FORM TWO BONDS.
THE HALOGENS, WE WON'T SPEND A LOT OF TIME WITH THE HALOGENS
BUT THEY ARE GROUP SEVEN
AND PRIMARILY THEY ONLY WANT TO FORM ONE BOND.
REMEMBER THAT HALOGENS ARE SYMBOLIZED WITH AN UPPER CASE X
AND AS WE PUT THE SEVEN VALENCE ELECTRONS AROUND,
WE SEE THAT THERE'S REALLY ONLY ONE VALENCE ELECTRON AVAILABLE
TO CREATE COVALENT BONDS.
AND THEN LAST BUT NOT LEAST, WE WOULD HAVE THE NOBLE GASSES
WHICH ARE GROUP EIGHT AND THEY THE FULL VALENCE SHELL
AND SO THAT'S WHY THEIR INERT
WHICH MEANS THEY'RE NOT REACTIVE.
SO HERE, THE MAIN POINT OF THIS PAGE IS TO REMIND YOU
THAT YOU ALWAYS KNOW HOW MANY VALENCE ELECTRONS ARE PRESENT
FOR ANY GIVEN ATOM,
SIMPLY BY NOTING THE GROUP NUMBER AT THE TOP OF THE COLUMN.
ALL RIGHTY.
SO NOW, LET'S TAKE THIS INFORMATION
AND APPLY IT TO ORGANIC MOLECULES
BECAUSE THAT'S THE EMPHASIS OF CHEM 309.
SO AND WE'RE GOING TO EMPHASIZE NEUTRAL BONDING PATTERNS
TO START.
THAT'S OUR STABLE GROUND.
YOU WANT TO HAVE YOUR NEUTRAL BONDING PATTERNS RECOGNIZED
FOR ORGANIC COMPOUNDS.
NOW, WHAT EXACTLY ARE ORGANIC COMPOUNDS?
THESE ARE CARBON COMPOUNDS.
CARBON CONTAINING COMPOUNDS. ALL RIGHTY.
SO, IF YOU RECOGNIZE THE NEUTRAL BONDING PATTERNS,
IF YOU THEN SEE AN ARRANGEMENT
THAT VIOLATES THE NEUTRAL BONDING PATTERN,
YOU WILL RECOGNIZE THAT THOSE ELEMENTS HAVE A CHARGE,
THEY'D EITHER BECOME CATIONS OR ANIONS.
NOW, WE'VE TALKED CARBON,
SO HERE WE HAVE THE ATOMIC NUMBER, RIGHT, CARBON SIX
AND BUT WE'RE IN GROUP FOUR.
SO--RIGHT.
SO CARBON HAS THE FOUR VALENCE ELECTRONS.
AND SO IT WILL SHARE THESE VALENCE ELECTRONS
TO FORM FOUR BONDS.
THE INTERESTING THING ABOUT THIS IS CARBON DOESN'T CARE
WHETHER IT HAS FOUR SINGLE BONDS, TWO SINGLE BONDS
AND A DOUBLE BOND OR A SINGLE BOND AND A TRIPLE BOND.
NONE OF THAT MATTERS TO CARBON.
IT JUST WANTS TO ALWAYS HAVE FOUR BONDS.
NOW, NITROGEN, RIGHT, GROUP FIVE HAS THE FIVE VALENCE ELECTRONS.
AND SO IT WILL WANT TO--
NITROGEN WILL WANT TO SHARE THESE THREE VALENCE ELECTRONS
TO FORM THREE BONDS AND NITROGEN WILL HAVE ONE LONE PAIR.
IT'S IMPORTANT TO RECOGNIZE THE PRESENCE OF THESE LONE PAIRS.
THEY'RE OFTEN NOT DRAWN IN WHEN YOU LOOK AT A STRUCTURE.
SO YOU MENTALLY WANT TO ADD THEM.
AND NITROGEN IS JUST LIKE CARBON,
IT DOESN'T CARE HOW IT GETS THE THREE BONDS.
IT DOESN'T CARE IF IT'S THREE SINGLE BONDS,
A SINGLE AND A DOUBLE OR A TRIPLE.
SO NOW THEN THE NEUTRAL BONDING PATTERN FOR NITROGEN
IS THREE BONDS WITH A LONE PAIR.
NOW, WHEN WE LOOK AT OXYGEN, OXYGEN IS GROUP SIX,
IT HAS THE SIX VALENCE ELECTRONS
AND SO, OXYGEN WHEN IT'S IN ITS NEUTRAL FORM
IN ORGANIC MOLECULES,
YOU WILL HAVE TWO LONE PAIRS AND TWO BONDS.
BUT JUST LIKE CARBON AND NITROGEN, OXYGEN DOESN'T CARE
WHETHER IT'S TWO SINGLE BONDS OR A DOUBLE BOND.
AND THEN, NOW, HERE WE HAVE ALL OF OUR EMPHASIS ON THE OCTET.
REMEMBER THAT HYDROGEN IS AN EXCEPTION TO THE OCTET RULE,
RIGHT, HYDROGEN WANTS A DUET.
HYDROGEN WANTS TO FILL THE FIRST SHOW, RIGHT,
THE FIRST ELECTRON SHOW.
SO, HERE'S HYDROGEN WITH ITS ONE VALENCE ELECTRON
AND HYDROGEN CAN ONLY FORM ONE BOND.
IF YOU EVER HAVE A SITUATION
WHERE YOU HAVE MORE THAN ONE BOND OF HYDROGEN,
AN ALARM SHOULD GO OFF IN YOUR HEAD THAT SOMETHING IS WRONG.
ALL RIGHT.
SO NOW THAT WE ARE AWARE OF THE BONDING PATTERNS OF THE ATOMS,
WE WILL BE USING FOR ORGANIC CHEMISTRY, WE CAN TAKE THIS
AND COMBINE THIS WITH A STRATEGY FOR DRAWING THE LEWIS STRUCTURE
FOR COVALENT COMPOUNDS.
SO I WILL GIVE YOU AN OVERVIEW OF THIS STEP BY STEP STRATEGY
AND THEN WE WILL PRACTICE TOGETHER.
SO THE FIRST THING WE NEED TO DO
IS DETERMINE THE NUMBER OF VALENCE ELECTRONS FOR EACH ATOM.
THESE ARE ALL OF THE ELECTRONS THAT WE GET.
IT'S ONLY THE VALENCE ELECTRONS THAT ARE AVAILABLE FOR BONDING.
SO REMEMBER THAT THE GROUP NUMBER
EQUALS THE NUMBER OF VALENCE ELECTRONS.
AND THEN THE SECOND STEP IS WE WRITE THE SYMBOL FOR EACH ATOM
IN THE MOLECULAR ARRANGEMENT AROUND THE CENTRAL ATOM.
SO IN MOST CASES, MOSTLY, RIGHT?
SO THE EXCEPTION IS INORGANIC ACIDS. OKAY?
BUT SINCE WE'RE FOCUSING ON ORGANIC MOLECULES,
WE DON'T HAVE TO WORRY ABOUT THE EXCEPTION, RIGHT.
THE FIRST ATOM LISTED IS THE CENTRAL ATOM. ALL RIGHT.
SO THEN WE WANT TO ARRANGE THE ATOM,
SO THERE'S A SINGLE COVALENT BOND
BETWEEN EACH OF THE BONDED ATOMS.
WE HAVE TO CREATE THE MOLECULE, SO THE ATOMS ARE HELD TOGETHER
BY A MINIMUM OF ONE COVALENT BOND.
REMEMBER THAT ONE COVALENT BOND EQUALS ONE ELECTRON PAIR.
AND OF COURSE THERE'S TWO ELECTRONS IN A PAIR.
SO THE COVALENT BOND, WE CAN WRITE IT AS A LINE
OR WE CAN SHOW IT AS TWO DOTS.
EITHER WAY, THIS EQUAL-- THIS WOULD BE A COVALENT BOND.
NOW, WE HAVE--IN MOST CASES,
WE DON'T USE UP ALL OF OUR AVAILABLE VALENCE ELECTRONS,
SO THEN WE TAKE ANY REMAINING VALANCE ELECTRONS
AND WE ADD THEM AS LONE PAIRS TO MAKE SURE
THAT ALL OF THE ELEMENTS HAVE THEIR OCTETS.
IF AN ATOM DOES NOT HAVE AN OCTET,
THEN WE SHIFT LONE PAIRS TO FORM MULTIPLE BONDS.
AND SO I'M GOING TO DEMONSTRATE THIS IN JUST A MOMENT.
AND THEN WE WANT TO VERIFY THAT EACH ATOM HAS AN OCTET
EXCEPT FOR HYDROGEN WANTS A DUET.
ALL RIGHT.
SO IF WE LOOK HERE, I'M GOING TO--
BECAUSE YOU SHOULD HAVE ALREADY PRACTICED THIS A LITTLE BIT
BEFORE LOOKING AT THE TUTORIALS.
I'M GOING TO DO A FEW MORE CHALLENGING MOLECULES
TO HELP EXPAND AND BUILD YOUR COMFORT WITH LEWIS STRUCTURES
ESPECIALLY FOR ORGANIC MOLECULES.
SO AS WE LOOK HERE, WE SEE A CARBON
AND SO THAT WILL BE SURROUNDED BY THE THREE HYDROGENS
AND THEN HERE'S THE OXYGEN.
SO WE WILL HAVE THE CARBON AND THE OXYGEN
AND THEN WE KNOW CARBON LIKES TO HAVE FOUR BONDS,
SO IT WILL BE SURROUNDED BY THE THREE HYDROGENS AND THE OXYGEN
AND THEN THE OXYGEN WILL BE BONDED TO THE HYDROGEN.
NOW, HOW MANY ELECTRONS DO WE HAVE TO PLAY WITH?
WELL, CARBON IS GROUP 4, SO IT GIVES US FOUR.
HYDROGENS, ALTOGETHER WE HAVE FOUR HYDROGENS
AND THEY EACH GIVE US ONE VALANCE ELECTRON.
AND THEN OXYGEN HAS SIX,
SO ALTOGETHER WE HAVE FOURTEEN ELECTRONS.
SO WE HAVE OUR NUMBER OF VALENCE ELECTRONS,
WE'VE ARRANGED THE ATOMS TO CREATE THE SHAPE OF THE MOLECULE
AND THEN THIS IS WHERE OUR UNDERSTANDING
OF BONDING PATTERNS COULD BE VERY HELPFUL
IN FIGURING OUT HOW TO PUT THE ATOMS TOGETHER.
SO THE FIRST THING WE HAVE TO DO NOW
IS CONNECT ALL OF THESE ATOMS WITH A SINGLE COVALENT BOND.
SO WE HAVE FOURTEEN ALTOGETHER.
TWO, FOUR, SIX, EIGHT, TEN. OKAY?
SO WE JUST USED THE TEN ELECTRONS.
WE ONLY HAVE FOUR ELECTRONS LEFT.
AS WE LOOK AT THIS HYDROGEN IT HAS A DUET.
THIS HYDROGEN HAS A DUET.
AND IF WE LOOK AT THIS CARBON, TWO, FOUR, SIX, EIGHT,
THE CARBON HAS AN OCTET.
NOW, DON'T USE THIS COUNTING SYSTEM WHEN YOU DO YOUR TAXES,
YOU'LL GO TO JAIL OR PAY BIG FINES,
BUT AMONG MOLECULES, IT'S OKAY.
THIS SHARED PAIR IS SHARED IN THE PUREST SENSE OF THE WORD.
THE HYDROGEN COUNTS THESE TWO ELECTRONS AS BELONGING TO IT
AND THE CARBON ALSO COUNTS THOSE TWO ELECTRONS.
SO WE CAN SEE THAT THE CARBON HAS ITS OCTET.
AND NOW WHEN WE GET TO THE OXYGEN, WE HAVE TWO, FOUR.
SO THE OXYGEN ONLY HAS FOUR VALENCE ELECTRONS RIGHT NOW.
LUCKILY, WE HAVE FOUR ELECTRONS LEFT OVER
AND WE REMEMBER THE BONDING PATTERN FOR OXYGEN IS TWO BONDS
AND TWO LONE PAIRS.
SO NOW WE'VE USED UP ALL OF OUR ELECTRONS
AND IF WE LOOK AT THIS LAST ATOM HERE, HYDROGEN HAS IT'S DUET.
SO, EVERY ATOM, CARBON AND OXYGEN BOTH HAVE THEIR OCTETS,
THE OXYGEN, TWO, FOUR, SIX, EIGHT.
ALL OF THE HYDROGENS HAVE THEIR DUETS.
WE ONLY USED THE FOURTEEN VALANCE ELECTRONS
AVAILABLE TO US AND WE HAVE OBSERVED THE BONDING PATTERNS
THAT WE RECOGNIZED FOR NEUTRAL COMPOUNDS.
SO WE'RE COMPLETE.
AND NOW I'D LIKE TO DO AN EXAMPLE
WHERE WE'RE GOING TO HAVE TO CREATE DOUBLE BONDS.
SO HERE WE HAVE A MOLECULE, SO WE SEE THE CARBONS.
CARBONS ARE ALWAYS GOING TO BE CENTRAL ATOMS.
SO WE'RE GOING TO HAVE THE TWO CARBON ATOMS.
EACH CARBON ATOM IS GOING TO BE SURROUNDED BY TWO HYDROGENS.
ALL RIGHT.
SO THERE'S HOW WE INTERPRET THE FORMULA AND ARRANGE THE ATOMS.
HOW MANY VALENCE ELECTRONS DO WE HAVE?
WELL, THERE'S TWO CARBONS AND THEY ARE GROUP FOUR
AND WE HAVE HYDROGENS, WE HAVE FOUR HYDROGENS
AND THEY'RE GROUP ONE, SO WE HAVE EIGHT PLUS FOUR,
ALTOGETHER WE HAVE 12 ELECTRONS.
SO THE FIRST THING WE WANT TO DO IS FORM SINGLE BONDS
TO HOOK ALL THE ATOMS TOGETHER.
OKAY. SO HOW MANY DID WE USE?
TWO, FOUR, SIX, EIGHT, TEN.
OH MY GOODNESS, WE'VE USED--
ALREADY USED UP 10 OF OUR ELECTRONS.
WE ONLY HAVE TWO ELECTRONS LEFT.
SO IN THIS CASE, WE COULD NAIVELY PUT THE TWO ELECTRON--
THE TWO ELECTRONS HERE TO CREATE A LONE PAIR
BUT WHAT DO WE KNOW ABOUT CARBON.
CARBON ALWAYS WANTS FOUR BONDS
AND WE KNOW IF WE PUT THE TWO ELECTRONS HERE,
THIS POOR CARBON IS GOING TO HAVE AN INSUFFICIENT NUMBER.
SO WE CAN JUST TAKE THOSE TWO ELECTRONS
AND I'LL CHANGE COLORS HERE.
WITH THE--THESE ELECTRONS AND WE'LL CREATE THE DOUBLE BOND.
SO WE HAVE IN BLUE, WE HAVE TWO, FOUR, SIX, EIGHT, TEN
AND THEN THERE'S THE LAST TWO ELECTRONS TO CREATE TWELVE.
SO, OUR CARBON ATOM ON THE LEFT, TWO, FOUR, SIX, EIGHT.
OUR CARBON IN THE LEFT HAS AN OCTET.
OUR CARBON ON THE RIGHT TWO, FOUR, SIX, EIGHT
AND EACH OF THE HYDROGENS HAS A DUET.
SO WE HAVE COMPLETED OUR SECOND LEWIS STRUCTURE.
SO NOW IF YOU'RE UP FOR THE CHALLENGE
GO AHEAD AND PAUSE THE VIDEO
AND PRACTICE DRAWING THE LEWIS STRUCTURES FOR THESE MOLECULES
AND I WILL FORGE AHEAD.
SO RECOGNIZING, WE SEE CARBON AND NITROGEN.
SO WE KNOW THESE TWO ATOMS WILL BE LINK TOGETHER
AS TWO CENTRAL ATOMS AND CARBON LIKES TO HAVE FOUR BONDS,
SO IT WILL BE SURROUNDED BY THE THREE HYDROGENS
AND THEN THE NITROGEN WILL ALSO BE SURROUNDED BY TWO HYDROGENS.
ALRIGHTY. HOW MANY ELECTRONS DO WE HAVE?
CARBON, FOUR, HYDROGEN THIS TIME WE HAVE FIVE HYDROGENS
THE EACH BRING ONE,
AND THEN WE HAVE NITROGEN GROUP FIVE,
SO ALL TOGETHER WE HAVE 14 ELECTRONS.
THE FIRST THING WE'RE GOING TO DO
IS HOOK ALL OF THE ATOMS TOGETHER,
TWO, FOUR, SIX, EIGHT, TEN, TWELVE.
AM I RIGHT?
SO HERE WE ARE WE ONLY HAVE TWO ELECTRONS LEFT
BUT IF WE TAKE ADVANTAGE OF OUR BONDING PATTERNS
WE NOTICED CARBON IS FULL WITH AN OCTET
AND ALL OF THE HYDROGENS ARE HAPPY
AND WE KNOW NITROGEN WANTS THREE BONDS WITH A LONE PAIR.
SO NOW WE USE THE LAST TWO ELECTRONS
AND WE HAVE FULFILLED THE OCTET RULE OR DUETS FOR HYDROGEN.
AND NOW THIS LAST ONE IS A VERY CHALLENGING PROBLEM OR COMPOUND
YOU CAN CONSIDER THIS A MASTERY
IF YOU'RE ABLE TO DO THIS LAST COMPOUND,
THEN YOU ARE VERY STRONG AND SOLID ON YOUR LEWIS STRUCTURES,
RIGHT.
SO THERE IS OUR TWO CARBONS,
WE KNOW THEY'RE GOING TO CONNECT TOGETHER
AND THEN THE CARBON ON THE LEFT IS SURROUNDED BY THREE HYDROGENS
AND THE CARBON ON THE RIGHT HAS ONE HYDROGEN AND ONE OXYGEN.
NOW, IT'S IMPORTANT TO NOTE THE DIFFERENCE HERE.
IN THIS EXAMPLE UP ABOVE, NOTICE THAT THE OXYGEN WAS LISTED FIRST
IMPLYING THAT THE OXYGEN WAS A CENTRAL ATOM TO THE HYDROGEN.
HERE, NOTICE THAT THE HYDROGEN AND OXYGEN ARE REVERSED.
THIS TELLS US THAT BOTH THE HYDROGEN AND THE OXYGEN
ARE BONDED TO THE CARBON, IT'S A VERY SUBTLE DIFFERENCE
BUT HAS HUGE IMPLICATIONS IN THE CHEMISTRY OF THE TWO MOLECULES.
ALRIGHTY.
SO WE'VE PRACTICED ARRANGING THE ATOMS
BASED OFF THE CHEMICAL FORMULA.
LET'S FIGURE OUT HOW MANY VALENCE ELECTRONS WE HAVE.
SO WE HAVE TWO CARBON ATOMS
AND THAT EACH CONTRIBUTE FOUR HYDROGEN,
WE HAVE FOUR HYDROGENS THAT EACH CONTRIBUTE ONE OXYGEN.
WE HAVE ONE OXYGEN THAT CONTRIBUTES SIX.
SO WE HAVE EIGHT AND TEN.
SO, WE HAVE 18 ELECTRONS AVAILABLE.
THE FIRST THING WE DO,
HOOK ALL THE ATOMS TOGETHER TO FORM THE MOLECULE.
TWO, FOUR, SIX, EIGHT, TEN, TWELVE.
OKAY. DID I COUNT THEM ALL RIGHT? YEAH. ALRIGHTY.
SO WE JUST USE TWO, FOUR, SIX, EIGHT, TEN, TWELVE.
SO WE JUST USE 12 ELECTRONS.
OKAY. SO THAT MEANS THAT WE HAVE SIX ELECTRONS LEFT.
AS WE LOOK AT THE CARBON ON THE LEFT WE CAN SEE IT HAS AN OCTET,
TWO, FOUR, SIX, EIGHT.
THE HYDROGENS ARE HAPPY.
SO IT'S THE RIGHT SIDE OF THE MOLECULE.
IF YOU LOOK AT THIS OXYGEN HERE
IT ONLY HAS TWO ELECTRONS RIGHT NOW.
SO IT'S A VERY SHORT OF AN OCTET.
AND OUR CARBON ONLY HAS TWO, FOUR, SIX.
SO TAKING ADVANTAGE OF OUR UNDERSTANDING
OF BONDING PATTERNS,
WE KNOW CARBON WANTS TO HAVE FOUR BONDS
AND OXYGEN WANTS TO HAVE TWO.
SO THE FIRST THING WE'RE GOING TO DO IS TAKE AWAY TWO ELECTRONS
TO MAKE A DOUBLE BOND.
NOW WE HAVE ONLY FOUR ELECTRONS LEFT.
BUT THEN IF WE CONTINUE BUILDING OFF OF OUR UNDERSTANDING
OF BONDING PATTERNS WE LOOK AT OXYGEN AND WE SEE, OKAY,
OXYGEN HAS THE TWO BONDS NOW
BUT WE KNOW IT ALSO NEEDS THE TWO LONE PAIRS.
SO THOSE FOUR ELECTRONS WILL CREATE THE LONE PAIRS
ON THE OXYGEN ATOM.
SO NOW WE HAVE--WE'VE USED ALL 18 OF OUR VALENCE ELECTRONS
AND BOTH CARBON ATOMS AND THE OXYGEN ATOM,
ALL THREE HAVE AN OCTET AND THE HYDROGENS HAVE THEIR DUETS.
SO WE HAVE CORRECTLY DRAWN THE LEWIS STRUCTURE
FOR THIS LAST CHALLENGING MOLECULE.
ALRIGHTY.
SO BEFORE YOU GET STARTED ON YOUR HOMEWORK LET'S LOOK AT--
LET'S WEAVE IN POLYATOMIC IONS.
SO THESE ARE VERY IMPORTANT
BECAUSE BIOLOGICALLY THEY-- CARBONATE AND PHOSPHATE
ARE VERY IMPORTANT BIOLOGICAL POLYATOMIC IONS
AND THIS IS A BLENDING OF COVALENT COMPOUNDS
BUT THEY HAVE A NET CHARGE.
SO THIS IS WHERE IONIC AND COVALENT COMPOUNDS
COME TOGETHER.
SO THE EASIEST WAY TO APPROACH THIS,
I BELIEVE, IS WE BASICALLY ADD OR REMOVE ELECTRONS
FROM THE TOTAL NUMBER OF VALENCE ELECTRONS
DEPENDING ON THE TYPE OF ION, RIGHT?
SO, THE MAIN THING TO KEEP IN MIND IS ELECTRONS ARE NEGATIVE.
SO FOR CATIONS, RIGHT, CATIONS HAVE A POSITIVE CHARGE
THAT MEANS WE TAKE AWAY ELECTRONS.
ANIONS HAVE NEGATIVE CHARGE, SO WE ADD ELECTRONS.
REMEMBER THAT AT THIS POINT NEUTRAL BONDING PATTERNS
NO LONGER APPLY.
AND BECAUSE WE'LL BE WORKING WITH PHOSPHATE, BIOLOGICALLY,
I WANTED TO REMIND YOU OF THE EXCEPTION TO THE OCTET RULE
BOTH SULFUR AND PHOSPHORUS, THEY ARE THIRD SHELL ELEMENTS
WHICH MEANS THAT THEY WRITE THAT AS WE GO FROM THE FIRST SHELL
TO THE SECOND SHELL TO THE THIRD SHELL,
THE SHELLS ARE LARGER AND LARGER AND CAN HOLD MORE ELECTRONS.
SO THE OCTET RULE IS REALLY A FOCUS
ON THE SECOND SHELL ELECTRONS OR THE SECOND SHELL ELEMENTS
AND SO HERE WE CAN HAVE EXPANDED OCTETS
WHICH BASICALLY MEANS MORE THAN EIGHT VALENCE ELECTRONS.
ALL RIGHT. OKAY. SO LET'S PRACTICE.
SO HERE WE HAVE CARBONATE.
THE CARBONS ARE GOING TO BE IN THE CENTER
AND IT WILL BE SURROUNDED BY THE THREE OXYGENS.
CARBON BRINGS IN FOUR, THE OXYGENS THERE'S THREE.
THEY EACH BRING IN SIX.
NOW WE HAVE A MINUS TWO CHARGE,
SO WE HAVE TWO ADDITIONAL ELECTRONS
BECAUSE OF THE MINUS TWO CHARGE SO THAT GIVES US 18, 20,
WE HAVE 24 VALENCE ELECTRONS.
WE BRING THE FIRST SIX ELECTRONS IN TO CONNECT THE ATOMS
INTO THE POLYATOMIC ION
SO THEN THAT LEAVES US WITH 18 ELECTRONS.
SO I'LL START FILLING THIS IN.
SO EVEN THOUGH I'VE MENTIONED
THAT THE NEUTRAL BONDING PATTERNS NO LONGER APPLY,
TYPICALLY, I SHOULD CLARIFY THAT, RIGHT.
FOR OXYGEN IN PARTICULAR, OKAY.
SO, CARBON DOES NOT LIKE TO HAVE A CHARGE,
SO IT'S GOING TO STILL HAVE THE OCTET.
IT DOESN'T MATTER WHICH OXYGEN ATOM WE PICK
WE'LL PUT OUR FOURTH BOND FOR CARBON.
SO WE'VE USED UP TWO MORE ELECTRONS.
NOW, WE'RE DOWN TO 16.
SO CARBON NOW HAS IT'S OCTET.
SO WE WANT TO BRING IN THE LAST 16 ELECTRONS
TO COMPLETE THE LEWIS STRUCTURE FOR CARBONATE.
SO THE DOUBLE BONDED OXYGEN IT HAS FOUR ELECTRONS
SO WE CAN BRING IN TWO LONE PAIRS
AND SO NOW THIS OXYGEN HAS EIGHT,
SO WE JUST USED UP FOUR MORE ELECTRONS THAT PUTS US AT 12.
SO FOR EACH OF THESE OXYGENS DOWN BELOW,
THEY ONLY HAVE TWO ELECTRONS.
THEY STILL NEED THE OCTET.
SO, TWO, FOUR, SIX, EIGHT, TEN AND TWELVE.
SO WE JUST USED UP THE LAST OF THE 12 ELECTRONS
AND SO NOW YOU HAVE AN OPTION, NOTICED THAT THESE OXYGEN ATOMS
THEY'RE NOT FOLLOWING THE NEUTRAL BONDING PATTERN.
NOTICED THAT ONE OF THE SHARED PAIRS IS A LONE PAIR,
SO WE CALL THIS A FORMAL CHARGE
WHICH YOU'RE NOT RESPONSIBLE FOR IN CHEM 309
BUT I'LL INTRODUCE IT ANYWAY.
SO WE CAN THINK OF EACH OF THESE OXYGEN ATOMS
CARRYING A MINUS ONE CHARGE.
THE OTHER WAY TO THINK ABOUT IT IS HOW WE DRAW,
WE CAN PUT SQUARE BRACKETS,
WE'RE GOING TO MAKE SURE THAT I ONLY USE TWO.
OKAY.
WE CAN PUT THE SQUARE BRACKETS
AND THEN WE CAN PUT A TWO MINUS HERE.
SO, EITHER WAY IT'S CORRECT AND IT JUST DEPENDS
ON HOW DEEPLY YOU WANT TO DIG INTO THE DETAILS,
KNOWING THAT AN OXYGENS ARE HOLDING MINUS ONE EACH
OR JUST RECOGNIZE THAT CARBONATE
OVERALL CARRIES A MINUS TWO CHARGE.
AND NOW PHOSPHATES GOING TO BE VERY IMPORTANT
BECAUSE OF ATP AND DNA.
SO WE WANT TO BE COMFORTABLE WITH PHOSPHATE.
SO IT'S WRITTEN IN THE FORM OF AN INORGANIC ACID.
SO HERE'S THE EXCEPTION I TALKED ABOUT, RIGHT,
HYDROGEN WILL NEVER BE A CENTRAL ATOM, IT CAN ONLY FORM ONE BOND.
SO THE PHOSPHATE IS THE CENTRAL ATOM.
SO, WE'LL PUT THE PHOSPHORUS IN THE CENTER
AND WE WILL SURROUND IT WITH THE FOUR OXYGENS
AND THEN THIS IS THE TRICKY PART,
THIS HYDROGEN IS AN ACIDIC HYDROGEN,
SO IT IS GOING TO BE BONDED TO ONE OF THE OXYGEN.
ALRIGHTY.
NOW, WE HAVE THE ARRANGEMENT OF THE ATOMS FROM THE FORMULA.
LET'S COUNT UP OUR VALENCE ELECTRONS.
HYDROGEN BRINGS IN ONE,
PHOSPHORUS BRINGS IN FIVE AS A GROUP FIVE,
OXYGEN FOUR TIMES SIX AND WE HAVE A MINUS TWO CHARGE
WHICH MEANS WE ADD TWO ELECTRONS.
SO ALL TOGETHER 24, 25, 26, 26 AND 6 WOULD BE 32 ELECTRONS.
ALL RIGHTY.
LET'S HOOK THESE ATOMS TOGETHER TO CREATE OUR POLYATOMIC ION.
TWO, FOUR, SIX, EIGHT, TEN.
WE JUST USED UP 10 ELECTRONS WE'RE DOWN TO 22.
HERE, BECAUSE THIS OXYGEN HAS THE TWO BONDS,
LET'S GO AHEAD AND ADD THE LONE PAIRS,
SO WILL BE MINUS FOUR ELECTRONS, SO THAT BRINGS US DOWN TO 18
AND NOW WE SEE THAT WE HAVE ONLY A MINUS TWO CHARGE.
SO WE CAN TAKE ADVANTAGE OF OUR EXPERIENCE WITH CARBONATE.
THAT'S GOING TO LET US KNOW THAT TWO OF THE OXYGENS
ARE CARRYING NEGATIVE CHARGE.
SO, WE CAN GO AHEAD AND ADD THE SIX VALENCE ELECTRONS.
SO THAT'S MINUS 12, SO THAT PUTS US AT SIX ELECTRONS BEING LEFT
AND SO WE HAVE SIX ELECTRONS LEFT
AND WE NEED TO GET THIS OXYGEN FILLED WITH IT'S OCTET.
SO, THE ONLY WAY THAT WE CAN GET THIS OXYGEN TO HAVE AN OCTET
WITH SIX ELECTRONS, RIGHT,
AND WE'LL TAKE ADVANTAGE OF OUR BONDING PATTERNS, RIGHT,
A DOUBLE BOND AND TWO LONE PAIRS.
SO, NOW WE'VE USED UP ALL OF OUR VALENCE ELECTRONS
AND HERE'S AN EXAMPLE,
THE PHOSPHORUS HAS TWO, FOUR, SIX, EIGHT, TEN.
PHOSPHORUS IS ALLOWED TO HAVE THE EXPANDED OCTET
BECAUSE IT'S A THIRD SHELL ELEMENT.
IT OCCURS IN THE THIRD PERIOD
WHICH IS ROWS OF THE PERIODIC TABLE.
SO EACH OF THE OXYGENS HAS AN OCTET
AND THE HYDROGEN HAS A DUET.
AND OF COURSE IF WE WANT IT TO WRITE THIS OTHER WAY,
WE COULD SIMPLY, OOPS, PUT A SMALL BOND THERE.
WE WANT TO MAKE SURE TO HAVE ALL OUR LONE PAIRS,
SO WE GET FULL CREDIT FOR OUR WORK.
WE COULD PUT THIS INTO SQUARE BRACKETS AND SAY MINUS TWO.
SO, EITHER WAY, EITHER REPRESENTATION WILL BE CORRECT.
SO I HOPE THAT YOU HAVE FOUND THIS VIDEO TUTORIAL HELPFUL
AND THAT YOU'LL TAKE SOME TIME NOW
TO WORK A FEW PRACTICE PROBLEMS TO GET STRONG
AT BUILDING LEWIS STRUCTURES FOR ORGANIC MOLECULE.