- WELCOME TO THE VIDEO TUTORIAL ON MOLECULAR SHAPE
AND APPLICATION OF VSEPR,
THE VALENCE-SHELL ELECTRON-PAIR REPULSION THEORY.
ALL RIGHTY.
SO LET'S MAKE SURE THAT WE'RE ALL CLEAR
ON WHAT WE NEED. RIGHT.
SO THE SHAPE OF THE MOLECULES
IS THE THREE DIMENSIONAL ARRANGEMENT.
LET'S SEE, ARRANGEMENT, RIGHT, OF THE ATOMS.
ALL RIGHT.
SO THAT'S THE MOLECULAR SHAPE.
SO THE MOLECULAR SHAPE IS GOING TO DEPEND ON THE ELECTRON
GEOMETRY SO WE WILL BUILD UP TO THAT.
BUT LET'S SAY, LIKE WHY DO WE EVEN CARE ABOUT THIS?
WELL, IT'S VERY IMPORTANT, THE SHAPES OF OUR MOLECULES
AND THEY'RE CHARGE DISTRIBUTION IS A WAY
THAT OUR CELLS CAN COMMUNICATE WITH EACH OTHER.
A COUPLE EXAMPLES IS HORMONES AT RECEPTORS SITES
ON THE SURFACE OF CELLS AND DRUG MOLECULES INTERACTING
WITH OTHER MOLECULES WITHIN THE CELL.
SO IT'S VERY IMPORTANT
THAT WE CAN VISUALIZE THESE THREE-DIMENSIONAL SHAPES.
SO YOU CAN THINK OF THIS AS BRAIN YOGA.
SO WE WILL BEGIN WITH THE ELECTRON GEOMETRY.
THIS WILL BE THE ARRANGEMENT
OF THE ELECTRONS AROUND THE CENTRAL ATOM.
SO THIS IS GOING TO MAKE USE OF OUR LEWIS STRUCTURES. RIGHT.
SO THE LEWIS STRUCTURES ARE HOW WE'RE GOING TO FIGURE OUT
THE ELECTRON GEOMETRY.
ONCE WE KNOW THE ELECTRON GEOMETRY,
THEN WE CAN MAKE PREDICTIONS ABOUT THE MOLECULAR SHAPE
WHICH WILL BE THE ARRANGEMENT
OF THE ATOMS AROUND THE CENTRAL ATOM.
SO THERE'S REALLY ONLY ONE RULE HERE,
ELECTRONS WANT TO STAY AS FAR APART AS POSSIBLE
BECAUSE LIKE CHARGES REPEL.
SO WHEN WE'RE LOOKING AT THE CENTRAL ATOM
OF A LEWIS STRUCTURE,
WE CAN PREDICT THE ELECTRON GEOMETRY.
IF THERE ARE FOUR GROUPS
OF ELECTRONS AROUND THE CENTRAL ATOM,
WE DESCRIBE THAT AS A TETRAHEDRAL ELECTRON GEOMETRY.
NOW THIS IS THE ONLY ONE THAT'S TRULY THREE-DIMENSIONAL.
SO TO DRAW THIS THREE-DIMENSIONAL SHAPE,
WE WILL HAVE OUR CENTRAL ATOM WHICH WILL USUALLY BE CARBON
AND WE CAN DRAW TWO ATOMS IN THE PLANE AND THEN
ONE ATOM IS GOING TO POKE OUT OF THE PAPER AS A WEDGE.
SO THIS WILL--
THIS POKES OUT AND THEN
WE'LL HAVE A DASHED LINE REPRESENTING POKING IN,
POKES IN. ALL RIGHT.
SO LET ME GRAB A MODEL
AND SO THAT I CAN SHOW YOU THIS TETRAHEDRAL GEOMETRY.
SO HERE IS THE TETRAHEDRAL SHAPE
AND WE CAN SEE THAT NO MATTER HOW WE SPIN THE MOLECULE,
ALL OF THESE ELECTRON GROUPS,
THESE SINGLE BONDS ARE RADIATING OUT INTO THE CORNER
OF THE TETRAHEDRON.
SO THE WAY I'VE DRAWN IT ON THE PAPER,
WE'VE PUT THESE THREE ATOMS IN THE PLANE,
AND SO THE ATOMS IN THE PLANE GET THE STRAIGHT LINE.
NOW, THIS ATOM THAT'S POKING OUT TOWARDS YOU, IT GOT THE WEDGE,
AND THEN THIS ATOM HERE,
THIS BOND GOT THE DASHED LINE SHOWING
THAT IT'S GOING BACK INTO THE PAPER.
SO IF I SLIDE THE PAPER OVER, RIGHT,
WE CAN SEE THE WEDGE POKING OUT AND NOW THE DASH IS HIDDEN
BECAUSE IT'S BEHIND THE PLANE OF THE PAPER.
ALL RIGHTY.
SO NOW WE'LL GO BACK TO OUR NOTES.
ALL RIGHTY.
THE OTHER TWO ELECTRON GEOMETRIES,
ALL THE ANIMALS REST--
ATOMS REST IN THE SAME PLANE.
SO WE COULD HAVE TRIGONAL PLANAR AND LINEAR.
NOW ASSOCIATED WITH THESE ELECTRON
GEOMETRIES ARE THE BOND ANGLES.
SO WITH THE TETRAHEDRAL, THE BOND ANGLE FOR--
BETWEEN EACH AND EVERYONE OF THESE ATOMS,
THEY'RE ALL ABOUT A HUNDRED AND NINE DEGREES.
WITH THE TRIGONAL PLANAR, THEY'RE ALL 120 AND FOR LINEAR,
IT'S A HUNDRED AND EIGHTY.
ALL RIGHT.
SO FROM THE LEWIS STRUCTURE, WE DETERMINE HOW MANY GROUPS
OF ELECTRONS ARE AROUND THE CENTRAL ATOM
AND THAT TELLS US OUR ELECTRON GEOMETRY.
THERE ARE ONLY THREE POSSIBLE ELECTRON GEOMETRIES,
TETRAHEDRAL, TRIGONAL PLANAR, AND LINEAR.
AND THEY HAVE THESE CORRESPONDING BOND ANGLES.
SO NOW LET'S EXPLORE THIS WITH ACTUAL MOLECULES
AND THEN CONNECT THE ELECTRON GEOMETRY TO THE MOLECULAR SHAPE.
SO WE'LL LOOK ON OUR NEXT HANDOUT
AND WE'LL START WITH THE SIMPLEST
AND WE'LL WORK OUR WAY UP TO THE MORE COMPLEX.
ALL RIGHT.
SO REMEMBER THAT THE ELECTRON GEOMETRY
IS GOING TO COME FROM THE LEWIS STRUCTURE. ALL RIGHTY.
SO FIRST WE'LL LOOK AT WHEN WE HAVE A LINEAR ELECTRON GEOMETRY,
NOW THIS CAN OCCUR IN TWO DIFFERENT WAYS.
WE COULD HAVE TWO DOUBLE BONDS AROUND THE CENTRAL ATOM
SO THAT IT COULD BE LIKE CARBON DIOXIDE
AND SO THIS CAN BE AN INTERESTING THING
WHEN WE'RE FIRST WORKING
WITH THIS THEORY IS THAT THIS IS A DOUBLE BOND,
IT'S CONSIDERED ONE GROUP OF ELECTRONS.
SO WE HAVE TWO GROUPS OF ELECTRONS
AND THEY'RE GOING TO ORIENT THEMSELVES TO BE AS FAR APART
AS POSSIBLE SO WE GET OUR 180 DEGREE BOND ANGLES.
NOW, WHEN WE'RE TALKING ABOUT THE ELECTRON GEOMETRY,
WE'RE LOOKING AT THE ELECTRONS.
WHEN WE'RE TALKING ABOUT MOLECULAR SHAPE,
NOW WE'RE ONLY LOOKING AT ATOMS, ATOMS ONLY.
SO IF WE LOOK AT THE ATOMS, THE CARBON AND THE TWO OXYGENS,
WE SEE THAT THEY'RE ALL FORMING A LINE.
SO THE MOLECULAR SHAPE IS THE SAME LINEAR
AS THE ELECTRON GEOMETRY.
THAT'S WHAT MAKES THIS ONE THE SIMPLEST.
NOW HERE'S ANOTHER EXAMPLE.
WHAT IF WE HAD A--
WE COULD HAVE A TRIPLE BOND AND A SINGLE BOND.
SO HERE'S AN EXAMPLE.
IT'S LITTLE BIT LARGER MOLECULE.
WE ACTUALLY HAVE TWO CENTRAL ATOMS.
SO IF WE FOCUS ON THE CARBON ON THE LEFT,
RIGHT, IF WE LOOKED AT THE CARBON ON THE LEFT,
WE WOULD SEE THAT THERE'S A SINGLE BOND AND A TRIPLE BOND.
SO THIS HYDROGEN AND THE TWO CARBONS ARE
ALL GOING TO BE IN A STRAIGHT LINE SO WE WOULD DESCRIBE THAT--
THE MOLECULAR SHAPE AS LINEAR.
SO NOTICE THAT A GROUP OF ELECTRONS CAN BE A SINGLE BOND,
A DOUBLE, OR A TRIPLE BOND,
AND THAT THE BOND ANGLE HERE WILL BE THE 180 DEGREES.
SO LET ME SHOW YOU A MODEL OF THIS ARRANGEMENT.
SO HERE'S A MODEL OF THE LAST MOLECULE WE WORKED WITH,
THE BLACK SIGNIFY THE CARBONS, THE WHITES, THE HYDROGEN.
SO IF WE FOCUS HERE, OF THE ATOM-- THIS CARBON ATOM,
IF WE CONSIDER IT THE CENTRAL ATOM,
WE SEE THAT THERE'S TWO GROUPS OF ELECTRONS,
A SINGLE BOND AND A TRIPLE BOND.
IF YOU WANT,
WE CAN SIMPLY IGNORE THIS HYDROGEN FOR A MINUTE.
SO THE CENTRAL ATOM THAT I'M HOLDING HERE, THIS CARBON,
WE WOULD DESCRIBE IT AS HAVING A LINEAR ELECTRON GEOMETRY
BECAUSE THERE ARE TWO GROUPS OF ELECTRONS,
A SINGLE BOND AND A TRIPLE BOND.
AND WE WOULD DESCRIBE IT AS HAVING A LINEAR MOLECULAR SHAPE
BECAUSE NOTICE
THAT ALL OF THE ATOMS ARE IN A STRAIGHT LINE.
ALL RIGHT.
NOW WE'LL MOVE TO THE NEXT SET--
THE NEXT ELECTRON GEOMETRY,
THE TRIGONAL PLANAR.
SO HERE, WE HAVE THREE GROUPS
OF ELECTRONS AROUND THE CENTRAL ATOM.
SO FOR EXAMPLE,
IF WE COULD HAVE ONE DOUBLE BOND AND TWO SINGLE BONDS.
SO FOR EXAMPLE, IF HAVE CARBON WITH A DOUBLE BOND
TO SULPHUR AND THEN A HYDROGEN AND A HYDROGEN.
IF WE LOOK AT THIS CARBON,
IT HAS THREE GROUPS OF ELECTRONS,
TWO SINGLE BONDS AND A DOUBLE BOND.
SO THESE BOND ANGLES WILL ALL BE 120 DEGREES.
ANOTHER EXAMPLE TO MATCH OUR MODELS,
I'LL DO OVER HERE,
WE'LL DO A SLIGHTLY LARGER MOLECULE.
WE'LL LOOK AT WHEN WE HAVE TWO CARBON ATOMS. RIGHT.
SO IF WE FOCUS ON THE CARBON ATOM
ON THE LEFT AS OUR CENTRAL ATOM,
THAT MEANS WE'RE FOCUSING OUR ATTENTION RIGHT HERE.
SO WE WOULD SEE THAT THIS CENTRAL ATOM
IS SURROUNDED BY ONE,
TWO, THREE, THREE GROUPS OF ELECTRONS,
TWO SINGLE BONDS AND A DOUBLE BOND.
SO WE WOULD DESCRIBE THE ELECTRON GEOMETRY
AROUND THIS CARBON,
THIS CARBON, OR THIS CARBON AS BEING TRIGONAL PLANAR.
AND THESE BOND ANGLES WILL ALL BE APPROXIMATELY
A HUNDRED AND TWENTY DEGREES.
SO THE MOLECULAR SHAPE MATCHES THE ELECTRON GEOMETRY.
SO THE MOLECULAR SHAPE
WOULD ALSO BE DESCRIBED AS TRIGONAL PLANAR.
HOWEVER, NOW THERE IS THE POSSIBILITY
OF HAVING LONE PAIRS.
SO NON-BONDING--
A NON-BONDING PAIR IS ALSO CALLED A LONE PAIR.
SO WHEN WE HAVE A LONE PAIR AROUND THE CENTRAL ATOM,
THIS IS HOW WE GET A DIFFERENCE BETWEEN THE ELECTRON GEOMETRY
AND THE MOLECULAR GEOMETRY.
SO LET ME DRAW AN EXAMPLE HERE.
SO FOR EXAMPLE,
IF WE HAD SULFUR DIOXIDE, ALL RIGHT.
SO IF WE LOOK AT OUR CENTRAL ATOM,
LONE PAIRS ARE ALSO CONSIDERED A GROUP
OF ELECTRONS SO WE HAVE LONE PAIRS,
THAT'S THE FIRST GROUP, A SINGLE BOND,
THAT'S THE SECOND GROUP,
AND THE DOUBLE BOND IS THE THIRD GROUP.
SO THESE TWO CENTRAL ATOMS SHARE THE SAME ELECTRON GEOMETRY.
WE DESCRIBE IT AS TRIGONAL PLANAR.
HOWEVER, WHEN WE GO TO THE MOLECULAR SHAPE,
THEN REMEMBER THAT IT'S ATOMS ONLY.
SO WHEN WE LOOK AT THIS ATOMS,
THEY FORM A BENT OR ANGULAR ARRANGEMENT.
SO NOW LET ME SHOW YOU SOME MODELS
TO ILLUSTRATE THE TRIGONAL PLANAR ELECTRON GEOMETRY
AND MOLECULAR SHAPE.
SO HERE'S OUR FIRST EXAMPLE,
HERE'S THE TWO CARBONS WITH THE FOUR HYDROGENS.
SO IF WE WANT, WE'LL JUST DISREGARD.
SO HERE'S OUR CENTRAL ATOM,
SO WE CAN SEE THAT IT HAS TWO SINGLE BONDS AND A DOUBLE BOND,
THAT CREATES THE THREE GROUPS OF ELECTRONS.
SO WE DESCRIBE THE ELECTRON GEOMETRY AROUND THE CENTRAL ATOM
AS TRIGONAL PLANAR.
NOTICE THAT ALL OF THE ATOMS ARE LYING IN THE SAME PLANE.
AND THIS MOLECULE HAS SYMMETRY SO NOTICE THAT WE CAN--
THAT ALL SIX ATOMS ARE ALL LYING IN THE SAME PLANE.
SO EACH ONE OF THESE CARBON ATOMS WOULD BE
DESCRIBED AS HAVING TRIGONAL PLANAR ELECTRON GEOMETRY
AND THE MOLECULAR SHAPE. RIGHT.
THE ATOMS ARE RADIATING OUT INTO THE CORNERS
OF THE TRIANGLE AND WILL HAVE THE 120 DEGREE BOND ANGLES.
NOW WHERE THE DIFFERENCE OCCURS IS,
THIS WOULD BE THE SULFUR,
YELLOW IS SULFUR AND THE REDS ARE THE OXYGENS.
SO NOW HERE, THE LONE PAIR WILL SYMBOLIZE--
IT'S IN AN ORBITAL SO THERE'S THE LONE PAIR,
THE TWO NON-BONDING ELECTRONS.
SO IF WE COMPARE THESE CENTRAL ATOMS,
WE CAN SEE THAT EACH CENTRAL ATOM IS--
HAS THE SAME ELECTRON GEOMETRY.
THERE ARE THREE GROUPS OF ELECTRONS.
SO HERE, WE HAVE THE LONE PAIR, THE SINGLE BOND,
AND THE DOUBLE BOND.
THESE ELECTRONS WANT TO BE AS FAR APART FROM EACH OTHER
AS POSSIBLE.
THIS ELECTRON GEOMETRY WOULD ALSO BE CALLED TRIGONAL PLANAR.
HOWEVER, WHEN WE GO TO THE MOLECULAR SHAPE,
NOW WE IGNORE THE LONE PAIR,
AND SO WHEN WE LOOK AT THIS SHAPE,
WE SEE THAT THE THREE ATOMS ARE NOT IN A STRAIGHT LINE.
SO WE WOULD DESCRIBE THE MOLECULAR SHAPE
AROUND THIS CENTRAL ATOM AS BENT.
SO IF WE COMPARE THAT TO OUR LINEAR,
WE CAN SEE THE DIFFERENCE
BETWEEN ALL OF THE ATOMS BEING IN A STRAIGHT LINE VERSUS BENT.
SO THE SIMILARITY HERE THOUGH IS SAME ELECTRON GEOMETRY,
DIFFERENT MOLECULAR SHAPE.
WHAT THESE TWO HAVE IN COMMON?
THEY'RE BOTH TRIGONAL PLANAR ELECTRON GEOMETRIES.
NOW, WE'LL LOOK AT THE LAST GEOMETRY
WE'LL FOCUS ON FOR THIS COURSE.
WE WILL LOOK AT THE TETRAHEDRON
OR TETRAHEDRAL ELECTRON GEOMETRY.
NOW WE HAVE FOUR GROUPS OF ELECTRONS.
SO WE'VE LOOKED AT THIS SHAPE ALREADY BUT WE'LL REVIEW IT.
SO CARBON, WHEN CARBON HAS FOUR SINGLE BONDS,
THEY'RE GOING TO RADIATE OUT
INTO THE CORNERS OF THE TETRAHEDRON.
NOTICE THAT EVERY TIME THERE'S NO LONE PAIRS, RIGHT,
WHEN THE CENTRAL ATOM HAS NO LONG PAIRS,
THE MOLECULAR SHAPE
AND THE ELECTRON GEOMETRY ARE THE SAME.
SO HERE, WE HAVE FOUR GROUPS OF ELECTRONS, FOUR SINGLE BONDS
THAT CREATES THE TETRAHEDRAL ELECTRON GEOMETRY
WITH THE 109 DEGREE BOND ANGLES
AND BECAUSE OUR CENTRAL ATOM HAS NO LONE PAIRS,
THE MOLECULAR SHAPE IS THE SAME AS THE ELECTRON GEOMETRY.
HOWEVER, IT'S POSSIBLE TO HAVE THE NON-BONDING LONE PAIRS.
SO FOR EXAMPLE, IF WE LOOK AT AMMONIA,
SO THEN WE WOULD HAVE THE THREE HYDROGENS COMING
OFF OF THE NITROGEN AND THEY'RE GOING TO GO INTO THE CORNERS
OF THE TETRAHEDRON AND THEN WE HAVE THE LONE PAIR.
SO REMEMBER THOSE LONE PAIR ELECTRONS ARE WHIRLING ABOUT,
CREATING A FORCE FIELD OF NEGATIVE CHARGE.
SO HERE WE STILL--
THESE TWO MOLECULES,
WHAT DO THEY HAVE THAT'S THE SAME?
THEY BOTH HAVE THE SAME ELECTRON GEOMETRY.
WHETHER IT'S FOUR SINGLE BONDS OR SINGLE BONDS AND A LONE PAIR.
THAT CREATES THE FOUR GROUPS OF ELECTRONS.
AND NOW THE LAST VARIATION
OF OUR TETRAHEDRAL ELECTRON GEOMETRY
WOULD BE WHEN WE HAVE TWO LONE PAIRS--
TWO NON-BONDING PAIRS AND THIS IS WHAT WE SEE WITH WATER.
WATER HAS--
THE OXYGEN ON WATER HAS THE TWO SINGLE BONDS TO THE HYDROGEN
AND THEN THE TWO LONE PAIRS.
SO ONCE AGAIN, ONE, TWO, THREE,
FOUR, FOUR GROUPS OF ELECTRONS.
ELECTRONS FIND EACH OTHER REPULSIVE
SO THEY'RE GOING TO MOVE AS FAR APART AS POSSIBLE
AND WHEN THERE'S FOUR GROUPS OF ELECTRONS,
THE TETRAHEDRAL ELECTRON GEOMETRY
IS AS FAR APART AS THEY CAN GET.
AS SOON AS WE HAVE LONE PAIRS PRESENT,
THEN WE GET THE VARIATION.
NOTICE THAT AMMONIA,
IF WE'RE FOCUSING ON THE ATOMS ONLY,
HAS A DIFFERENT SHAPE FROM THE METHANE.
HERE, IT'S CALLED--
THERE'S-- NOW THERE ARE--
IT'S TRIGONAL BECAUSE THERE'S ONLY
THREE HYDROGENS SURROUNDING--
OR THREE ATOMS SURROUNDING THE CENTRAL ATOM.
SO WE HAVE TRIGONAL BUT IN THIS EXAMPLE,
THE NITROGEN IS NOT IN THE SAME PLAIN AS THE HYDROGEN.
SO IT'S CREATED THE PYRAMID.
AND WITH WATER,
THIS IS ANOTHER EXAMPLE OF THE BENT OR ANGULAR.
THE DIFFERENCE HERE
THOUGH IS BECAUSE WE HAVE THE TETRAHEDRAL ELECTRON GEOMETRY,
THIS IS GOING TO BE ABOUT A HUNDRED AND NINE DEGREES
VERSUS OUR EXAMPLE UP HERE, RIGHT?
THIS HAD THE TRIGONAL PLANAR GEOMETRY.
SO THIS BENT MOLECULE HAS A DIFFERENT BOND ANGLE
THAN THIS IS BENT MOLECULE
BECAUSE OF THE DIFFERENCES IN THE ELECTRON GEOMETRY.
SO HERE WE'RE BACK TO THE NAMES OF BENT OR ANGULAR.
SO NOW THAT WE'VE DESCRIBED IT FOR YOU IN PICTURES,
LET'S TAKE A LOOK AT THE MODELS.
SO HERE IS OUR TETRAHEDRAL ELECTRON GEOMETRY
AND MOLECULAR SHAPE.
SINCE THERE ARE NO LONE PAIRS AROUND THE CENTRAL ATOM
AND IT'S HELPFUL WHEN LOOKING AT THIS SHAPE,
RIGHT, TO THINK ABOUT PUTTING AS MANY ATOMS AS POSSIBLE
INTO THE PLAIN OF THE PAGE.
SO THE WE SEE ONE POKING OUT AND ONE COMING BACK.
NOW, THIS WOULD BE--
THIS IS OUR AMMONIA EXAMPLE.
SO THE BLUE IS THE NITROGEN AND THE WHITES ARE THE HYDROGENS
BUT WE HAVE TO REMEMBER THERE'S THE LONE PAIR.
SO THOSE TWO NON-BONDING ELECTRONS CREATE A FORCE FIELD
OF REPULSION THAT BENDS DOWN THESE SINGLE BONDS.
IT'S IMPORTANT
THAT YOU CAN DISTINGUISH BETWEEN THE TWO TYPES OF TRIGONALS.
SO IF WE LOOK HERE,
I'LL TAKE THESE HYDROGENS OFF TO TRY TO MINIMIZE THE CONFUSION.
HERE WE HAVE TO CENTRAL ATOMS.
THEY BOTH CAN BE DESCRIBED AS TRIGONAL BUT TRIGONAL
IS NOT ENOUGH INFORMATION TO DESCRIBE THEM COMPLETELY.
NOTICE WITH THIS MODEL HERE,
THE TRIGONAL PLANAR HOW ALL OF THE ATOMS--
ALL FOUR ATOMS ARE IN THE SAME PLANE,
TRIGONAL PLANAR VERSUS THE AMMONIA
OR THE NITROGEN TRIGONAL PYRAMIDAL.
SEE HOW THE CENTRAL ATOM IS RAISED
ABOVE THE FOUR HYDROGEN ATOMS?
SO THEY'RE BOTH TRIGONAL.
BOTH OF THEM HAVE THE CENTRAL ATOMS SURROUNDED
BY THREE OTHER ATOMS
BUT WE WOULD DESCRIBE THIS AS TRIGONAL PLANAR
AND TRIGONAL PYRAMIDAL.
AND THEN THE LAST OPTION IS OUR WATER MOLECULE.
THE OXYGEN IS RED.
THE HYDROGENS ARE WHITE.
WE HAVE THE TWO SINGLE BONDS
AND THEN WE WOULD HAVE THE LONE PAIRS.
SO NOTICE IF WE FOCUS ON THE ELECTRONS,
RIGHT, TETRAHEDRAL,
TETRAHEDRAL, AND TETRAHEDRAL.
WHAT DO THESE THREE--
WHAT DO THESE ALL THREE MOLECULES HAVE IN COMMON?
THEY ARE ALL TETRAHEDRAL ELECTRON GEOMETRY.
WHEN THERE ARE NO LONE PAIRS AROUND THE CENTRAL ATOM
THE MOLECULAR SHAPE IS THE SAME.
WHEN WE HAVE ONE LONE PAIR,
THE MOLECULAR SHAPE BECOMES TRIGONAL PYRAMIDAL
AND WHEN WE HAVE TWO LONE PAIRS,
THE MOLECULAR SHAPE BECOMES BENT.
SO WE SEE THAT THEY'RE NOT IN A LINE.
ALL RIGHTY. OKAY.
SO THERE WE--
WE'LL GO BACK TO THE NOTES
AND WE'LL TIE THIS ALL TOGETHER WITH SOME FOLLOW-UP QUESTIONS.
OKAY. SO THE OVERALL STRATEGY,
RIGHT, IS THAT THE ELECTRON GEOMETRY IS GOING TO DETERMINE
THE MOLECULAR SHAPE, RIGHT?
SO THE FIRST THING WE HAVE TO DO
IS DRAW THE LEWIS STRUCTURE, RIGHT?
THIS IS HOW WE KNOW, RIGHT,
THE NUMBER OF ELECTRON GROUPS, RIGHT?
AND THEN IT'S IMPORTANT
TO RECOGNIZE THAT ELECTRON GROUPS CAN BE IN LONE PAIRS,
RIGHT, SINGLE BINDS, DOUBLE BONDS OR TRIPLE BONDS.
ALL RIGHT.
SO WE'RE ONLY GOING TO FOCUS ON THREE ELECTRON GEOMETRIES,
TETRAHEDRAL, TRIGONAL PLANAR AND LINEAR.
WHEN WE WANT TO LOOK FOR THE MOLECULAR SHAPE, REMEMBER NOW,
WE'RE LOOKING FOR ATOMS ONLY.
SO WE IGNORE THE LONE PAIRS AND SO THE--
WE CAN GET A DIFFERENCE BETWEEN THE MOLECULAR SHAPE
AND THE ELECTRON GEOMETRY WHEN THERE ARE LONE PAIRS PRESENT.
OKAY? SO THE LONE PAIRS,
THEY CREATE THE DIFFERENCE BETWEEN THE MOLECULAR SHAPE
AND THE ELECTRON GEOMETRY. ALL RIGHT?
SO NOW, LET'S PRACTICE THIS A BIT
AND TIE IT ALL TOGETHER.
SO LEWIS, RIGHT?
SO THE FIRST THING WE HAVE TO DO
IS WE HAVE TO HAVE THE LEWIS STRUCTURE.
WE HAVE TO KNOW HOW MANY GROUPS OF ELECTRONS WE HAVE.
ALL RIGHTY.
SO WHEN WE LOOK HERE, PHOSPHORUS IS OUR CENTRAL ATOM
AND IT'S GOING TO BE SURROUNDED BY THE THREE HYDROGENS AND THEN
WE HAVE TO KNOW HOW MANY VALENCE ELECTRONS WE HAVE.
SO FOR PHOSPHORUS, IT BRINGS IN FIVE,
THE HYDROGENS, 3 X 1 SO WE HAVE EIGHT VALENCE ELECTRONS.
THE FIRST SIX GO INTO CONNECTING OUR ATOMS.
WE'VE GOT ONE PAIR LEFT WHICH WORKS OUT PERFECT
BECAUSE PHOSPHORUS ONE'S AN OCTET.
SO IF WE LOOK AT PHOSPHORUS,
THE ELECTRON GEOMETRY, ONE,
TWO, THREE, FOUR GROUPS.
WE WOULD DESCRIBE THIS AS A TETRAHEDRAL ELECTRON GEOMETRY.
HOWEVER, FOR THE MOLECULAR SHAPE, WE HAVE A LONE PAIR.
WITH ONE LONE PAIR PRESENT,
THE MOLECULAR SHAPE IS GOING TO BE TRIGONAL PYRAMIDAL.
AND REMEMBER THAT THE BOND ANGLES ARE DETERMINED
FROM THE ELECTRON GEOMETRY SO THEY WOULD BE ABOUT 109 DEGREES.
ALL RIGHT.
LET'S WORK ONE MORE PRACTICE PROBLEM
BEFORE YOU GET STARTED ON YOUR HOMEWORK.
CH2O, RIGHT?
SO THE CARBON IS THE CENTRAL ATOM AND THEN THE HYDROGENS
AND THE OXYGEN SURROUND THE CARBON.
HOW MANY VALENCE ELECTRONS DO WE HAVE?
THE CARBON GIVES US FOUR.
THE HYDROGENS, THERE'S TWO.
THEY EACH GIVE US ONE AND THE OXYGEN GIVES US SIX.
SO ALTOGETHER WE HAVE 12 ELECTRONS.
THE FIRST THING WE DO IS WE USE SIX ELECTRONS
TO CONNECT ALL OF THE ATOMS.
NOW WE HAVE SIX ELECTRONS LEFT.
USING OUR UNDERSTANDING OF BONDING PATTERNS
AND IF DRAWING THESE LEWIS STRUCTURES,
YOU'RE FINDING CONFUSING, THERE IS A VIDEO TUTORIAL
THAT DOES FOCUS SPECIFICALLY ON DRAWING THE LEWIS STRUCTURES.
SO IF YOU NEED MORE PRACTICE WITH DRAWING LEWIS STRUCTURES,
PLEASE TAKE ADVANTAGE OF THAT RESOURCE.
WE HAVE SIX ELECTRONS HERE THAT ARE LEFT OVER.
SO WE KNOW CARBON HAS TO HAVE FOUR BONDS
AND OXYGEN WANTS TWO BONDS.
WELL, WE'RE GOING TO NEED TO BRING IN THAT FOURTH BOND
FOR CARBON AND THAT SECONDS BOND FOR OXYGEN.
SO THAT LEAVES US THEN WITH FOUR ELECTRONS
AND THEN TAKING ADVANTAGE
OF OUR ORGANIC COMPOUND BONDING PATTERNS,
WE KNOW OXYGEN WANTS TWO BONDS AND TWO LONE PAIRS.
SO NOW WE HAVE USED UP ALL 12 OF OUR VALENCE ELECTRONS
AND WE'VE CREATED AN OCTET FOR CARBON,
TWO, FOUR, SIX, EIGHT,
AND AN OCTET FOR OXYGEN, TWO,
FOUR, SIX, EIGHT AND OF COURSE HYDROGEN JUST WANTS THE DUET.
SO NOW IF WE LOOK AT CARBON AS THE CENTRAL ATOM,
WE SEE ONE, TWO, THREE GROUPS OF ELECTRONS,
TWO SINGLE BONDS AND A DOUBLE BOND.
THAT WILL MAKE OUR ELECTRON GEOMETRY TRIGONAL PLANAR.
NOW, THERE ARE LONE PAIRS IN THIS COMPOUND,
BUT THEY ARE NOT ON THE CENTRAL ATOM.
ANYTIME THE CENTRAL CAN--
HAS ALL BONDS AND NO LONE PAIRS,
WE KNOW THAT THE MOLECULAR SHAPE IS IDENTICAL
TO THE ELECTRON GEOMETRY,
AND THE BOND ANGLES FOR TRIGONAL PLANAR ARE ABOUT 120 DEGREES.
SO HERE ARE--
SO THAT CONCLUDES THE OVERVIEW OF DETERMINING ELECTRON GEOMETRY
AND MOLECULAR SHAPE FROM LEWIS STRUCTURES.
PLEASE TAKE SOME TIME NOW TO REINFORCE YOUR UNDERSTANDING
WITH SOME PRACTICE PROBLEMS.