Discussion: Lewis Structures Discussion: Lewis Structures and VSEPR Theoryand VSEPR Theory

OBJECTIVE: Students Will OBJECTIVE: Students Will Understand Why Elements Form Understand Why Elements Form

Bonds With Other ElementsBonds With Other Elements

What are valence electrons?What are valence electrons?

Electrons in the highest energy levelElectrons in the highest energy levelElements in the same group on the Elements in the same group on the

periodic table have the same number periodic table have the same number A valence of 8 electrons is called a octetA valence of 8 electrons is called a octetAtoms try to get a valence of 8 electronsAtoms try to get a valence of 8 electrons

Not all elements try to get a Not all elements try to get a valence of eight electronsvalence of eight electrons

Hydrogen needs only two.Hydrogen needs only two.Beryllium needs on four Beryllium needs on four Boron needs only six.Boron needs only six.However, most elements like to have eight However, most elements like to have eight

electrons in their valence shell. electrons in their valence shell.

How the Valence Electrons Fit How the Valence Electrons Fit Around Group #1 AtomsAround Group #1 Atoms

Valence Electrons for Group #2Valence Electrons for Group #2

Valence for the restValence for the rest

How Atoms Gain an Octet by How Atoms Gain an Octet by Losing ElectronsLosing Electrons

Calcium atom loses two electrons to Calcium atom loses two electrons to achieve an octetachieve an octet

How Atoms Gain an Octet by How Atoms Gain an Octet by Gaining ElectronsGaining Electrons

Chlorine has seven electrons. It gains one Chlorine has seven electrons. It gains one electron to make eight. The addition of an electron to make eight. The addition of an electron make the ion negative.electron make the ion negative.

Nobel Elements already Nobel Elements already have an octet. have an octet.

Neon for example has 10 electronsNeon for example has 10 electronsNotice it already has an octet. That is Notice it already has an octet. That is

why it doesn't react with other elementswhy it doesn't react with other elements

Ions almost always have an octetIons almost always have an octet

NaNa++ ion for example, has 10 electrons just ion for example, has 10 electrons just like Neonlike Neon

2 82 8NaNa+ + | | lost 1| | lost 1

2 82 8OO2-2- | | gained 2 | | gained 2 2 82 8Ne | | Atoms try to get this Ne | | Atoms try to get this

electron configuration electron configuration

AlClAlCl33

Draw on boardDraw on board

PClPCl33

Count all the valance electronsCount all the valance electrons

Determine the central atom. Determine the central atom. Connect the ligands to the central atom. Connect the ligands to the central atom.

VSEPR TheoryVSEPR Theory

ValenceValenceShellShellElectron Electron PairPairRepulsionRepulsion

The VSEPR TheoryThe VSEPR Theory

Explains the geometric shape of a Explains the geometric shape of a molecule based solely on the repulsion of molecule based solely on the repulsion of electron pairs around the central atom.electron pairs around the central atom.

Geometric ShapesGeometric Shapes

LinearLinear

Planer TriangularPlaner Triangular

TetrahedralTetrahedral

Polar moleculesPolar molecules

Molecules that are unbalanced are polar.Molecules that are unbalanced are polar.

Planer TriangularPlaner Triangular Draw SODraw SO22

Total Electrons = 18Total Electrons = 18 Central Atom = SCentral Atom = S Connect the two oxygen Connect the two oxygen

atoms to the central atom. atoms to the central atom. Subtract 2 for each bond.Subtract 2 for each bond.

Put six around the O. Put the Put six around the O. Put the extra around the central extra around the central atom. atom.

Make the legends double Make the legends double share until all have octets.share until all have octets.

The Shape of SOThe Shape of SO22

Notice there are 3 pairs Notice there are 3 pairs of electrons around the of electrons around the central atom.central atom.

The Shape of HThe Shape of H22OO

Draw the Lewis Draw the Lewis structure for structure for WaterWater

NHNH33

Draw the Lewis Draw the Lewis structure for ammoniastructure for ammonia

CHCH44

Draw the Lewis Draw the Lewis structure for methanestructure for methane