Teori VSEPR

Present by :

Moch Nur Kholis Lailatul Isnaeni

Aris Tri SusantoMuzdalifah

Noor

The shape of a molecule plays a very The shape of a molecule plays a very important role in determining its important role in determining its properties. properties.

Who cares about molecular Who cares about molecular shape?shape?

Properties such as smell, taste, and Properties such as smell, taste, and proper targeting (of drugs) are all the proper targeting (of drugs) are all the result of molecular shape.result of molecular shape.

Lewis structures tell us nothing about how Lewis structures tell us nothing about how atoms in a molecule are arranged in 3-atoms in a molecule are arranged in 3-dimensional space. dimensional space.

Molecular ShapeMolecular Shape

Could you have predicted the arrangement of atoms on the right from just seeing it’s Lewis structure?

Main Premise of VSEPR Theory

Molecules will adopt a shape that is lowest in energy

A low energy shape is one that minimizes the valence shell electron pair repulsion (VSEPR) between adjacent atoms

Huh?

Atoms in a molecule try to spread out from one another as much as possible to reduce the “like charge repulsion” between their outer electrons.

C

H

H

H

Hmetmetanaana, CH, CH44

But if you think in 3 dimensions, this shape But if you think in 3 dimensions, this shape actually causes less repulsion between the actually causes less repulsion between the bonding pairs of electrons bonding pairs of electrons

109.5°109.5°90°90°

C

H

H

H

HYou might think this is the You might think this is the farthest that the hydrogens farthest that the hydrogens can get away from each can get away from each other other

Lima Bentuk Utama :

•Molecules adopt a geometry (shape) that Molecules adopt a geometry (shape) that minimizes e – e repulsions minimizes e – e repulsions this occurs this occurs when e- pairs are as when e- pairs are as farfar apart as possible. apart as possible.

Here are the STEPS for determining molecular geometry:• 1. Draw a Lewis structure

• 2. Count the # of bonds and # of lone pairs around the central atom (these are called “domains”)– Single, Double and Triple bonds count as ONE domain– Each lone pair counts as ONE domain

• 3. Use CHART to determine shape– Name of molecule shape is based on position of atoms

(not domains)

What the heck is an electron domain?• It’s a region in space where electrons are

likely to be– So it’s an area of high electron “concentration”– A domain is wherever there is a bond or a lone

pair around an atom

• Bonds = Bonding Domains

• Lone Pairs = Nonbonding Domains

How many “domains” around each atom?

Remember the BIG PICTURE?

• These “domains” are all negatively charged so they want to spread out from each other as much as possible within a molecule.– This minimizes “like charge repulsion”

between electrons and represents the lowest possible energy state for the molecule.

Ben

tuk

Geo

met

ri

Contoh VSEPR

Contoh VSEPR

Contoh VSEPR

Catatan:

• Lone pairs take up more space than bonding pairs and thus push atoms farther away from each other (decrease the predicted bond angles).

Susunan untuk e- tetrahedral:

Pasangan Elektron Bebas

0 1 2

Sudut ikatan

109.5 107 104.5

Contoh CH4 NH3 H2O

C

H

H

H

Hmemetanatana, CH, CH44

Gaya tolakan yang lemah Gaya tolakan yang lemah antara pasangan elektron antara pasangan elektron ikatikat

109.5°109.5° 90°90°

Let’s take a look at this phenomena…

mungkin banyak yang mengira mungkin banyak yang mengira bentuk bentuk NHNH33 adalah datar di adalah datar di tengah, tapi PEB dari NHtengah, tapi PEB dari NH3 3

mendorong hidrogen menuju ke mendorong hidrogen menuju ke bawahbawah

NH H

H

.

.

ammoniammoniaa

NHNH33

C

H

H

H

H NH H

H

OH H.. ..

..

109.5°109.5° 107°107° 104.5°104.5°

VSEPR Notation

• Also known as “AXE” notation

• A= central atom

• X = # of atoms bonded to central atom

• E = # of lone pair electrons on central atom

VSEPR Notation

• In AXE notation, the “X” and the “E” may have a subscript after them to tell how many.

• Example: H2O

– Has 2 atoms bonded to central atom– Has 2 lone pairs on central atom

– Its AXE notation is AX2E2

H

CH H

HN

H HH

OH H

Review

• If two atoms share electrons, but don’t share them equally, we call it a polar covalent bond– Electrons are pulled closer to the more

electronegative atom– One end of the molecule develops a partial

negative charge because it has a higher electron density

Momen Dipol

• A molecule such as H-Cl that has an area of partial positive charge and an area of partial negative charge is said to have a “dipole moment”

• The dipolar character is represented by an arrow pointing toward the (-) end of the molecule

Dipole Moments

• ALL diatomic molecules with a polar bond have a dipole moment– Ex. H-Cl

• SOME polyatomic molecules with polar bonds have a dipole moment– Ex. water (does); carbon dioxide (does NOT)– It depends on the molecular geometry!!!

Air itu Senyawa Polar

• It has polar H-O bonds

• Its geometry is “bent”

• So the molecule is polar– It has a dipole

BF3 termasuk Non-polar

• It has polar B-F bonds

• Bentuk geometri = trigonal planar

• So the molecule is non-polar– Tidak memiliki momen dipol– All the fluorines “cancel”

Cara Memisahkan.....

• “Melarutkan dengan yang Sejenis”– Molekul polar dilarutkan dalam larutan

polar– Molekul nonpolar dilarutkan dalam larutan

nonpolar • Larutan polar dan nonpolar solutions tidak

akan bereaksi– Contohnya : Air (polar) dan minyak (nonpolar)