Shapes of molecules & ions

VSEPR theory

VSEPR - the Valence Shell Electron Pair Repulsion theory is used to obtain the shape of simple molecules and ions

VSEPR theory

VSEPR - the Valence Shell Electron Pair Repulsion theory is used to obtain the shape of simple molecules and ions

It states that because of the repulsion between pairs of electrons around the central atom in the molecule or ion, the pairs of electrons are arranged to be as far away from each other as possible

Negative Charge Centres

For VSEPR, multiple bonds (double, triple) count as if they are just one pair of electrons

Negative Charge Centres

For VSEPR, multiple bonds (double, triple) count as if they are just one pair of electrons

So VSEPR uses repulsion between negative charge centres rather than between pairs of electrons

4 bonding pairs - tetrahedral

Bond Angle =

109.5º

E.g methane - CH4, CCl4, SiCl4

4 negative charge centres

From: http://library.thinkquest.org/3659/structures/commonstructures.html

www.chemvc.com/~tim/VSEPR.html , http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml

3 bonding pairs (& 1 non-bonding pair) trigonal pyramid

Bond Angle = 107º

E.g. ammonia - NH3, PCl3

4 negative charge centres

From: http://library.thinkquest.org/3659/structures/commonstructures.html

www.chemvc.com/~tim/VSEPR.html , http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml

The bond angle decreases because the non-bonding pair of electrons exerts a greater repulsion than the bonding pair(s)

From: http://www.examstutor.com/chemistry/resources/studyroom/bonding/shapes_of_molecules/

2 bonding pairs ( 2 non-bonding pairs)

bent or v-shaped

Bond angle = 105º

E.g. H2O, H2S

4 negative charge centres

From: http://library.thinkquest.org/3659/structures/commonstructures.html

www.chemvc.com/~tim/VSEPR.html , http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml

2 negative charge centres

Shape is always linear

Bond angle is 180º

E.g.

From: www.physchem.co.za/Bonding/Shape.htm

www.tulane.edu/~bmitche/book/structrj.html

3 negative charge centres 3 bonding electron

pairs, no non-bonding pairs

Trigonal planar

Bond angle is 120°

e.g. BF3, ethene, CO32-

2 bonding pairs, 1 non-bonding pair

Bent or v-shaped Bond angle is 120°

E.g. SO2, ozone - O3

From: http://library.thinkquest.org/3659/structures/commonstructures.html ,

www.chm.bris.ac.uk/motm/so2/so2h.htm

5 and 6 charge centres

Elements in the third period can have more than 8 electrons in their outer shell because the 3 d orbitals are close in energy to the 3p orbitals.

The VSEPR theory also applies

You do not have to know specific bond angles

5 negative charge centres (HL)

Trigonal bipyramidal e.g. PCl5

From: http://library.thinkquest.org/3659/structures/commonstructures.html

5 negative charge centres (HL)

4 bonding pairs and 1 non-bonding pair

Distorted tetrahedral

From: www.chemvc.com/~tim/Predictions.html

6 negative charge centres (HL)

Octahedral

e.g. SF6

From: http://library.thinkquest.org/3659/structures/commonstructures.html

6 negative charge centres (HL)

4 bonding pairs and 2 non-bonding pairs as far apart as possible (above & below plane)

Square planar

e.g. XeF4

From: www.chemvc.com/~tim/Predictions.html

VESPR theory

All the previous molecular shapes are based on VSEPR theory

VSEPR=Valence Shell Electron Pair Repulsion theory

Pairs of electrons arrange themselves around the central atom so that they are as far apart from each other as possible