Molecular Geometry 2-2 Ch. 6 – Molecular shape. Molecules are three-dimensional objects that...
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Transcript of Molecular Geometry 2-2 Ch. 6 – Molecular shape. Molecules are three-dimensional objects that...
• Molecules are three-dimensional objects that occupy a three-dimensional world; In general, only the smallest molecules can be said to have a fixed geometrical shape; the icosahedral C60 “soccer ball” is a rare exception.
VSEPR Theory
• Valence Shell Electron Pair Repulsion Theory- focuses on the bonding and nonbonding electron
pairs present in the outermost (“valence”) shell of an atom to which are connected two or more other atoms.
• Electron pairs orient themselves in order to minimize repulsive forces.
VSEPR Theory
• Types of e- Pairs– Bonding pairs - form bonds– Lone pairs - nonbonding e-
Lone pairs repel
more strongly than
bonding pairs!!!
• Draw the Lewis Diagram.
• Shape is determined by the # of bonding pairs and lone pairs.
Know the 5 common shapes !
Determining Molecular Shape
Linear molecules: AB2
• Ex: BeCl2 and CO2.
• -If you write out the electron dot formula for carbon dioxide, you will see that the C-O bonds are double bonds.
Double bonds are treated
like single bonds when
shape is determined!!!!!!
Be
180°
180°
Be90°
270°
The shape of BeCl2 is linear.
BeCl2
TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM
Cl Be Cl
THE MOLECULAR SHAPE IS BASED ON THE POSITION OF THE ATOMS!
Trigonal planar: AB3
• In the molecule BF3, there are three regions of electron density extending out from the central boron atom. The repulsion between these will be at a minimum when the angle between any two is 120°. This requires that all four atoms be in the same plane; the resulting shape is called trigonal planar.
B
120°
120°
120°
F F
F
B
THE SHAPE OF BF3 IS TRIGONAL PLANAR.
BF3 NO OCTET ON B !
THREE ELECTRON PAIRS AROUND THE BORON ATOM.
MOLECULAR SHAPE
F B F
F
ATTACH FLOURINES TO EPG.
Shape with lone pairs: AB2E2
2 bonding electrons and 2 lone pairs
or AB2E , 2 bonding electrons and 1
lone pair • Bent:
• The nonbonding electrons are also in orbitals that occupy space and repel the other orbitals.
Tetrahedral: AB4
• Methane, CH4, contains a carbon atom to which are connected four hydrogens. Consequently, the four equivalent bonds will point in four equivalent directions in three dimensions.
There is a better arrangement for four electron pairs:
109.5°
C
TETRAHEDRAL
The angle is…..
Put on the H-atoms…….
There is a better arrangement for four electron pairs:
TETRAHEDRAL
4 electron pairs tetrahedral EPG
C
H
HH
H
The shape of CH4 is tetrahedral.
Polyatomic ions
• The charge will dictate how many less or extra electrons will be added to the lewis dot diagrams.
• NO3-
• the negative charge tells you that you need to add one more electron to the diagram.
5 + 3(6) + 1 = 24
Polyatomic ions