MOLECULAR STRUCTURE
Objective
To be able to figure out the shape,3D structure of a molecule, starting from the chemical formula.
For instance: to predict that, explain why, H2O molecule is bentpolar, though CO2 molecule is linearnon-polar.
Why important?
From the shape of molecule, one can predict physical properties of material (say, that water is a strong dielectric).
Chemical properties are also determined by the shape of molecule:
how accessible are its electrons to attack by another agent?
Say, why CH4 molecule is rather non-reactive, though NH3 violently reacts with HCl, C2H4 forms polymers?
What molecules can exist, what can’t?
What do we already know?
Electrical charges of the same sign repel each other.
Especially important is that electrons repel each other.
There are valence (outermost) core (inner-shell) electrons in atoms.
Only valence electrons are shared by bonded atoms.
Covalent bond is a shared electron pair, occupying a MOLECULAR ORBITAL.
The physical reason of pairing electrons is that each electron is a small magnet. 2 electrons may cancel each other’s magnetic field (we say they have antiparallel SPINS, thi is a more stable situation:
an electron pair at the same molecular orbital) or sum up their magnetic field (unstable situation: both electrons cannot belong to the same orbital)
Electron octet configuration is especially stable.
Lewis dot formulasof molecules are compiled to satisfy
the OCTET RULE: Chemical compounds form so that each atom, by gaining, losing or sharing electrons has an octet (8) of its valence electrons (2e- for H)
H:H H2 H – H
Cl2 Cl–Cl
HF H – F
When more than 2 atoms bonded, there is a quest: which atoms are bound to each other which are not?
Methane, CH4
H
|
H – C – H
|
H
In SF2 F – S – F there are two S – F bonds, but none of F to F
Structural formula shows the order of connectivity of atoms.
Ammonium NH4+ H+
ion |
H - N - H
|
H
Sulfate ion, SO42- :Ö: 2-
|
:Ö - S - Ö:
|
:O:
Nitrate ion, NO3-|Ö|-
N = O|
|Ö
VALENCE-SHELL ELECTRON-PAIR REPULSION THEORY
VSEPR
predicts the shape of molecules.
1.Only outermost, or valence electrons, count.
Core electrons do not contribute to bonding, do not affect the shape of a molecule.
2.Valence electrons in molecules are arranged in pairs, when possible.
3. Lewis electron-dot formula is valid, with bonding lone electron pairs.
(H-Ö-H Oxygen in water has 2 bonding 2 non-bonding electron pairs)
- Allelectron pairs (both bonding non-bonding) repel each other occupymolecular orbitals (MO) located as far apart as possible.
5. Formation of double bonds do not affect the geometry.
Major Cases
Two electron pairs around central atom.
BeH2 total 2+ 2= 4 val.e-, 2 el. pairs & 2 bonds H-Be-H, no lone el. pairs, linear molecule
Bond angle 180o
2 double bonds no lone el. pairs around central atom give the same linear geometry, as in CO2
O=C=O
Bond multiplicity does not affect the basic geometry!
3 electron pairs around central atom
BH33+3=6e, 3 bonds are formed, no lone pairs,
trigonal planar H
|
B
HH
4 electron pairs around central atom*
The most important case
8 valence electrons (4 pairs) 4 bonds around central atom.
The most remote location of electron pairs – tetrahedral.
CH4tetrahedron
All bond angles 109.5o
4 electron pairs – 1 lone pair around central atom
:NH3 5+3=8e-, 4 MO, 3 bonds N-H, one lone e-pair on N
Lewis electron dot formula is
Electron configuration is still tetrahedral,
but the molecule geometry is different:
4 electron pairs – 2 lone pairs around central atom
H2O 8e-, 4 MO, 2 bonds, 2 lone pairs on O,
:O:
bent molecule: H H
while electron configuration is still tetrahedral
Electronic structures
with more than 8e- around central atom
Octet rule is strictly valid for the elements of 2nd Period. Starting with 3th period octet rule may be violated more than 8 electrons, more than 4 el. pairs surround the central atom.
For the surrounding atoms – ligands –
octet rule is still valid
5 electron pairs around central atom
Trigonal bipyramid
Bond angles: 90120º.
PF5 (but not NF5), SnCl5-
Lone pair, if present,repels other pairs stronger than bondingpairs. Therefore, lone pairoccupies anequatorial position (bond angle 120o), not an axial position (90o)
6 electron pairs around central atom
Octahedron
Review of the step-by-step procedure:
POLARITY OF MULTIATOMIC MOLECULES
Covalent bond between atoms of different electronegativity is polar:
H – O C – O H – N
However the net polarity of a multiatomic molecule depends on the bond angle between polar bonds.
O = C = O or