Normally between a metal and a non-metal:
They exchange electrons and become ions
(charged atoms) which attract each other by
A pair of ions does not stay alone but form crystals
Two non-metals share
(Remark: Transition metals can form covalent bonds also !)
Polar Covalent Bond
Two non-metals share electrons unevenly because
of electronegativity difference.
Electrons are closer to one atom than the other.
This results on partially negative and positive charges on the atoms
Metal atoms share all
their valence electrons,
which freely move
between all atoms which
form a network.
Therefore all metals can conduct electricity and look shiny
Split all bonds in the middle
=> “real” charge on atoms
(2) Octet Rule
Count all bonding electrons for
=> 8 is most stable
(3) Oxidation Number
Give all bonding electrons to
the more electronegative atom
Especially P and
S can use d-
orbitals to make
more than 3
resp. 2 bonds !
B and Al !
Part 2: Valence Bond Theory
“Valence Electrons are located in
bonds and lone pairs”
Acid or Base ?
Compare AlCl3 and PCl3 ?
Which acts as acid and which as base – and why ?
Why is FeCl3 a strong Lewis acid ?
PROBLEM: Describe the types of bonds and orbitals in acetone, (CH3)2CO.
PLAN: Use the Lewis structures to ascertain the arrangement of groups and
shape at each central atom. Postulate the hybrid orbitals taking note of
the multiple bonds and their orbital overlaps.
• Draw the Lewis Structures and the Hybrid
Orbitals for Ethane, Ethene and Ethyne
(mark the hybrid orbitals)
• Which hybridization has the central atom in:
H2O, O2, NH3, NH4+, N in pyridine, O in THF,
S in SOCl2, C in HCHO compared to CO
From the hybrid orbitals we can estimate if a
molecule acts as Lewis acid or base
(if there is an electrophilic or nucleophilic center)
Consider the “empty” pz orbital of C in HCHO vs. the
“filled” sp orbital of C in CO
-> in the first case, it acts as Lewis acid, in the second
as base !
The Central Themes of MO
A molecule is viewed on a quantum mechanical level as a collection of nuclei
surrounded by delocalized molecular orbitals.
Atomic wave functions are summed to obtain molecular wave functions.
If wave functions reinforce each other, a bonding MO is formed (region of
high electron density exists between the nuclei).
If wave functions cancel each other, an antibonding MO is formed (a node of
zero electron density occurs between the nuclei).
Amplitudes of wave
An analogy between light waves and atomic wave functions.
The energy level
is the lower, the
higher the EN of
the atom is !
Naming of MO’s: example O2 molecule
“g” = symmetric to C axis
“u” = anti-symmetric
Consider the EN of each atom – the higher the EN,
the lower is the energy of the orbitals !
The highest filled MO is called “HOMO”, the lowest
unoccupied MO “LUMO”
-> check example CO