Chemical Bonding 4.1 The Octet Rule

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The Octet Rule
Elements tend to gain, lose, or share
electrons to acquire a full octet.
es
Chemical Bonding 4.1
• In n = 1 a full octet consists of 2 electrons.
• There is only one 1s orbital.
• From n = 2 to n = 7, a full octet consists of
8 electrons.
• A full s-orbital and a full p subshell.
The Formation of Ions
ts
Electron Configurations of Ions
Non-metals gain
electrons to
become negatively
charged anions.
Na
Incomplete Octet
ud
en
Metals lose
electrons to
become positively
charged cations.
'L
ec
tu
re
N
ot
Octet Rule
Ionic Bonding
Lattice Energy
Covalent Bonding
St
Non-Metals
+
+
+
1s2 2s2 2p6 3s1
= [Ne] 3s1
1s2 2s2 2p6
= [Ne]
C
op
y
of
Metals
Na+
Complete Octet
Lost the electron from
n = 3 to acquire a full octet
pl
e
Electron Configurations of Ions
Sa
m
Cl
Incomplete Octet
ClComplete Octet
Gained an electron in
n = 3 to acquire a full octet
+
+
1s2 2s2 2p6 3s2 3p5
= [Ne] 3s2 3p5
1s2 2s2 2p6 3s2 3p6
= [Ne] 3s2 3p6 = [Ar]
The charges on ions are usually
related to the group they’re in.
Group 1A Æ +1 (H can also be –1)
Group 2A Æ +2
d-block Æ +1 to +4 (it’s hard to predict)
Group 3A Æ +3 (Tl can also be +1)
Group 4A Æ +4 (Sn and Pb can also be +2)
Group 5A Æ -3 non-metals, +3 and +5 metals
Group 6A Æ -2
Group 7A Æ -1
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es
• These elements lose electrons from their highest
s-sublevel first before losing from their d-sublevel.
Ionic Compounds are created when a metallic
cation forms an electrostatic bond with a
non-metal anion.
• Ex) Iron may form 2 types of ions.
• Fe [Ar] 4s2 3d6
• Fe2+ [Ar] 3d6 Æ Lost electrons from 4s
• Fe3+ [Ar] 3d5 Æ Lost electrons from 4s and
one in 3d.
Non-Metals
'L
ec
tu
re
Metals
N
ot
d-block cations
ud
en
1) Metals transfer electrons to non-metals, and the
two form bonds due to the electrostatic
attractions between them.
ts
Ionic Bonding
Ionic Bonding Defined
or
Na + Cl Æ Na+ + [ Cl ]-
C
op
y
of
St
2) Cations (metal ions) and anions (non-metal
ions) form electrostatic bonds based on
opposite charges.
• Cations and anions may be polyatomic
Lattice Energy
pl
e
Lattice Energy
m
• The energy required to break one mole of a
solid ionic compound into its individual
gaseous ions.
Sa
• This requires a lot of energy as ionic bonds are
very strong.
• Very Endothermic
NaCl(s)Æ Na+(g) + Cl-(g)
ΔHlattice = +788 kJ/mol
Na+(g) + Cl-(g) Æ NaCl(s)
ΔH = -788 kJ/mol
• Conversely, putting a lattice together is Very
Exothermic
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Factors Affecting Lattice Energy
Ex) Trends in Lattice Energy
d
E=
_
KQ1Q2
d
1) LiF
K = 8.99 x 109 Jm/C2
2) MgCl2
MgO
3) NaF
MgI2
re
d = distance between ionic centers
LiI
N
ot
+
es
Ex) Which compound from each set has the
highest melting temperature? Why?
Electrostatic Equation
Derived from Coulomb’s Law
'L
ec
tu
Q = the charge of a single ion
Properties of Ionic Solids
Covalent Bonds
ts
• High Lattice Energy
ud
en
– High melting point
• Cleaves along planes
– Brittle 3D structure
– Ions line up in a repetitive pattern.
• Ions
St
– Most are soluble in polar solvents.
– Conduct electricity when molten or dissolved in a polar
solvent
One atom shares electrons with
another atom so that both acquire
full octets.
Occurs between two non-metals
C
op
y
of
• The higher the concentration of ions in a
solution, the higher the electrical conductivity.
Electronegativity and
Bond Polarity
m
pl
e
Types of Bonds
Sa
Covalent
Bond
0
0.5
F
Polar
Covalent
Bond
F
H
F
Na+ F-
Ionic Bond
1.9
3.5
Covalent
Bond
Polar Covalent
Bond
Ionic
Electronegativity Difference
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Determining Bond Type
H-H
H-Br
2.1
2.1
2.8
N
ot
2.1
es
Determining Bond Type
Electronegativity difference = 0.7
'L
ec
tu
re
Electronegativity difference = 0.0
ud
en
O-C-O
3.5
Determining Bond Type
ts
Determining Bond Type
St
2.5
1.0
4.0
Electronegativity difference = 3.0
C
op
y
of
Electronegativity difference = 1.0
LiF
pl
e
Molecular Compounds Defined
m
Molecules:
Sa
• Two or more non-metals bonded together to form
a compound.
• Bonds between atoms are covalent or polar
covalent.
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