Recap – Bonding
Ionic
Bonds
Polar Covalent Bonds
• Electrons transferred
• Electrostatic attraction
between cations & anions
• Non-directional
Covalent
Bonds
• Electrons shared
• Attraction between
electrons and nuclei
• Defined bond axis
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Metallic Bonding
• Outer electrons of a metal atom
are not fixed to that atom but
spread around surrounding
atoms or ‘delocalized’.
• Malleable and ductile – atoms
able to slide past each other in a
“sea of electrons”.
• Mobile electrons – good electrical
and thermal conductivity in solid
and molten state.
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Types of Formula
Empirical formula
• Lowest ratio of atoms of different types present.
•
•
•
Ionic Compounds, eg NaCl, K2O
Covalent compounds, eg glucose CH2O
Elements, always used except for diatomic elements
(H2, N2, O2, F2, Cl2, Br2, I2)
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Formulae for Molecules
Molecular formula:
• Actual number of atoms of each type in molecule.
Eg. Glucose has molecular formula C6H12O6.
• Implies covalent bonding.
• Always a simple multiple of empirical formula.
Eg. Empirical formula:
Molecular formula:
CH
C2H2 (ethyne)
C6H6 (benzene)
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Formulae for Molecules
Structural formula:
• Shows connectivity of atoms.
• Eg glucose
Empirical: CH2O
Molecular: C6H12O6
• Eg hydrogen peroxide
Empirical: HO
Molecular: H2O2
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Allotropes
• Allotropes: Elements with different bonding
diamond
Toby Hudson (School of Chemistry, University of Sydney)
Example:
Carbon
commons.wikimedia.org/wiki/File:Apollo_synthetic_diamond.jpg
arrangement of atoms.
• Usually one form is more stable at a given temperature
and pressure.
graphite
buckminsterfullerene
nanotube
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Molecular vs Network Solids
Covalent bonded substances maybe ‘molecular’ or
‘network’ materials.
• Eg the elements iodine and carbon
The purple colour of the vapour above solid
iodine is due to I2 molecules in the gas phase.
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Molecular vs Network Solids
Molecular materials are:
• Soft when solid (eg wax)
with a low melting point
• May also be gases or liquids
at room temperature
• Eg CO2
Network materials are:
• Hard solids with a high
melting point
• Covalent bonds broken
when solid melts
• Eg SiO2
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Learning Outcomes:
• By the end of this lecture, you should:
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understand the basis of bonding in metals
appreciate the differences between empirical, molecular
and structural formulae
know the context in which the different types of formula
are used (elements, ionic, covalent compounds)
know what an allotrope is
understand the different characteristics of a network and
molecular solids
be able to complete the worksheet (if you haven’t already
done so…)
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Questions to complete for next lecture:
1. When a material is heated the energy provided results
in increased motion of the particles present. Why are
metals good conductors of heat?
2. Name the two allotropes of the element with atomic
number 8.
3. Silicon has a melting point of 1410 C and phosphorus
has a melting point of 44 C. What does this suggest
about the nature of bonding in the two solids?
4. Silicon carbide (SiC) is a solid at temperatures up to
about 3000 C. What does this suggest about the type
of structure of this material?
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