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Recap – Bonding

Polar Covalent BondsIonic Bonds

Covalent Bonds

• Electrons transferred• Electrostatic attraction

between cations & anions

• Non-directional

• 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.

Types of Formula

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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: CHMolecular formula: C2H2

(ethyne)C6H6 (benzene)

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Formulae for Molecules

Structural formula: • Shows connectivity of atoms.• Eg glucose

• Eg hydrogen peroxide

CC

OH

H

C

OH

H

C

OH

H

C

OH

H

C

H

OH

H

O

H

Empirical: CH2O

Molecular: C6H12O6

H O O H

Empirical: HO

Molecular: H2O2

Allotropes• Allotropes: Elements with different bonding

arrangement of atoms. • Usually one form is more stable at a given

temperature and pressure.

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Example: Carbon

Molecular vs Network Solids

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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.

Molecular vs Network Solids

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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

• By the end of this lecture, you should:− 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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Learning Outcomes:

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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?