OCR AS Chemistry F332: Molecules and Networks

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Molecules and Networks
Carbon and Silicon Oxides
Carbon and silicon are both in group 4 of the periodic table.
CO2 is a gas at room temperature whereas SiO2 is a hard solid with a high melting point.
The reason for this difference in physical properties is the difference in bonding between
carbon and oxygen on one hand, and silicon and oxygen on the other.
Both are covalent compounds, but the small size of the carbon atom makes it possible for
carbon to form double bonds with oxygen, so that carbon dioxide is composed of individual
molecules.
Bonds between the carbon dioxide molecules are weak.
Little energy is needed to separate individual molecules in the solid and liquid phases to
form a gas, so CO2 is a gas at room temperature.
It freezes at 195K to a white solid called dry ice.
It is soluble in water, giving an acidic solution.
Silicon atoms are larger than carbon atoms and they normally bond to four oxygen atoms.
SiO2 is an extended network of SiO4 units in which the central silicon is covalently bonded
to each of four oxygen atoms.
Every silicon atom has a half-share in four oxygen atoms.
Because of its extended network structure, SiO2 is insoluble in water and has high melting
and boiling points.
The reason for these properties is the strong covalent bonding which exists throughout the
SiO2 structure.
Considerable energy is needed to break bonds within the structure so that a very high
temperature is needed to melt the SiO4 units.

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Comparing properties
Carbon Dioxide CO2 Silicon Oxide SiO2
Gas at room temperature Solid at room temperature
Covalent Covalent
Small atom Large atom
Forms double bonds with oxygen Each silicon atom has a half share in four
oxygen atoms
Simple molecule Giant structure
Soluble in water Insoluble in water
Low boiling point High boiling point
Little energy is needed to break bonds Considerable energy is needed to break
bonds
Two types of covalent structures
Covalent structures can exist as two types of structures, depending on how…read more

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