Chemistry
Properties of small molecules Learning objectives: • identify small molecules from formulae • explain the strength of covalent bonds • relate the intermolecular forces to the bulk properties of a substance.
KEY WORDS covalent bond free electrons intermolecular force molecule (a)
(a) (b) Researchers have found that the polar caps of Mars are probably made of both water ice and dry ice from carbon dioxide. However, at room temperature on Earth, carbon dioxide is a gas and water is a liquid. We meet them as (b) Figure 2.21 Models of small molecules bulk substances made up of small molecules and their (a) carbon dioxide (b) water properties are related to the size of their molecules.
Small molecules
KEY INFORMATION
Carbon dioxide and water are examples of simple, small molecules. You can often tell a small molecule from its formula. It has two or three atoms joined together, with no charge. Atoms
O
Mg
Cl
Na
S
Ions
O2–
Mg2+
Cl –
Na+
SO42–
Molecules
H2
Cl2
CO2
Br2
H2O
The subscript tells you how many atoms of the same type are bonded. For example, H2O means there are two hydrogen atoms bonded to one oxygen atom in one molecule of water.
Look at the table and notice that the molecules have no charge and more than one atom. Substances that are made up of small molecules are usually gases or liquids. They have relatively low melting points and boiling points. This is because simple molecules such as carbon dioxide and water have weak forces between the molecules. These weak forces easily break to allow the molecules to move randomly around away from each other. These substances do not conduct electricity because the molecules do not have an overall electric charge. 1
solid
Explain why carbon dioxide has a boiling point of –78.5 °C.
Intermolecular forces Carbon dioxide and water are simple molecules that have strong covalent bonds within the molecule. The carbon atom does not break its bonds with the oxygen atoms when carbon dioxide changes state. These are strong covalent bonds formed by electrons sharing. These substances have only weak forces between the molecules solid (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.
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gas
Figure 2.22 CO2 at low temperatures and CO2 above boiling point
AQA GCSE Chemistry for Combined Science: Trilogy: Student Book
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