An element is a substance made of only one kind of atom.
Atoms join, or bond, to other atoms of the same element,
or to atoms of other elements. They do this by sharing or
exchanging electrons. In many cases, the bonded atoms
form groups called molecules. When atoms of one
element combine with atoms of other elements,
they form compounds.Molecules
ETHANOL (C 2 H 5 OH) ▲
Ethanol is a compound with molecules
containing two carbon (C) atoms, six
hydrogen (H) atoms, and one oxygen
(O) atom. At room temperature, it is a
colourless liquid used as a disinfectant,
preservative, and the alcohol in drinks.92
VITAMIN C (C 6 H 8 O 6 ) ▶
Like ethanol, vitamin C (ascorbic
acid) contains carbon (C), hydrogen
(H), and oxygen (O) atoms. However,
because the molecule contains different
quantities of the elements arranged
in a different way it forms an entirely
different substance – a compound
that is solid at room temperature.◀ METHANE (CH 4 )
Molecules of the gas methane
contain four hydrogen (H) atoms
bonded to a carbon (C) atom. Compounds
of carbon and hydrogen are called
hydrocarbons. They include fuels, such as
methane, oil, and coal, and artificial
materials, such as polystyrene.◀ OXYGEN (O 2 )
Some elements can bond in
several ways to form different
substances. Atoms of the element
oxygen (O) bond together in pairs
to form molecules of oxygen gas.
High in the atmosphere, however,
oxygen atoms also bond in threes
to form the gas ozone.◀ SALT (NaCl)
Table salt, or sodium
chloride, is made of equal
numbers of sodium (Na)
and chlorine (Cl) atoms.
Salt does not form as
small individual molecules
containing one atom each
of sodium and chlorine.
Instead, many atoms of the
two elements link to form a
rigid lattice called a crystal.Sodium and chlorine
atoms always join up
in the same way to
form cubic crystalsHydrogen atomCarbon
atomOxygen atomChlorine atomSodium atomOxygen
atomOxygen
atomCarbon atom
Hydrogen atomCarbon atomHydrogen
atom092_093_Molecules.indd 92 10/01/19 2:26 PM