• A chemical symbol is a one or two letter symbol used to refer to any element listed in the Periodic Table.
• They are used not just because they are 'shortcut' ways of writing the element’s name, but because the chemical symbol of separate elements can be combined to make chemical formulae
• Chemical formulae describe more complicated chemicals called molecules
, which are chemicals made of more than one atom combined together, and compounds
, which are chemicals made of more than one type of atom (elements!) combined together.
• Chemical formulae for chemical compounds obey Proust’s law
, or the law of definite proportions:
A chemical formula shows the ratio of each element
(measured by mass) in a chemical compound. This ratio is always true regardless of how the chemical was prepared.
• The advantage of chemical formulae over written names of chemicals is that they can specify the exact number of each atom in that chemical, as a ratio of all the elements it’s made of. For example: magnesium hydroxide, Mg(OH)
and sodium hydroxide, NaOH. Here, the chemical formulae clarifies the ‘hydroxide’ term as the first compound has two hydroxide groups, and the second has only one.
• Be careful with capital letters in chemical formulae. The second letter of chemical symbols are always lower case
, so a capital letter always shows a new element symbol
. For example: CO and Co are two completely different chemicals. CO is a compound
of carbon (C) and oxygen (O) called carbon monoxide, whereas Co is symbol for the metal element Cobalt.
• Numbers in
are used to specify how many of the atom
are present in a molecule of the chemical. Do not confuse this with
which is not used
in chemical formulae. For example: Na
O is a compound
that is made of two atoms of sodium and one atom of oxygen.
( ) followed by numbers in chemical formulae are used to show that every atom contained in the brackets
is present in that quantity. For example: the chemical formula Mg(OH)
shows that, as a ratio, this compound contains two of both O and H atoms for every one Mg atom.
• If there is no number in
written after a chemical formula, it means that only one of that type of atom
is present in this chemical.
• When writing simpler chemical formulae, the metal (and hydrogen) atoms are normally written first
, followed by non-metal atoms or groups of atoms in a formulae.
In this lesson, we will learn:
• What chemical symbols and formulae are, and how to read them correctly.
• Why they are used in chemistry and their advantage.
• How they are used to describe more complex chemicals.