Page 63 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

F O R M U L A E IR-4.2

IR-4.1 I N T R O D U C T I O N

Formulae ( empirical, m olecular and structural formulae as described below) provide a
simple and clear method of designating compounds. T hey are of particular importance
in chemical equations and in descriptions of chemical procedures. In order to avoid
ambiguity and for many other purposes, e.g. in databases, indexing, etc., standardization is
recommended.

IR-4.2 D E F I N I T I O N S O F T Y P E S O F F O R M U L A

IR-4.2.1 Empirical formulae

The empirical formula of a c ompound is formed by juxtaposition of the atomic symbols with
appropriate (integer) subscripts to give the simplest possible formula expressing the
composition. For the order of citation o f s ymbols in formulae, see Section IR-4.4, b ut, in the
absence o f a ny other ordering criterion (for example, if little structural i nformation is
available), the alphabetical order of atomic s ymbols should b e u sed in an empirical formula,
except t hat in carbon-containing compounds, C a nd H a re usually cited first and second,
respectively. 1

Examples:

1. BrClH 3 N 2 NaO 2 Pt
2. C 10 H 10 ClFe

IR-4.2.2 Molecular formulae

For compounds consisting of discrete molecules, the molecular formula, a s o pposed t o t he
empirical formula, may be used to indicate t he actual composition of the molecules. For the
order of citation o f s ymbols in molecular formulae, see Section IR-4.4.
The choice of formula d epends o n t he chemical context. I n s ome cases, the empirical
formula may also c orrespond to a m olecular composition, in which case the only possible
difference between t he two f ormulae is the ordering o f t he atomic s ymbols. If it is not
desirable or possible t o s pecify the composition, e.g. in the case of polymers, a l etter
subscript such as n may be used.

Examples:
Molecular formula Empirical formula
S
1. S 8
S
2. S n
F 6 S
3. SF 6
ClS
4. S 2 Cl 2
H 2 O 3 P
5. H 4 P 2 O 6
ClHg
6. Hg 2 Cl 2
7. N 2 O 4 NO 2

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