WRITING FORMULAS AND NAMING COMPOUNDS
All forms of matter are composed of elements. Diamond and graphite are minerals that
are formed from only one element, carbon. Water is formed from the combination of two
elements, hydrogen and oxygen, in fixed proportion two hydrogen atoms to every oxygen atom).
Chemical Symbols
An element is represented by a symbol which may be one or two letters: the first is
capitalized and the second is in the lower case. The symbols may be derived from Greek, German
or Latin names of the elements. (Refer to Table 6.1 for the complete list of names, symbols and
derivation of the names of the elements.)
Example:
Name
Symbol
Hydrogen
H
Helium
He
Carbon
C
Cobalt
Co
Chromium
Cr
Iron(ferrum)
Fe
Gold(aurum)
Au
Periodic Table
The periodic table is one of the most versatile tools used for understanding chemistry. It
arranges the elements according to the similarities in chemical and physical properties. The more
elements are written on the left. Metallic character decreases from left to right.
I.
BINARY COVALENT COMPUNDS
Binary covalent compounds are formed between two non-metals.
A. Naming binary covalent compounds
Example: CO2
1.
Identify the elements present in the compound given by the chemical formula.
CO2
carbon oxygen
The name of the more metallic element is written first. In this case C (group 14) is
more metallic than O (Group 16).
2.
Change the suffix of the less metallic element to –ide. In accordance with
established practice, the elements in a covalent compound are placed and named
first in the sequence: Rn, Kr, Si, Sb, B, As, P, H, Te, At, Se, C, I, Br. N. Cl, O, F.
CO2
carbon oxide
The –ygen is dropped and –ide is substituted.
3.
Use the prefix corresponding to the number of atoms present in the compound.
Refer to Table 1.1 for the list of prefixes. The prefix for one is mono- and for two,
di-, therefore,
CO2 monocarbon dioxide or carbon dioxide
The mono- prefix is frequently omitted, particularly for well-known substances. If
no prefix is used, it is usually implies that the number of atoms of the elements
is one. However, experts on nomenclature caution that this can be dangerous
and suggest that it is better to include the mono- prefix.
Example
Formula
Name
N2O4
dinitrogen tetroxide
CO
carbon monoxide
PCl3
phosphorus tricloride
Some compounds are known only by their common names. The most common
of these are:
Formula
Name
H2O
water
NH3
ammonia
PH3
phosphine
B. Writing formulas of binary compounds
1.
Represent each kind of element in a compound with the correct symbol of the
element.
2.
Indicate by a subscript the number of atoms of each element in a molecule of the
compound.
3.
Write the symbol of the more metallic elements first. (H is an exception to this rule.)
Example:
Sulfur dioxide implies one atom of sulfur, and two atoms of oxygen
Symbol:
S
O2
Therefore, the chemical formula of sulfur dioxide is SO2
II.
IONIC COMPOUNDS
Compounds formed between metals and non-metals are called ionic compounds.
Tables 1.2 and 1.3 give a list of the more common cations while Tables 1.4 and 1.5
give a list of the common anions.
A. Naming Ionic Compound
1.
Write the name of the cations first, followed by the name of the anions.
2.
Unlike binary covalent compounds, PREFIXES ARE NO ALLOWED to indicate the number
of ions present in the formula.
Example:
Formula
Name
KBr
potassium bromide
AlI3
aluminum iodide
Na2O
sodium oxide
Note that for ionic compounds, the prefixes are not attached to the chemical name to
denote the number of atoms of the elements. The number of atoms is implied by the charges of
the cations and the anion. It is therefore important to know the charges of the common cations
and anions.
3.
Most transition metals (Table 1.3) can exist in more than one ionic form. Thus, it is
important to know the charges of the common cations and anions.
Example:
Formula
Stock system
Old system
SnCl4
tin (IV) chloride
stannic chloride
SnBr2
tin (II) bromide
stannous bromide
The method of indicating the charge of the cations involves placing a Roman numeral
equivalent to the magnitude of the charge of the cations in the parenthesis after the English
name, is called the Stock System of Nomenclature.
Some ionic compounds form crystals that contain a certain proportion of water
molecules apart from the ions of the compounds. Such compounds are called hydrates.
Hydrates are names just like other ionic compounds except for the addition of the word
“hydrate” with a Greek prefix indicating the number of water molecules per unit of the ionic
compound.
Example:
Formula
Name
Na2CO3 ‘ 10H2O
sodium carbonate decahydrate
CaSO4 ‘2H2O
calcium sulfate dehydrate
CuSO4 ‘ 5H20
copper (II) sulfate pentahydrate (Stock)
OR cupric sulfate pentahydrate (Old)
B. Writing Formulas of Ionic Compounds
Example: sodium oxide
1.
Write of the positive ion (cation) first, followed by the symbol of the negative ion
(anion).
Symbol:
2.
Sodium
oxide
Na
O
Write the charge of each ion over the symbol of that ion. Usually, for the main group
elements, the group number usually gives the charge of the monoatomic ion.
Remember that Group 1 elements would have a change of (+); Group 2 (+2); most
Group 3 elements, (+3); Group 16 (-2); Group 17 (-1) and Group 18 (0) – unless
indicated.
3.
+1
-2
Na
O
Choose a subscript that will make the net change zero.
+1
-2
Na2
O1
or Na2O
(+1x2) + (-2x1) = 0
The simplest procedure is to use the absolute value of the charge of the anions as the
subscript for the cation; and the absolute value of the cation charge as the subscript for the
anions (cross-over rule)
Example: magnesium oxide
+2
-2
First Step:
Mg2
O2
Final Formula:
MgO
then divide subscript by 2
When both subscripts in the formula can be divided by same number to simplify the
formula, you should do so, unless you know the actual molecule represented (e.g. H2O2).
4.
For hydrates follow the same steps, then add a centered dot, followed by the number
of water molecules (indicated by the prefix) and the chemical formula of water.
Example: magnesium sulphate heptahydrate
+2
-2
First Step:
Mg2
(SO4)2
Second Step:
MgSO4
Final Formula:
MgSO4 * 7H20
then divide subscript by 2
hepta indicates 7 water molecules
III. ACIDS
A. Naming Binary Acids
Binary acids contain only two different elements – hydrogen and a non-metal.
Binary acids are named hydro_____ic acid, where the stem of the non-metal is inserted in
place of the line. Thus,
HF – hydrofluoric acid
HBr – hydrobromic acid
The names hydrogen fluoride and hydrogen bromide are also used for HF and
HBr, respectively. Both names are correct although the present in aqueous solutions.
Thus, HF in aqueous solution is hydrofluoric acid, but pure HF is referred to as hydrogen
fluoride.
B. Naming Oxyacids
Another type of acid is the oxyacids derived from the oxyanions listed in Table
1.6 and other oxyanions, like SO4 2 and others. Examples are H2SO4 and H2CO3. Since
some elements from more than one oxyanion, they also form more than one oxyacid.
The name of the oxyacid is derived from the name of the oxyanion with a change in the
suffix using the following rules:
1.
If the name of the oxyanion ends in –ate the name of the oxyacid will be of the form
_______ic acid. Examples are:
Anions
Name of Anion
Acid
Name of Acid
SO4
sulphate
H2SO4
sulfuric acid
perchlorate
HClO4
perchloric acid
2-
ClO4
2.
If the name of the oxyanions ends in –ite, the name of the oxyacid will be of the
form _______ous acid. Examples are:
Anions
Name of Anion
Acid
Name of Acid
SO3
2-
sulphite
H2SO3
sulfuous acid
ClO
-
hypoclorite
HClO
hypochkorous acid