FORMULA WRITING AND NOMENCLATURE OF BINARY COMPOUNDS
I. WRITING FORMULAS OF BINARY COMPOUNDS
A binary compound is formed by two elements.Using the oxidation numbers, it is not difficult to write the formula of binary compounds.There is one important rule that must be remembered:
The algebraic sum of the oxidation numbers of the elements in a chemical compound is zero.
The following examples will demonstrate how this rule is applied in writing the formula of a compound:
a) Write the formula for potassium bromide
Potassium, K, has an oxidation number of +1 and bromine, Br, has an oxidation number of -1. Writing the symbol of the positive element or polyatomic ion first, the formula is:
The algebraic sum of the oxidation numbers is +1 + (-1) = 0 Thus, the positive and negative oxidation numbers match and the formula of potassium bromide is correct as written above.
The formula of potassium bromide is interpreted to mean that the compound contains the proportion: one atom of potassium/one atom of bromine.
b) Write the formula of iron(II) bromide.
In this example, it is found that the oxidation number of iron, Fe, is +2 (as indicated by the Roman numeral) and the oxidation number of bromide, Br, is -1. If the formula of iron(II) bromide is written as:
the algebraic sum of the oxidation numbers of the elements in this formula is +2 + (-1) = +1 There is an excess of the positive oxidation number and the addition of a second bromine will be needed to make the sum zero. Thus, we put:
the algebraic sum of the oxidation numbers is +2 + [2 x (-1)] = 0. This formula, as written, is in an inconvenient form since the formula of bromide appears twice. In order to simplify the formula, we use a subscript is used to indicate the number of bromine atoms required. In this example, two bromine atoms are needed, so the proper formula for iron(II) bromide is written as:
The formula of iron(II) bromide is interpreted to mean that a molecule of the compound contains one atom of iron and two atoms of bromine. Make note of the fact that the subscript applies only to the element it directly follows.
II. DETERMINING THE OXIDATION NUMBER OF AN ELEMENT IN A BINARY COMPOUND
In a chemical compound, some elements exhibit oxidation states other than their most common oxidation state. It is desirable to be able to determine these less common oxidation states as well as to determine which state an element with several oxidation numbers is in. To do this, we must make use of the rule that the algebraic sum of the oxidation numbers is zero along with the use of elements that have only a single oxidation state as references for the calculation.
The elements with single oxidation states are:
Hydrogen has an oxidation number of +1 except in the case of metal hydrides where it is -1.
Lithium, sodium, potassium, rubidium and cesium (the alkali metals) always have oxidation numbers of +1.
Beryllium, magnesium, calcium, strontium, and barium (the alkaline-earth metals) always have oxidation numbers of +2.
Boron and aluminum always have oxidation numbers of +3.
Oxygen has oxidation number of -2 except in peroxides where it is -1.
Fluorine always has an oxidation number of -1.
Chlorine, bromine, iodine always have oxidation number of -1 in binary compounds.
The following examples will demonstrate how to determine the oxidation number of an element in a compound:
a) Determine the oxidation number of Mn in Mn2O7 .
Using the rules given above, the oxidation number of O is -2. Thus, we put:
and solving, is clear that x=+7. The name of the compound is manganese(VII) oxide as we shall learn below.
II. NAMING BINARY COMPOUNDS
In the nomenclature of binary compounds the following rule is used:
Name first the electropositive element in the formula (the one with the positive oxidation number) followed by the stem name of the electronegative element (the one with the negative oxidation number) with an -ide ending.
There are two different types of binary compounds, those formed from a metal and a non-metal and those formed from two non-metals. In this last case we treat in particulary form the binary acids wich are compounds of hydrogen and a a few non-metals.
A. Binary compounds formed from a metal and a non-metal.
These compounds consist of a metal and a non-metal in an atom ratio determined by their oxidation numbers. It is customary to write the metal first and the non-metal second. The rule for naming these compounds is:
Name the metal followed by the stem name of the non-metal with an -ide ending.
NaCl = sodium chloride
AgI = silver iodide
CaO = calcium oxide
If the metal has more than one oxidation number, the oxidation number of the metal, in the compound of interest, is indicated by a Roman numeral placed in parentheses following the name of the metal.
HgBr is named mercury(I) bromide
HgBr2 is named mercury(II) bromide
(mercury can have oxidation states of +1 and +2)
CoCl2 is named cobalt(II) chloride
CoCl3 is named cobalt(III) chloride
cobalt can have oxidation states of +2 and +3
An alternative metjod (called systematic methiod, uses prefixes to indicate the number of atoms of each element present in the compound. Some of them are:
Mn2O7 = dimanganese heptaoxide
Fe2O3 = diiron trioxide
B. Binary compounds formed from two non-metals.
In compounds that occur between non-metals, the positive and negative oxidation numbers are assigned to the elements according to their electronegativities. The element with the lowest electronegativity or positive oxidation number is the first one in the formula and we name it first.
To name a binary non-metal compound name each element in the order they appear in the formula using the appropriate prefix to indicate the number of atoms of the element in the compound. The second element uses an -ide ending. The prefix mono- is generally omitted unless it is needed to distinguish between two or more compounds of the same elements.
CO (mono)carbon monoxide
CO2 carbon dioxide.
PCl3 phosphorus trichloride
PCl5 phosphorus pentachloride
NO nitrogen oxide
NO2 nitrogen dioxide
N2O dinitrogen monoxide
N2O3 dinitrogen trioxide
N2O5 dinitrogen pentoxide
here is an alternate method to naming binary non-metal compounds. Instead of using numerical prefixes, this method uses the oxidation numbers of the first element in parentheses, similar to the methodused for binary compounds composed of a metal and a non-metal. Using this method, the above examples would be named:
CO carbon(II) oxide
CO2 carbon(IV) oxide.
PCl3 phosphorus(III) chloride
PCl5 phosphorus(V) chloride
NO nitrogen(II) oxide
NO2 nitrogen(IV) oxide
N2O nitrogen(I) oxide
N2O3 nitrogen(III) oxide
N2O5 nitrogen(V) oxide
C. Naming of Binary or Non-oxygen Acids
A binary acid is a compound consisting of hydrogen combined with a some non-metallic elements (F,Cl,Br,I,S,Se,Te), in the formula for an acid, hydrogen is always listed as the first element.
Although we can name them in the terms described in B (for example: HCl, hydrogen chloride), they have also an especific nomenclature. Binary acids are named byusing the prefix hydro- followed by the stem name of the non-metal element (the second element in the formula) with an -ic ending. The name is followed by the word acid.
Alternatively we can name using the stem name of the non metal with an -hydric ending followed by the word acid.
HCl is named hydrochloric acid or chlorhydric acid
HBr is named hydrobromic acid or bromhydric acid
HI is named hydroiodic acid or iodhydric acid
H2S is named hydrosulfuric acid or slufhydric acid