WRITING THE NAME OF GIVEN COMPOUND
Compounds are made up of elements chemically combined together. The chemical formula of a compound represents this combination. The single unit can be molecules or groupings of ions. Every molecule, ion and ionic compound has a name. A system of naming chemical substances is called a chemical nomenclature. There are various ways of naming substances. The systematic naming is the most useful. A systematic system follows a set of rules. The set of rules will give one name for one formula. For example, the name sulphur oxide is not a systematic name because both SO2 and SO3 molecules exist. The systematic name sulphur(VI) oxide, however, can only mean SO3. The systematic naming has the added advantage of giving other information about the compound. This includes oxidation state and number of atomscombining to form the compound.
Trivial or common names are old, traditional names. They are not necessarily based on any rules. As an example, the common name for the formula H2SO4 is sulphuric acid and the systematic name is tetraoxosulphate(VI) acid. The West African Examination Council(WAEC) recommends an adapted IUPAC systematic naming which will be used in this book. However, the IUPAC system still includes some trivial names which have been found to be simple and useful. For example, water for H2O and ammonia for NH3 are trivial names used in the IUPAC naming system. Some common or traditional names may be put in parentheses beside their IUPAC names.
Both inorganic and organic compounds can be named in the IUPAC system. Inorganic compounds are mainly compounds of metals and non-metals (with the exception of carbon). The naming of inorganic Compounds relies on oxidation numbers.
Oxidation number (O.N.) of the element is defined as the residual charge which its atom has or appears to have when all other atoms from the molecule are assumed to be removed as ions by counting the shared electrons with more electronegative atom.
For example, in hydrogen chloride molecule, chlorine more electronegative than hydrogen. Therefore, the shared pair is counted towards chlorine atom as shown below:
As a result of this, chlorine gets one extra electron and acquires a unit negative charge. Hence, oxidation number of chlorine is -1. On the other hand, hydrogen atom without electron has a unit positive charge. Hence, oxidation number of hydrogen in hydrogen chloride is + 1.
It may be noted that electrons shared between two similar atoms are divided equally between the sharing atoms. For example, in chlorine molecule (Cl2), the electron pair is equally shared between the chlorine atoms. Therefore, one electron is counted with each chlorine atom as shown below:
Now, there is no net charge on each atom of chlorine. In other words, oxidation number of chlorine in Cl2 molecule is zero.
Thus, atoms can have positive, zero or negative values of oxidation numbers depending upon their state of combination. Now since it is not always possible to remember or make out easily in a compound or ion which element is more electronegative than the other, therefore, a set of rules have been formulated to determine the oxidation number. If two or more than two atoms of an element are present in the molecule ion, the oxidation number of the atom of the element will than be average of all the atoms at that element.
All elements have oxidation numbers in whatever state they occur. Some common ones are given in Table 12.5. Arabic numerals are used in the algebraic exercises to determine oxidation numbers, e.g., + 2, 3, +4, but capital Roman numerals, e.g., I, II, IV, are used in the naming (see Table 12.1). A combination of the knowledge of oxidation numbers, and how to indicate the number of atoms, ions or radicals, enables the correct formula of a compound to be written. The charge on a species is written as a right superscript followed by its positive or negative sign. For example, Fe2+ and o2- and not Fe+2 and o-2 since the oxidation number is not an addition or subtraction process.
RULAR FOR ASSIGNING OXIDATION NUMBER TOANATI
The oxidation number of various elements in their compounds can be calculated by applying following rules. These rules have been formulated on the basis of the assumption that electrons in a covalent bond belong entirely to the more electronegative atom.
- The oxidation number of the element in the free or elementary state is always zero-irrespective its allotropic form.
Oxidation number of helium in He = 0
Oxidation number of chlorine in Cl = 0
Oxidation number of sulphur in S8 =0
Oxidation number of phosphorus in P4 =0
- The oxidation number of the element in monoatomic ion is equal to the charge on the ion. For example, in K+ Cl-, the oxidation number of K is + 1 while that of Cl is -1. In the similar way, oxidation number of all alkali metals is + 1 while those of alkaline earth metals is +2 in their compounds.
- The oxidation number of fluorine is always -1 in all its compounds. Other halogens (Cl, Br and I) also have an oxidation number of -1, when they occur as halide ions in their compounds. However, in oxoacids and oxoanions they have positive oxidation numbers.
- Hydrogen is assigned oxidation number + 1 in all its compounds except in metal hydrides. In metal hydrides like NaH, MgH2 , CaH2, LiH, etc., the oxidation number of hydrogen is -1.
- Oxygen is assigned oxidation number -2 in most of its compounds, however; in peroxides (which contain 0-0 linkage) like H2O 2, BaO2, Na2O 2, etc., its oxidation number is -1. Similarly, the exception also occurs in compounds of fluorine and oxygen like OF 2 (F–0-F) and 0 2F2 (F-0-0-F) in which the oxidation number of oxygen is +2 and+ 1 respectively.
- In accordance with the principle of conservation of charge, the algebraic sum of the oxidation numbers of all the atoms in molecule is zero. But in case of polyatomic ion the sum of oxidation numbers of all its atoms is equal to the charge on the ion.
- In binary compounds of metal and non-metal, the metal atom has positive oxidation number while the nonmetal atom has negative oxidation number. For example, O.N. of Kin KI is + 1 but O.N. ofl is -1.
- In binary compounds of non-metals, the more electronegative atom has negative oxidation number; but less electronegative atom has positive oxidation number. For example, O.N. of Cl in C1F3 is positive (+3) while that in ICI is negative (-1).
Table 12.5. Oxidation Numbers in Various Species
WRITING NAMES OF SIMIPLE COMPOUNDS
The simplest compounds are binary compounds. A binary compound is a compound that contains only two elements. For example, NaCl is a binary compound of sodium and chlorine. CaF2 is a binary compound of calcium and fluorine. The following set of rules are followed while writing the correct IUPAC names:
The symbol of the more electronegative element is written on right hand side while that of less electronegative element is written on left hand side.
The number of atoms of each element are indicated by subscripts on the right hand side bottom of the symbol. For example, for a compound formed by combination of a metal with a non-metal, the symbol of the metal element is written first (left hand side) and the symbol of the non-metal element is written on right hand side.
- The first element (less electronegative element) is named as such while the name of the second element (more electronegative element) is written with anide ending. For example,
KI is named as potassium iodide
MgO is named as magnesium oxide
NaH is named as sodium hydride
CaCl2 is named as calcium chloride.
The names of some non-metallic elements with –ide endings are given below:
While naming binary compounds of metals and nonmetals, the subscript numerals are ignored. For example, BaCl2 is named barium chloride and not barium dichloride.
In naming the binary compounds of two non-metals, the subscript numerals have to be taken into consideration and are indicated as a part of the name. The reason for using prefixes is that the same two non-metallic elements may combine to form many compounds. For example, phosphorus and chlorine combine to form two compounds PC13 and PCly
A subscript 2 is indicated by the prefix di; subscript 3 by tri; subscript 4 by tetra, and so on. There should be no gap between the prefix and the name of the element. Some examples are given in Table 12.6.
Table 12.6. Names of Some Binary Compounds of Two Non-Metals
IUPAC NAMES OF BINARY AND TERNARY COMPOUNDS (WAEC ADAPTED)
The word acid replaces the word ion in the name of the oxoanion. Examples are shown in Table 12.7.
Table 12.7. Names of some common inorganic acids
Acid Name of acid
H2SO4 tetraoxosulphate(VI) acid
HNO3 trioxonitrate(V) acid
HNO2 dioxonitrate(Ill) acid
H2CO3 trioxocarbonate(IV) acid
HCIO oxochlorate(l) acid
H3PO4 tetraoxophosphate(V) acid
Aqueous HCl is called hydrochloric acid. It does not contain an oxoanion.
BASES AND SALTS
Bases can be divided into two groups, namely oxides and hydroxides. The salts can also be divided into two groups, namely the binary salts and the oxoacid salts.
1. Oxides and binary salts are named as binary compounds (see Table 12.6 and 12.7).
2. A hydroxide is made up of a cation and the hydroxide ion, OH-. Hydroxides are named in a similar way to binary compounds. The name of the cation is written first, followed by the oxidation number (if it varies). Followed by the word hydroxide, some examples are given in the following table.
Formula Common name WAEC Adapted IUPAC Name
NaOH Sodium hydroxide Sodium hydroxide
Cu(OH)2 Cupric hydroxide Copper(II) hydroxide
Fe(OH)3 Ferric hydroxide Iron(III) hydroxide
Ca(OH)2 Calcium hydroxide Calcium hydroxide
3. An aqueous solution of NH3 is a basic solution. It is preferably called aqueous ammonia or ammonia solution instead of the common name ammonium hydroxide. It is because it exists mainly as dissolved NH3 molecules and not as ammonium NH4 + and hydroxide OH- ions.
4. Oxoacid salts are named by writing the cation first followed by its oxidation number, (if it varies) and in the last the name of the oxoanion is ·added. Some examples are tabulated below:
Formula Common name WAEC Mapted IUPAC name
Na2sO3 Sodium sulphate Sodium trioxosulphate(IV)
ZnCO3 Zinc carbonate Zinc trioxocarbonate(IV)
KMnO4 Potassium perman- Potassium tetraoxomang ganate
NaClO Sodium hypochlorate Sodium oxochlorate
5. The names of salts, other than above, are a combination of the names of cations and the anions. For example, NaCN is named as sodium cyanide.
6. Some salts and compounds have water molecules as part of their structures.
(i) The name of the hydrated substance is followed by the number of water molecules. For example, penta-(5), tetra-( 4), hexa-(6), hepta-(7).
(ii) The number of water molecules can also be represented as a pure number separated by a dash plus the word water. For example, CuSO4O is named as copper(II) tetraoxosulphate(VI) pentahydrate or copper(II) tetraoxosulpahte(VI)-
The water molecules in the substance are called water of crystallization
ORMULAE AND IUPAC NAMES OF SIMPLE DROCARBONS
The formulae of some simple hydrocarbons are:
Ethane: C2H6 or C-CH3
Propane: C3H8 or CH3-C-CH3
Butane: C4H10 or CH3-C~-CH2-CH2
IUPAC Names of Hydrocarbons
The IUPAC names of some simple hydrocarbons are:
1. CH3 CH CH3 : 2 –Methy1 propane
2. CH3 CH2 CH2 CH2 CH3 : Pentane
3. CH3-CH2-CH3 : Propane
4. CH3 – CH – CH3 : 2, 3-Dimethyl butane
5. CH3-CH = CH2 : Propene
6. HC = CH : Ethyne