APPENDIX A: CHEMICAL NOMENCLATURE

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1 BULDNG CHEMSTRY LABRATRY SESSNS APPENDX A: CHEMCAL NMENCLATURE Chemical elements are substances which contain atoms that have the same atomic number (number of protons in the atomic nucleus). Each element is described by an international UPAC name and corresponding symbol. The combination of two or more atoms results in the formation of compounds. Their chemical formulae are always composed of symbols of the elements which the compound contains and indices which indicate the number of corresponding atoms of a given element. E.g. H S (sulphuric acid) is a compound that contains hydrogen atoms, 1 atom of sulphur and oxygen atoms in its molecule. f an element or a group of elements bears an electric charge we call them ions. The charge of an ion is represented by a superscript just after the chemical formula. Positively charged ions (Na +, Ca +, Fe +, Fe 3+, NH + ) are called cations and negatively charged ions (Cl, CN, S, S 3 ) are anions. The nomenclature of inorganic compounds is based on the oxidation states of elements. The oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. xidation states can be positive, negative, or zero. t should be remembered that the oxidation state of an atom does not represent the "real" charge on that atom: this is particularly true for high oxidation states, where the ionisation energy required to produce a multiple positive ion is far greater than the energies available in chemical reactions. xidation states are typically represented by oxidation numbers in the form of Roman numerals, which are placed either as a right superscript to the element symbol, e.g. Fe, or in parentheses after the name of the element, e.g. iron(): in the latter case, there is no space between the element name and the oxidation number. The sum of oxidation states of all atoms in an electroneutral molecule is equal to zero. Similarly, the oxidation state of free elements is also zero (He 0, Na 0, 0, H 0 ). An overview of common oxidation states in selected elements: Element xidation state xidation state Element negative positive negative positive H Li, Na, K, Cs, Ag - Al - Mg, Ca, Sr, Ba, Zn - Cr -, V Cu, Hg -, - Fe, Co -, C V, V Sn, Pb -, V Si V V Mn -,, V, V, V N,,, V, V P, As, Sb, V S V, V Cl, Br,,, V, V F -

2 APPENDX A: CHEMCAL NMENCLATURE How can an oxidation state be derived? carbon dioxide There are two atoms of oxygen and one atom of carbon in the molecule of carbon dioxide (C ). xygen atoms are more electronegative, their oxidation state in oxides is always. Therefore, if there are four negative charges on the oxygen atom, the formal charge on the carbon atom must be +V so that the whole molecule is electroneutral: C C - - V V - C sum of oxidation numbers: 1 (V) + ( ) = 0 dinitrogen pentoxide n the case of odd numerical indices we can also use a cross rule to derive the oxidation state of bonded atoms or a group of atoms: V N 5 sum of oxidation numbers: (V) + 5 ( ) = 0 sulphuric acid There are two atoms of hydrogen, two atoms of oxygen and one atom of sulphur in a molecule of sulphuric acid (H S ). xygen atoms are more electronegative, hence they would bear a negative charge: H H S H H - - V S - - V - H S sum of oxidation numbers: () + 1 (V) + ( ) = 0 Many substances have been more widely known for years by their trivial or common names but all compounds can be described by their systematic names. Some of these examples are listed in the table below. Formula Trivial name Systematic name NH 3 ammonia nitrogen hydride Al 3 alumina aluminum oxide CH 3 CH H alcohol ethanol Ca burnt lime calcium oxide Ca(H) slaked lime calcium hydroxide CaS H gypsum calcium sulphate dihydrate CaS 0.5H plaster of Paris calcium sulphate hemihydrate CuS 5H blue vitriol copper () sulphate pentahydrate Si silica silicon oxide n English nomenclature, the name of inorganic compounds is composed of the name of the more electropositive part, usually a cation, followed by the name of the more electronegative part, usually an anion. 5

3 BULDNG CHEMSTRY LABRATRY SESSNS Some compounds may contain several particles of the same type. The number of such particles is expressed by a numerical prefix. The list of common prefixes is shown in the following table. The prefix mono- is only used in some special cases (e.g. carbon monoxide) and is usually omitted. Prefix Number Prefix Number hemi- 1/ hepta- 7 mono- 1 okta- 8 di- nona- 9 tri- 3 deca- 10 tetra- undeca- 11 penta- 5 dodeca- 1 hexa- 6 trideca- 13 Nomenclature of cations When an element only forms one monatomic cation, the ion is named by taking the unchanged name of the element: E.g. Na + sodium cation Al 3+ aluminum cation Ca + calcium cation When an element forms more cations with different oxidation states, each ion is named so that it is differentiated from the others. There are two ways to do this the Stock system and the ous-ic system. Stock System The oxidation state of the element is indicated by means of a Roman numeral in parentheses after the name of the element. us-ic System This is an older system, but still permitted by UPAC and often used. When the element forms two different cations, the lower oxidation state is indicated by the suffix -ous, and the higher oxidation state by the suffix ic, following the root of the Latin name of the element. Element Name English Latin copper cuprum tin stannum iron ferrum arsenic arsenium lead plumbum xidation Formula of Name State Cation ous-ic s. Stock s. + Cu + cuprous ion copper() ion + Cu + cupric ion copper() ion + Sn + stannous ion tin() ion +V Sn + stannic ion tin(v) ion + Fe + ferrous ion iron() ion + Fe 3+ ferric ion iron() ion + As 3+ arsenous ion arsenic() ion +V As 5+ arsenic ion arsenic(v) ion + Pb + plumbous ion lead() ion +V Pb + plumbic ion lead(v) ion 6

4 APPENDX A: CHEMCAL NMENCLATURE A special type of complex cation is the ammonium ion (NH + ), which is formed by the addition of hydrogen cation (H + ) to a neutral molecule of ammonia. t has the formal oxidation state +, (NH ). xides Naming oxides xides are binary compounds with oxygen where the oxidation state of oxygen is -. xides of metals are named as if they were salts. The name is composed of the cation name and the word oxide. E.g. Na sodium oxide Ca calcium oxide Sn tin() oxide or stannous oxide Sn tin(v) oxide or stannic oxide Cu copper() oxide or cuprous oxide Cu copper() oxide or cupric oxide xides of non-metals are named by stating the name of the element first, followed by the word oxide. Numeral prefixes are used where necessary: E.g. S sulphur dioxide S 3 sulphur trioxide N nitrogen oxide or nitrogen monoxide N 5 dinitrogen pentoxide Cl dichlorine oxide Cl 5 dichlorine pentoxide Cl 7 dichlorine heptoxide P 3 diphosphorus trioxide P 5 diphosphorus pentoxide Some elements form only one type of oxide. n these cases, the use of numerical prefixes is not necessary: E.g. Al 3 aluminum oxide Si silicon oxide Deriving formulas To derive a formula for an oxide, write the symbols of the elements together with indices according to the numerical prefix that indicates the number of specified atoms in the molecule. E.g. dinitrogen oxide N carbon dioxide C n the case of metal oxides, where the numerical prefixes are not usually used, we have to obey the rule of electroneutrality. E.g. boron oxide B B 3 3 7

5 BULDNG CHEMSTRY LABRATRY SESSNS lead(v) oxide antimony(v) oxide Pb V Pb As V 5 As 5 Hydroxides Hydroxides are compounds which have an H group in their molecules. Since the oxidation state of oxygen is and the oxidation state of hydrogen is, the formal oxidation state of the hydroxide group is, (H) -. n the nomenclature of hydroxides the metal ion is named first, followed by the word hydroxide. f the oxidation state of the metal is >, we have to put the H group into brackets. E.g. sodium hydroxide Na (H) - NaH calcium hydroxide Ca ( H ) Ca(H), not CaH aluminum hydroxide Al ( H Al(H) 3, not AlH 3 ) 3 Binary acids Binary acids are compounds of hydrogen with non-metal. The oxidation state of hydrogen in acids is always +. These acids are actually aqueous solutions of gaseous binary compounds that have the same formula. The most common binary acids are summarised in the following table: Formula Name of Pure Compound Name of Aqueous Solution HF hydrogen fluoride hydrofluoric acid HCl hydrogen chloride hydrochloric acid HBr hydrogen bromide hydrobromic acid H hydrogen iodide hydroiodic acid H S hydrogen sulphide sulphane hydrosulphuric acid xoacids xoacids are ternary compounds that can be generally formed by the reaction of non-metal oxides with water. xoacids are always formed of hydrogen H, oxygen, and a central atom that can have various oxidation states. Such acids are named according to the rules of ous-ic system. When the central atom forms only one acid, the suffix -ic is used: E.g. H C 3 carbonic acid H Si 3 silicic acid n the case that the central atom forms two oxoacids with different oxidation states, the suffixes -ous for the lower and -ic for the higher oxidation states are used. 8

6 APPENDX A: CHEMCAL NMENCLATURE Formula xidation state Systematic name HN + nitrous acid HN 3 +V nitric acid H S 3 +V sulphurous acid H S +V sulphuric acid Up to four different oxidation states can be differentiated by using the prefix hypo- to indicate a state lower than the -ous state and the prefix per- to indicate a state higher than the -ic state. Formula xidation state Systematic name HCl + hypochlorous acid HCl + chlorous acid HCl 3 +V chloric acid HCl +V perchloric acid Some acids have more than two hydrogen atoms or more than one central atom in their molecules. These compounds are called polyprotic acids or polyacids, respectively. The number of hydrogen and central atoms is then expressed by numerical prefix. Formula xidation state Systematic name H 3 P +V trihydrogenphosphoric acid H Si +V tetrahydrogensilicic acid H S 5 +V disulphurous acid H S 7 +V disulphuric acid H P 7 +V tetrahydrogendiphosphoric acid Deriving formulas from corresponding oxides by adding molecule(s) of water and subsequent cancelling of indices S V + H H S V 3 S V 3 + H H S V sulphur dioxide sulphurous acid sulphur trioxide sulphuric acid C V + H H C V 3 N V 5 + H HN V 3 carbon dioxide carbonic acid dinitrogen pentoxide nitric acid by applying the rule of electroneutrality (sum of oxidation states must be equal to zero) V perchloric acid HCl H Cl x V + x ( ) = 0 x = HCl nitric acid V HN H N x V + x ( ) = 0 x = 3 HN 3 f the sum of positive oxidation numbers is an odd integer the number of hydrogen atoms must be two. sulphurous acid HS H S V - V ( + V = V) H S x + 1 V + x ( ) = 0 x = 3 H S 3 9

7 BULDNG CHEMSTRY LABRATRY SESSNS disulphuric acid V V HS H S x ( + V = X) H S x + 1 V + x ( ) = 0 x = H S Salts The name of a salt consists of the name of its cation followed by that of its anion. An anion is always formed by splitting one or more hydrogen atoms from an acid. Some hydrogen atoms may stay fixed in the anion part, which is expressed by the prefix hydrogen- and, if necessary, a numerical prefix. HCl H + + Cl chloride H S H + + H 3 P H + + HS H + + hydrogensulphate S sulphate H H + + P HP H P dihydrogenphosphate hydrogenphosphate phosphate Salts of binary acids n the case of salts derived from binary acids the name of the anion consists of the stem of the central atom plus the suffix ide. Binary acid Anion Formula Name Formula Name HF hydrofluoric acid F - fluoride HCl hydrochloric acid Cl - chloride HBr hydrobromic acid Br - bromide H hydroiodic acid - iodide H S hydrosulphuric acid S - sulphide (HS) - hydrogensulphide E.g. Sn (NH ) S Ca(HS) FeCl 3 Sn V tin(v) iodide or stannic iodide ( NH S ammonium sulphide Ca ) ( HS calcium hydrogensulphide ) Fe Cl3 iron() chloride Deriving formulas The rules for deriving formulas for salts are similar to those used for the nomenclature of metal oxides. Complex anions that consist of more than one atom must be enclosed in brackets if the numerical index is greater than one. E.g. sodium chloride Na Cl - NaCl iron() bromide Fe Br3 FeBr 3 magnesium hydrogensulphide Mg ( HS Mg(HS), not MgHS ) 50

8 APPENDX A: CHEMCAL NMENCLATURE Salts of oxoacids The name of an oxoanion is derived from the name of the corresponding oxoacid. When the name of an acid ends with ous the anion ends with the suffix ite and when the acid ends with ic the corresponding suffix is ate. The system of prefixes for even higher or lower oxidation states is the same as for the naming of acids. The formal oxidation state of an anion can be obtained using the cross rule. Na C 3 K K Ca(Cl) hypochlorite Na ( C3 ) derived from H C 3 (carbonic acid) sodium carbonate ) Cl) ( derived from H (periodic acid) potassium periodate Ca ( derived from HCl (hypochlorous acid) calcium Note: Phosphates are usually derived from trihydrogenphosphoric acid H 3 P. E.g. Mg 3 (P ) Mg3 ( P ) magnesium phosphate K Cr 7 K ( Cr 7) potassium dichromate Na HP Na ( HP sodium hydrogenphosphate Deriving formulas To derive the formulas for ternary salts it is necessary to obtain the formula for the corresponding oxoacid. The anion can be obtained by splitting one or more hydrogen atoms from the acid. To obtain the whole formula, put cation first followed by the anionic part together with numerical indices so that the sum of the oxidation numbers is zero. Complex anions that consist of more than one atom must be enclosed in brackets if the numerical index is greater than one. potassium nitrate derived from nitric acid HN 3 (N 3 ) - K ( N3 ) KN 3 sodium sulphate derived from sulphuric acid H S (S ) - Na ( S ) Na S aluminum sulphite derived from sulphurous acid H S 3 (S 3 ) - Al S3 ) 3 ( Al (S 3 ) 3, not Al S 3 calcium dihydrogenphosphate derived from trihydrogenphosphoric acid H 3 P Ca ( H P ) Ca(H P ), not CaH (P ) magnesium hydrogencarbonate derived from carbonic acid H C 3 Mg ( HC3) Mg(HC 3 ), not MgH(C 3 ) Hydrates Many inorganic salts and minerals contain weakly bonded molecules of water in their structure. These compounds are called hydrates. The chemical formula is formed from two parts: the formula of a salt and the formula of water, separated by a dot. The number of water molecules is expressed by a capital numeral just before the water formula and is identified in the name by a numerical prefix. E.g. CuS 5H copper() sulphate pentahydrate Na C 3 10H sodium carbonate decahydrate 51