JEE Chemistry Basic Inorganic Nomenclature Notes Set 01

Oxidation Number: Meaning and Rules

Oxidation number is the apparent charge an atom would carry if every shared pair of electrons went wholly to the more electronegative atom. It is a bookkeeping device for tracking electron shifts.
1. Fluorine is always -1 in compounds. Oxygen is -2 in oxides, -1 in peroxides, -1/2 in superoxides, -1/3 in ozonides, +2 in \( \ce{OF2} \) and +1 in \( \ce{O2F2} \).
2. Hydrogen is +1, but -1 in metal hydrides such as NaH. Other halogens are -1 unless bonded to a more electronegative atom (as in \( \ce{KClO3} \), \( \ce{HClO4} \)).
3. Alkali metals are +1, alkaline earth metals +2 and aluminium +3. Atoms in an element, in any allotrope, are 0.
4. The oxidation numbers in a neutral molecule add up to zero; in an ion they add up to the charge on the ion.
5. For a p-block element in group n, the range is from (n - 18) to (n - 10). The maximum is never more than the number of valence electrons.

Common Oxidation States

Nitrogen: -3 to +5. Chlorine: -1, +1, +3, +5, +7. Sulphur: -2, +2, +4, +6. Manganese: +2 to +7. Chromium: +2, +3 (most stable), +6. Iron: +2, +3. Copper: +1, +2. Tin and lead: +2 and +4. Fluorine shows only -1. Peroxide-type average values such as +8/3 for iron in \( \ce{Fe3O4} \) and -1/3 for N in azides are fractional averages.

Naming Elements, Acids and Cations

1. Hydracids contain hydrogen and a non-metal but no oxygen; the name is hydro + stem + ic (hydrochloric, hydrocyanic, hydrosulphuric). Their anions end in -ide.
2. Monatomic cations of fixed charge end in -ium or take the element name (sodium ion). For variable charge use a Roman numeral, or -ous for the lower and -ic for the higher state: cuprous Cu(I), cupric Cu(II), stannous, stannic, ferrous, ferric, mercurous \( \ce{Hg2^2+} \) and mercuric \( \ce{Hg^2+} \).
3. Cations of non-metals use -ium: \( \ce{NH4^+} \) ammonium, \( \ce{H3O^+} \) hydronium, \( \ce{NO^+} \) nitrosonium, \( \ce{NO2^+} \) nitronium, \( \ce{N2H5^+} \) hydrazinium, \( \ce{C6H5NH3^+} \) anilinium, \( \ce{C5H5NH^+} \) pyridinium.

Simple and Miscellaneous Anions

Hydride \( \ce{H^-} \), fluoride, chloride, bromide, iodide, oxide, sulphide, selenide, telluride, nitride \( \ce{N^3-} \), phosphide, arsenide, antimonide, carbide \( \ce{C^4-} \), silicide, boride.
Others: hydroxide \( \ce{OH^-} \), peroxide \( \ce{O2^2-} \), superoxide \( \ce{O2^-} \), ozonide \( \ce{O3^-} \), disulphide \( \ce{S2^2-} \), triiodide \( \ce{I3^-} \), azide \( \ce{N3^-} \), imide \( \ce{NH^2-} \), amide \( \ce{NH2^-} \), cyanide, acetylide \( \ce{C2^2-} \), thiocyanate \( \ce{SCN^-} \), thiosulphate \( \ce{S2O3^2-} \), acetate, oxalate \( \ce{C2O4^2-} \), manganate \( \ce{MnO4^2-} \), permanganate \( \ce{MnO4^-} \).

Oxyacids and Their Anions

1. If an element forms one oxyacid, it takes -ic (\( \ce{H2CO3} \) carbonic). With two, the one with more oxygen is -ic and the one with less is -ous (sulphuric and sulphurous, nitric and nitrous).
2. In a series of four, the sequence is hypo...ous, ...ous, ...ic, per...ic: \( \ce{HClO} \), \( \ce{HClO2} \), \( \ce{HClO3} \), \( \ce{HClO4} \).
3. Anions: -ic acid gives -ate, -ous acid gives -ite, and hypo-/per- prefixes are kept (hypochlorite, perchlorate).
4. Ortho and meta differ in water content: \( \ce{H3BO3} \) is ortho, losing one water gives meta \( \ce{HBO2} \). Only Si, P and B form meta acids.
5. Pyro acids come from two molecules of the ortho acid less one water: \( \ce{H4P2O7} \), \( \ce{H4B2O5} \), \( \ce{H2S2O7} \), \( \ce{H6Si2O7} \). N, C, Cl and Br do not form pyro acids.
6. Peroxo replaces -O- by -O-O-: \( \ce{H2SO5} \), \( \ce{H2S2O8} \), \( \ce{HNO4} \), \( \ce{H3PO5} \). Thio replaces an oxygen by sulphur: \( \ce{H2S2O3} \) thiosulphuric acid.
7. Dithionic is \( \ce{H2S2O6} \) and dithionous is \( \ce{H2S2O4} \). Xenon: \( \ce{H4XeO6} \) perxenic acid, \( \ce{H2XeO4} \) xenic acid, with xenate and perxenate anions.
8. Acid salts: remaining replaceable hydrogen is named hydrogen or bi: \( \ce{HCO3^-} \) hydrogencarbonate (bicarbonate), \( \ce{HPO4^2-} \) monohydrogen phosphate, \( \ce{H2PO4^-} \) dihydrogen phosphate.

Writing Formulae of Ionic Compounds

Write the cation first, then the anion; write each charge over the other symbol (criss-cross) and cancel any common factor. Examples: calcium chloride \( \ce{CaCl2} \), aluminium oxide \( \ce{Al2O3} \), magnesium nitride \( \ce{Mg3N2} \), ammonium sulphate \( \ce{(NH4)2SO4} \). Calcium oxide is CaO, not \( \ce{Ca2O2} \).
When both elements are non-metals, the more electronegative one is written as the anion part: \( \ce{OF2} \) oxygen difluoride, \( \ce{ICl3} \) iodine trichloride, \( \ce{As2O3} \) arsenic(III) oxide.

Atoms Compared with Ions

Atoms are neutral (protons = electrons); cations have more protons than electrons and anions fewer. Ions usually reach a stable octet (ns2np6), for instance \( \ce{Na^+} \) 2,8 and \( \ce{Cl^-} \) 2,8,8, and their chemistry is governed by charge, with opposite ions held by electrostatic attraction.