Naming a compound from its molecular formula is definitely an useful skill to master for students studying Chemistry. This guide will help you become familiar with that skill. You can practice these skills and master them.
General Terms
An inorganic compound can either be described as binary, ternary and quarternary. A binary compound is the one that contains two different elements for example KCl; a ternary compound is the one that contains three different elements, for example \(K_2SO_4\) and a quarternary compound is the one that has four elements, for example \(KHSO_3\).
Number Prefixes
The number of atoms in a compound is usually denoted by a prefix. For example in CO, there is an oxygen atom for every carbon atom and hence it is called Carbon Monoxide. “Mono” is the term that indicates one.
Here are the terms that indicate One to Ten. Mono is One; Bi is Two; Tri is Three; Tetra is four; Penta is five; Hexa is Six, Hepta is Seven; Octa is Eight; Nona is Nine and Deca is Ten.
In \(B_2O_3\) there are \(1\frac{1}{2}\) oxygen atoms for every boron atom. This is indicated by the term “sesqui“. So, \(B_2O_3\) is called Boron Sesquioxide.
Examples: \(SO_2\) is Sulphur Dioxide, \(CCl_4\) is Carbon Tetrachloride and \(P_2O_3\) is Phosphorus Sesquioxide.
Metallic Cations
Some metal atoms show a single valency (oxidation state) and some metal atoms show multiple valencies (oxidation states). For example, Magnesium shows +2 oxidation state in all its compounds and Copper can show +1 and +2 oxidation states.
If the metal atom shows only on oxidation state, its name in the cationic state is the name of the metal itself. For example, \(Mg^{2+}\) is called Magnesium ion.
For metals showing multiple oxidation states the naming can be done in any of the following two ways.
(a) The name or the Latin name of the metal ion can me suffixed in such a way that the oxidation state can be identified from it. If the metal is present in its lowest oxidation state its name is suffixed with -ous and if the metal is present in its highest oxidation state its name is suffixed with -ic. Following are some examples;
Fe is Ferrum. If Fe is present in its lower oxidation state +2, it is called Ferrous ion and if Fe is present in its higher oxidation state, +3, it is called Ferric ion.
Cu is Cuprum. If Cu is present in its lower oxidation state +1, it is called Cuprous ion and if Cu is present in its higher oxidation state, +2, it is called Cupric ion.
Similarly, Mercury is called Mercurous ion in its +1 state and Mercuric ion in its +2 state.
(b) The name of the metal is followed by the oxidation state indicated by Roman Numeral in parentheses.Following are some examples;
\(Cu^+\) can be called Copper (I) ion and \(Cu^{2+}\) can be called Copper (II) ion. Similarly, \(Pb^{2+}\) can be called Lead (II) ion and \(Pb^{4+}\) can be called as Lead (IV) ion.
Anions
The anion in a compound could either be elemental or a group of atoms called radicals. For example, \(Cl^-\) is an elemental anion and \(ClO_3^-\) is a radical.
(a) The names of elemental anions end with the suffix -ide. SO, \(Cl^-\) is called chloride and \(S^{2-}\) is called sulphide.
(b) Typically in an anionic radical the central atom will be connected to few oxygen atoms. If the radical is monovalent (i.e. has an overal charge of -1), it will usually found to be connected to three oxygen atoms. If the radical is polyvalent (i.e. has an overall charge of more than -1), it will usually found to be connected to four oxygen atoms. In both the cases described the name of the anionic radical will end with the suffix -ate. For example, \(ClO_3^-\) is called chlorate and \(PO_4^{3-}\) is called phosphate.
If there are less oxygen atoms than expected, then the name of the anionic radical will end with a different suffix to indicate it. If there is one oxygen atom less than expected, the name of the anionic radical will end with -ite. Examples are \(SO_3^{2-}\) ion - sulphite ion and \(NO_2^-\) ion - nitrite ion.
If there is one oxygen more than expected, the name of the anionic radical gets a prefix -per along with the suffix -ate. For example, \(BrO_4^-\) is called perbromate ion.
If there are two oxygen atoms less than expected, the name of the anionic radical gets a prefix -hypo along with the suffix -ite. For example, \(ClO^-\) is called hypochlorite ion.
© If an anion is formed by removal of hydrogen atoms from a molecule (conjugate base of acids), the name of the anionic radical starts with a prefix -bi. For example, \(HS^-\) is called bisulphide ion.
To name an inorganic compound the name of the cation appears first followed by the name of the anion. For example, \(Ba SO_4\) is called Barium Sulphate and \(KClO_4\) is called Potassium Perchlorate.