Ions And Formula

Names and Formulae of a Simple Compounds

The simplest compounds are binary compounds. A binary compound is a compound that contains atoms of only two different elements. For example, NaCl is a binary compound of sodium and chlorine. CaF₂ is a binary compound of calcium and fluorine. It may be noted that a binary compound may contain more than two atoms.
While writing the formula of a binary compound 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 symbols of the non-metal element is written on right hand side.
While naming the binary compounds, the first element (less electronegative element) is named as such while the name of the second element (more electronegative element) is written with an ide 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 as follows :
    Hydrogen – Hydride            Nitrogen – Nitride
    Fluorine – Fluoride            Carbon – Carbide
    Chlorine – Chloride            Boron – Boride
    Bromine – Bromide            Iodine – Iodide
    Oxygen – Oxide                Sulphur – Sulphide
    Phosphorus – Phosphid

While naming binary compounds of metals and non-metal, the subsript numericals are ignored. For example, BaCl₂ is named barium chloride and not barium dichloride.

On the other hand while naming the binary compounds of two non-metals, the subscript numericals 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 PCl₃ and PCl₅. 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 :

Imp points to be noted that :
    = ‘–o’ or ‘–a’ at the end of the prefix is often dropped before another vowel. For example, tetroxide, pentoxide, etc. However, in case of iodides ‘–o’ or ‘–a’ at the end of the prefix is not dropped. For example, PI₅ is named phosphorus pentaiodide and not phosphorus pentiodide.

    = The prefix mono is generally not used for the first element in the formula.

    = When hydrogen is the first element in the formula, no prefix is added before hydrogen irrespective of the number of hydrogen atoms in the formula. For example, H₂S is named hydrogen sulphide and not dihydrogen sulphide. Similarly, H₂Se is named hydrogen selenide.

    = A few compounds of hydrogen with non-metals are named by their trivial or common names. For example, H₂O is called water and NH₃ is called ammonia