Ionic compound is a compound composed of cations and anions.For example; sodium chloride consists of sodium ion and chloride ion.
Writing formula of an ionic compound:
Formula of an ionic compound is written by knowing the charges of the two ions involved.Then you can apply the principal of neutrality, which requires that the positive charge of the cations in the formula must equal the total negative charge of the anions.
For example: The ionic compound calcium chloride .The ions present are Ca2+ and Cl- . For compund to be electrically neutral,there must be two Cl- and 1 Ca2+ ion.Formula of calcium chloride must be CaCl2, indicating the simplest ratio of Cl- and Ca2+.
Generally speaking, you use the magnitude of charge on one ion to obtain the subscript for the other ion and you may need to simplify the formula obtained this way so that it expresses the simplest ratio of ions.
Lets see an example to understand this;
Chromium (II) oxide composed of Cr^3+ and O^2- ions, what is its formula?
Problem strategy:Because a compound is netral, the sum of positive & negative charges equals zero.Consider ionic compound CaCl2 , which consists of one Ca2+ and two Cl- ions.The sum of charge is;
1 x +2 + 2 x (-1) = 0
Note that the number of calcium ions in CaCl2 equals the magnitude of charge on the chloride ion (1), whereas the number of chloride ions in CaCl2 equals the magnitude of charge on the calcium ion (2).
Solution ; You can achieve electrical neutrality by taking as many cations as there are units of charge on the anion and as many anions as there are units of charge on the cations.Two Cr3+ ions have a total charge of 6+ and three O2- ions have a total charge of 6- , giving the combination a net charge of zero. The simplest ration of Cr3+ to O2- is 2:3 , and the formula is Cr2O3 . Note the charge without its sign on one ion becomes the subscript of the other ion.
let see one more example :
Strontium oxide is a compound composed of Sr2+ and O2- . Write the formula of this compound.
solution: you see the equal numbers of Sr2+ and O2- ions will give a neutral compound.Thus, the formula is SrO . If you use the units of charge to find the subscripts , you get Sr2O2 . But the final formula is SrO because this gives the simplest ratio of ions.
Naming an ionic compound :
The name of an ionic compound consists of two words .The first word names the cation and second names the anion.This is of course the same order on which the ions appear in the formula.
Monoatomic cations take the name of the metal from which they are derived . For example ; Na+ sodium K+ potassium .
However, there is one complication . Certain metals in the transition and post transition series form more than on e cation , for example : Fe2+ and Fe3+ .To distinguish between these cations , the charge must be indicated in the name . This is done by putting charge as a Roman numeral in the parentheses after the name of the metal :
Fe2+ iron(II) and Fe3+ iron (III)
An older system also uses suffixes - ic for the ion of higher charge and -ous for the ion of lower charge .These were added to the stem of the latin name of the metal , so that Fe2+ ion was referred to as ferrous and the Fe3+ ions was reffered as ferric .
Monoatomic anions are named by adding the suffix - ide to the stem of the name of the nonmetal from which they are derived.
N3- nitride , O2- oxide , F - flouride , H - hydride
The polyatomic ions (check out the download for the polyatomic ion table) are given special names.Certain polyatomuc ion containing oxygen (oxoanions) .The names of several such oxoanions are shown in table 2.3 . From the entries in the table, you should be able to deduce the following rules:
1) Whe a non metal forms two oxoanions, the suffix -ate is used for the anion with the larger number of oxygen atoms. The suffix -ite is used for the anion containing fewer oxygen atoms.
ClO- hypochloraite ion and ClO2- chlorite ion
2) When a non metal forms more than two oxoanions, the prefixes per - ( largest number of oxygen atoms) and hypo - ( fewest oxygen atoms) are used as well.
ClO3- chlorate ion and ClO4- perchlorate ion
Naming of the ionic compound consists of two words : the first word names the cation and the second names the anion .This is ofcourse , the same order in which the ions appear in the formula.
In naming the compounds of transitiion and post transition metals , we ordinarily indicate the charge of the metal cation by a Roman numeral:
Cr(NO3)3 chromium (III) nitrate SnCl2 tin (II) chloride
in contrast we never use Roman numberals with compounds of group I and II they always form cations with charges of +1 and +2 respectively.
CaS - calcium sulfide