Chemistry

Atoms and Molecules


Writing Of Formulae Of Ionic Compounds


We will now describe the method of writing the chemical formulae of ionic compounds. First of all we write down the name of compound whose formula is required. Then we write down the symbols of its ions. As a convention, the cation (positive ion) is written on the left hand side and the anion (negative ion) is written on the right hand side.The number of cations and anions is adjusted in such a way that the total number of positive valencies of cations becomes equal to the total number of negative valencies of anions.In other words, the number of cations and anions is adjusted in such a way that the total number of positive charges of cations becomes equal to the total number of negative charges of anions (because an ionic compound is electrically neutral, having no over-all charge). The number of cations required (2, 3, 4,etc.) is written on the right side bottom of the symbol of cation but without showing the charge on the cation. For example, if 2 aluminium ions are required to balance the charges in a compound, then 2Al3+ is written as Al2 in the formula of the compound. Similarly, the number of anions required (2, 3, 4, etc.) is written on the right side bottom of the symbol of the anion but without showing the charge on the anion. For example, if three sulphate ions are needed to balance the charges in a compound, then 3SO42- is written as (SO4)3 in the formula of the compound. Please note that if only 1 cation or anion is required to write the formula of a compound then the digit 1 is not written with the symbol of the ion. For example, if 1 sodium ion Na+ is needed to write the formula of a compound, then we write just Na for it (and not Na1). Similarly,if one chloride ion Cl- is required, then we write just Cl for it (and not Cl1).Another point to be noted is that the final formula of an ionic compound is written without showing the charges on the ions involved in it.Let us take an example to learn the writing of formulae of ionic compounds.
Suppose we have to write the formula of sodium carbonate.Now, sodium carbonate is made up of two types of ions : sodium ion, Na+,and carbonate ion, CO32-.We find that the sodium ion,Na+,has 1 unit of positive charge whereas carbonate ion,CO32-, has 2 units of negative charge.This means that two Na+ions are needed to balance the two negative charges (or valencies) of a carbonate ion CO32-.So, the sodium carbonate compound is made up of 2Na+ ions and one CO32- ion, that is,2Na+ CO32-.Now, when we write the formula of sodium carbonate then 2Na+ is written as Na2 and CO32- is written as CO3(because charges are not shown in the formula), so that the formula of sodium carbonate becomes Na2CO3.
We will now describe the second method of working out the formula of sodium carbonate by the crossing-over of valencies (or charges) of the sodium ion and carbonate ion. First we write down the symbols of sodium ion and carbonate ion (without showing the charges on them). Below the symbol of sodium ion we write the valency (or charge) of sodium ion which is 1+. And below the carbonate ion we write the valency (or charge) of carbonate ion which is 2-. This is shown below :
We now cross-over the valencies (or charges) of the sodium ion and carbonate ion.The crossed-over valencies are written as subscripts with the ions (but without their charges). In this way we get Na2 and CO3 which on joining give Na2CO3. This *s the formula of sodium carbonate.
A very important point to note here is that in the "crossing-over of valencies' method of writing the formulae of ionic compounds, we usually do not write the charges (plus or minus) on the symbols of the ions. This is done just for the sake of convenience and to avoid confusion. The plus and minus charges on the valencies are, however, shown. We will now solve some problems based on the writing of formulae of ionic compounds.
Sample Problem 1. Work out the formula for magnesium chloride.
Solution. Magnesium chloride is an ionic compound made up of magnesium ions (Mg2+) and chloride ions (Cl-).Magnesium ion has a valency (or charge) of 2+ whereas chloride ion has a valency (or charge) of
1.The formula for magnesium chloride can be worked out as follows :
Sample Problem 2. Work out the formula for aluminium sulphate.
Solution. Aluminium sulphate is made up of aluminium ions (Al3+) and sulphate ions (SO42-).The valency (or charge) of aluminium ion is 3+ whereas the valency or charge of sulphate ion is 2-. The formula for aluminium sulphate can be worked out as follows :
Thus, the formula of aluminium sulphate is Al2(SO4)3
Sample Problem 3. The symbols of some of the ions are given below :K+,Al3+,Br-,O2- using this information, write down the formulae of :
(i) Aluminium bromide
(ii) Potassium oxide
Solution. (i) Formula for aluminium bromide
Thus, the formula for aluminium bromide is AlBr3
Please write the formula for potassium oxide yourself. The correct formula will be K2O.
Sample Problem 4. The valencies of two elements A and B are given below :ElementValency
A 1+
B 2+
(i) What is the formula of sulphate of A ?
(ii) What is the formula of nitrate of B ?
Solution.(i) Formula of sulphate of A
Thus, the formula of sulphate of element A is A2SO4
(Please note that the element A of valency 1 is like sodium. So, the sulphate A2SO4 may be sodium sulphate, Na2SO4).
(ii) Formula of nitrate of B
Thus, the formula of nitrate of element B is B(NO3)2.
[It should be noted that the element B of valency 2 is like magnesium, so that the nitrate B(NO3)2 may be magnesium nitrate,Mg(NO3)2]
Sample Problem 5. The formula of the chloride of a metal is MCl2.What will be the formula of its sulphate ?
Solution.(i) First of all we have to find out the valency of metal M from the formula of its ionic chloride MCl2.Now, we know that the valency of chlorine (Cl) is 1. Since one atom of M has 2C1 atoms attached to it, so the valency of M is 2 or 2+.
(ii) The formula of sulphate of metal M of valency 2 can be found as follows :
Thus,the formula of sulphate of metal M will be MSO4.
(The metal M of valency 2 is like magnesium, so the chloride MCl2 is like magnesium chloride,MgCl2,and the sulphate MSO4 is like magnesium sulphate,MgSO4).
Before we go further and discuss mole concept please answer the following questions :

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Notes
Atoms and Molecules - Notes
1. Drawbacks of Dalton’s Atomic Theory
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2. Monovalent Cations (Cations Having a Valency of 1+)
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3. Divalent Cations (Cations Having a Valency of 2+)
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4. Trivalent Cations (Cations Having a Valency of 3+)
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5. Dalton’s Symbols of Elements
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6. an example, let us give the significance of symbol C
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7. Formulae of Some Molecular Compounds
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8. An Important Discussion
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9. Explanation of the Law of Conservation of Mass
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10. Molecular Formulae of Some Common Elements
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11. 2. A Negatively Charged Ion is Known as Anion
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12. Molecules
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13. As an example, let us give the significance of the formula H2O
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14. Simple Ions and Compound Ions (Polyatomic Ions)
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15. Molecules of Compounds
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16. Chemical Formulae
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17. Formulae of Elements
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18. Some Ionic Compounds
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19. Symbols of Elements
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20. Molecular Mass
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21. Gram Atomic Mass And Gram Molecular Mass
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22. Atoms
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23. Formulae of Compounds
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24. Formula Mass
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25. Atoms, Molecules and Ions
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26. 1. Molecules of Elements
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27. Mole of Atoms
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28. 1. A Positively Charged Ion is Known as Cation
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29. Ions
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30. Mole of Molecules
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31. Dalton's Atomic Theory
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32. Laws Of Chemical Combination
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33. Calculation of Molecular Mass
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34. Divalent Anions (Anions Having a Valency of 2-)
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35. Molecular Masses of Some Common Elements
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36. Some Common Ions
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37. Explanation of the Law of Constant Proportions
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38. John Dalton
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39. Significance of the Formula of a Substance
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40. Formula Unit of Ionic Compounds
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41. Ionic Compounds
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42. Symbols Derived from Latin Names of the Elements
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43. Atomic Mass Of An Element
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44. Modern Symbols of Elements
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45. Valencies of Some Common Metal Elements Valencies of Some Common Non-Metal Elements
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46. Atomic Masses of Some Common Elements
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47. Writing Of Formulae Of Molecular Compounds
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48. Laws Of Conservation of Mass
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49. Problems Based On Moles Of Atoms
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50. Writing Of Formulae Of Ionic Compounds
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