Question

Write down the formulae of

(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide

NCERT Class IX
Sciences - Main Course Book

Chapter 3. Atoms and Molecules

Answer 1

Crossing over of valencies to write the formula of ionic compounds:

We first write the symbols of the ions which form the ionic compound.
Below the symbol of each ion we write down its valency.
Finally we cross over the valencies of the combining ions that is with first ion we write a valency of 2nd ion ( as subscript) and with the second ion we write the valency of first ion (as subscript).


i)
Sodium is an ionic compound made up of sodium ions and oxide ions. Sodium ion has a valency or charge of 1+ where is oxide Ion has a valency or charge of 2-

Na O
1+ 2- (cross over valencies)

Formula: Na2O

Hence, the formula of sodium oxide is Na2O.


ii)

Aluminium chloride is an ionic compound made up of aluminium ions and chloride ions. Aluminium Ion has a valency of 3+ whereas chloride Ion has a valency of1-.

Al Cl
3+ 1- (cross over valencies)

Formula: AlCl3

Hence, the formula of aluminium chloride is AlCl3.

iii)
Sodium sulphide ionic compound made up of sodium ions and sulphide ions. Sodium Ion has a valency of 1+ ion has a valency of 2-

Na S
1+ 2- (cross over valencies)

Formula: Na2S

Hence, the formula of sodium sulphide is Na2S.

iv)
Magnesium hydroxide is an ionic compound made up of magnesium and Hydroxide ions. Magnesium iron has a valency of 2+ and Hydroxide has a valency of 1-

Mg OH
2+ 1- (cross over valencies)

Formula: Mg(OH)2

Hence, the formula of magnesium hydroxide is Mg(OH)2.

In the crossing over of valencies method of writing the formula of ionic compounds we usually to do not write the charges plus or minus on the symbols of the ions.

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Answer 2

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