The resulting chemical formulae are CaBr2, Al2(CO3)3, K2SO4 and NH4Cl. In each ionic compound, the valency numbers are criss-crossed so that positive and negative charges balance out to maintain overall electrical neutrality.
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(i) Aluminium nitrate has the chemical formula Al(NO3)3. (ii) Calcium oxide has the chemical formula CaO. (iii) Ferric oxide has the chemical formula Fe2O3. Each formula balances the combining valencies of the constituent ions.
Statement (iv) is true. Statements (i), (ii) and (iii) are false. Elements contain identical atoms, compounds contain atoms of different elements and nitrogen gas exists as diatomic molecules with two atoms.
The correct option is (iii). Two Fe3+ ions provide a total positive charge of 6+ and three O2- ions provide a total negative charge of 6-, making the combination neutral and fully balanced.
Element X needs two electrons to complete its valence octet. Two X atoms share two electron pairs equally, establishing a double covalent bond to create a stable diatomic molecule with the structure X double bond X.