Option (iii) correctly represents the chloride ion Cl-. A neutral chlorine atom has 17 electrons with arrangement 2, 8, 7. Gaining one electron forms Cl- with 18 electrons distributed as 2, 8, 8 in three shells.
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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.
(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.
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 A loses one electron to form a unipositive cation A+. Element B gains two electrons to form a dipositive anion B2-. They combine through an ionic bond to form the compound A2B.