The calculated formula unit masses are 80 u for ammonium nitrate NH4NO3, 98 u for phosphoric acid H3PO4 and 84 u for sodium hydrogencarbonate NaHCO3, found by adding the individual atomic masses.
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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.
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.
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.