Questions: Writing Chemical Formulas for Ionic Compounds

Question 1

What is the correct formula for calcium nitrate, formed from Ca²⁺ and NO₃⁻?

• A. CaNO₃
• B. Ca(NO₃)₂
• C. CaN₂O₆
• D. Ca₂NO₃

Answer: (B)

Ca²⁺ requires two NO₃⁻ ions to achieve charge neutrality (+2 and 2×−1). The entire nitrate unit must be enclosed in parentheses before adding the subscript: Ca(NO₃)₂. Writing CaNO₃ omits the second nitrate; writing CaN₂O₆ incorrectly expands the polyatomic ion instead of using parentheses. The parentheses are essential — the subscript applies to the whole NO₃⁻ group.

Question 2

Applying the criss-cross method to Mg²⁺ and O²⁻ produces the preliminary formula Mg₂O₂. What is the correct final ionic formula?

• A. Mg₂O₂, because ionic formulas preserve the subscripts from the criss-cross method exactly
• B. MgO, because ionic formulas must be reduced to the simplest whole-number ratio
• C. MgO₂, because the oxygen subscript is always kept when it exceeds 1
• D. Mg₂O, because the cation subscript is retained and the anion subscript is dropped when equal

Answer: (B)

The criss-cross method gives a starting point, but you must always reduce the ratio to its simplest form. Mg₂O₂ has a 2:2 ion ratio, which simplifies to 1:1, giving MgO. A common error is treating the criss-cross result as final without checking for simplification.

Question 3

The formula CaCl₂ represents a discrete molecule in which one calcium atom is covalently bonded to two chlorine atoms.

• T. True
• F. False

Answer: False

Ionic formulas represent the simplest ratio of ions in an extended crystal lattice, not discrete covalently bonded molecules. In solid CaCl₂, each calcium ion is surrounded by multiple chloride ions and vice versa — there is no identifiable 'molecule.' The formula unit CaCl₂ simply expresses that the Ca²⁺ to Cl⁻ ratio in the lattice is 1:2.

Question 4

When writing the formula for a compound that requires more than one polyatomic ion, the polyatomic ion must be enclosed in parentheses before adding the subscript.

• T. True
• F. False

Answer: True

Parentheses are required because the subscript must apply to the entire polyatomic ion as a unit. For example, Ca(NO₃)₂ correctly indicates two complete NO₃⁻ groups. Without parentheses, CaNO₃₂ is ambiguous and chemically meaningless — it could be misread as a formula with 32 oxygen atoms or other errors.

Question 5

Why must you reduce the ratio to its simplest whole numbers after applying the criss-cross method? Give an example where failing to do so would produce a wrong formula.

Model answer: Ionic formulas express the simplest whole-number ratio of ions — the empirical ratio in the crystal lattice. The criss-cross method ensures charge balance but can produce non-simplified ratios. For example, Mg²⁺ and O²⁻ criss-cross to Mg₂O₂, but the correct formula is MgO (ratio 1:1). Reporting Mg₂O₂ implies a different compound than MgO and would give incorrect molar masses and stoichiometric calculations.

Simplification is required because ionic formulas are empirical — they describe the fundamental ratio, not the absolute count of ions. Two formulas with the same ratio (MgO and Mg₂O₂) describe the same compound; only the simplified form is the accepted convention.