Answered

Which of the following is a correct formula unit of an ionic compound?


A$Al _2 O _3$
B$Ca _2 O$
CMgOH
D$NaCl _2$

Verified Answer :

  • Analyze the net charge of each chemical formula: \(Al_2O_3\), \(Ca_2O\), \(MgOH\), and \(NaCl_2\).
  • Determine ion charges based on their periodic table groups: \(Al^{3+}, O^{2-}, Ca^{2+}, Mg^{2+}, OH^-, Na^+, Cl^-\).
  • Verify electrical neutrality: the sum of charges must be zero.
  • Identify the correct formula as \(\boxed{Al_2O_3}\).

Explanation

  1. Analyzing the Requirements for Ionic Compounds

An ionic compound is a chemical compound formed by the electrostatic attraction between a positively charged cation (usually a metal) and a negatively charged anion (usually a nonmetal). A fundamental property of stable, solid ionic compounds is that they must be electrically neutral, meaning the sum of the positive charges from the cations and the negative charges from the anions in the formula unit must equal zero. We will evaluate each of the given options: \(Al_2O_3\), \(Ca_2O\), \(MgOH\), and \(NaCl_2\) to determine which one satisfies this neutrality condition based on common ion charges.

  1. Evaluating Each Chemical Formula
  • For \(Al_2O_3\): Aluminum (Al) is in group 13, typically forming a \(+3\) ion (\(Al^{3+}\)). Oxygen (O) is in group 16, forming a \(-2\) ion (\(O^{2-}\)). The total charge is \((2 \times +3) + (3 \times -2) = +6 - 6 = 0\). This formula is neutral and correct.
  • For \(Ca_2O\): Calcium (Ca) is in group 2, forming a \(+2\) ion (\(Ca^{2+}\)). Oxygen (O) is a \(-2\) ion (\(O^{2-}\)). The total charge is \((2 \times +2) + (1 \times -2) = +4 - 2 = +2\). This is not neutral (the correct formula would be \(CaO\)).
  • For \(MgOH\): Magnesium (Mg) is in group 2, forming a \(+2\) ion (\(Mg^{2+}\)). Hydroxide (OH) is a polyatomic ion with a \(-1\) charge (\(OH^{-}\)). The total charge is \((1 \times +2) + (1 \times -1) = +1\). This is not neutral (the correct formula would be \(Mg(OH)_2\)).
  • For \(NaCl_2\): Sodium (Na) is in group 1, forming a \(+1\) ion (\(Na^{+}\)). Chlorine (Cl) is in group 17, forming a \(-1\) ion (\(Cl^{-}\)). The total charge is \((1 \times +1) + (2 \times -1) = 1 - 2 = -1\). This is not neutral (the correct formula would be \(NaCl\)).
  1. Conclusion

Based on our analysis, the only formula unit that results in a net charge of zero is \(Al_2O_3\). Therefore, it is the only correct representation of a stable ionic compound among the choices provided. The final answer is \(\boxed{Al_2O_3}\).

Examples

The principles of ionic charge balancing are essential in fields like materials science and pharmaceuticals. For instance, when chemists formulate a new salt for a medicine, they must ensure the ionic formula is neutral to guarantee stability and biological safety. Using the periodic table to balance charges, they can predict how different ions will combine to form stable compounds like \(Al_2O_3\), which is also widely known as the mineral corundum (a component of rubies and sapphires). Understanding these fundamental rules allows scientists to build complex structures from the atomic level up.

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