In basic inorganic chemistry, what is the correct chemical formula of aluminium nitride, the binary compound formed between aluminium and nitrogen?

Introduction / Context:
This question tests your understanding of writing chemical formulas for binary ionic compounds, specifically aluminium nitride. In many entrance and board examinations, candidates are expected to translate a compound name into its correct empirical formula by using valency rules and charge balance. Aluminium nitride is an important ceramic material and a classic example used in textbooks to illustrate the bonding between a metal cation and a nonmetal anion. Knowing its correct formula strengthens your grasp of ionic compounds, oxidation states, and the systematic way in which formulas are derived from element symbols and charges.

Given Data / Assumptions:
- The compound name given is aluminium nitride.
- Aluminium is a metal belonging to group 13 of the periodic table.
- Nitrogen is a nonmetal from group 15 and forms nitride ions in ionic compounds.
- You must select the correct chemical formula from four closely related options.

Concept / Approach:
The main concept here is charge balance in ionic compounds. Aluminium usually forms Al3+ ions, while nitrogen forms nitride ions, written as N3 minus. In a stable ionic compound, the total positive charge must balance the total negative charge so that the overall compound is electrically neutral. Once you know the common charges on aluminium and nitrogen, you can combine the ions in the simplest whole number ratio that produces a neutral compound. The formula is written by placing the metal symbol first followed by the nonmetal symbol, with subscripts indicating the number of each ion required for neutrality.

Step-by-Step Solution:
Step 1: Identify the cation and anion involved. Aluminium forms Al3 plus ions and nitrogen forms N3 minus ions called nitride. Step 2: Apply the neutralization rule for ionic compounds. The algebraic sum of all ionic charges must equal zero in the final formula. Step 3: Observe that one Al3 plus ion and one N3 minus ion already balance each other because 3 plus and 3 minus add to zero. Step 4: Since a one to one ratio of aluminium to nitrogen gives a neutral compound, there is no need for any additional subscripts. Step 5: Write the formula with the metal first and nonmetal second, giving AlN as the correct chemical formula for aluminium nitride.

Verification / Alternative check:
As a quick alternative check, you can use the criss cross method that many students use to write ionic formulas. Take the valency or charge magnitude of aluminium, which is 3, and of nitride, which is also 3. Criss cross the numbers as subscripts to get Al3N3 and then reduce the subscripts to the simplest whole number ratio by dividing both by 3. This simplification gives AlN. Standard chemistry reference tables and textbooks also list aluminium nitride with the formula AlN, confirming that this choice is correct and consistent with practice.

Why Other Options Are Wrong:
Al2N would imply a total positive charge of 2 multiplied by 3 plus, which is 6 plus, on aluminium and a total negative charge of 3 minus on nitrogen, giving an overall 3 plus charge, so it is not neutral. AlN2 would correspond to 3 plus from aluminium and 2 multiplied by 3 minus which is 6 minus from nitrogen, leading to a net negative charge. AlN3 is even more unbalanced. None of these formulas satisfies charge neutrality, so they are not acceptable empirical formulas for aluminium nitride.

Common Pitfalls:
Students often forget to consider the magnitude of ionic charges and instead write formulas by guesswork or by simply matching subscripts to group numbers. Another common mistake is not simplifying larger subscripts like Al3N3 down to the lowest ratio. Some learners may also confuse nitrides with nitrates and mistakenly think aluminium nitride should contain oxygen, which is incorrect. Always remember to identify the ionic charges clearly, enforce overall charge neutrality, and then reduce the subscripts to the simplest whole number ratio.

Final Answer:
The correct empirical chemical formula of aluminium nitride is AlN.