Aluminum sulfide formula
The compound described by the keyword aluminum sulfide formula is a binary ionic compound made from aluminum (a metal) and sulfide (a nonmetal anion). The correct formula is found by making the total ionic charge equal to zero using the smallest whole-number ratio of ions.
Final result: \( \mathrm{Al_2S_3} \).
Step 1: Identify the ions and their charges
Aluminum commonly forms a \(+3\) cation because it is in Group 13 and tends to lose three electrons: \( \mathrm{Al^{3+}} \). Sulfur forms the sulfide anion with a \(−2\) charge (Group 16 gains two electrons): \( \mathrm{S^{2-}} \).
| Ion | Charge | Meaning |
|---|---|---|
| Aluminum ion | \( \mathrm{Al^{3+}} \) | One aluminum ion contributes \(+3\) charge. |
| Sulfide ion | \( \mathrm{S^{2-}} \) | One sulfide ion contributes \(−2\) charge. |
Step 2: Balance charges using the least common multiple
The charge magnitudes are \(3\) and \(2\). The least common multiple is \(6\). Make \(+6\) total from aluminum ions and \(−6\) total from sulfide ions:
- \(2 \times (+3) = +6\) → use 2 aluminum ions.
- \(3 \times (−2) = −6\) → use 3 sulfide ions.
| Ion | Charge per ion | Number of ions | Total charge |
|---|---|---|---|
| \( \mathrm{Al^{3+}} \) | \(+3\) | \(2\) | \(2 \times (+3) = +6\) |
| \( \mathrm{S^{2-}} \) | \(−2\) | \(3\) | \(3 \times (−2) = −6\) |
| Sum | — | — | \(+6 + (−6) = 0\) |
Step 3: Write the formula unit
The cation is written first, then the anion. The subscripts come from the balanced ion counts: \[ \mathrm{Al_2S_3} \]
A compact way to show the combination of ions into a neutral formula unit is: \[ 2\,\mathrm{Al^{3+}} + 3\,\mathrm{S^{2-}} \rightarrow \mathrm{Al_2S_3} \]
Common mistakes and quick checks
-
Mistake: writing \( \mathrm{Al_3S_2} \) by placing charges directly as subscripts without balancing.
Fix: enforce neutrality: \(2 \times 3 = 3 \times 2 = 6\) so the correct subscripts are 2 and 3. -
Mistake: reducing incorrectly or leaving charges in the final formula.
Fix: the final ionic formula contains only element symbols and subscripts; the net charge must be \(0\). - Quick check: compute total charge: \(2(+3) + 3(−2) = 0\). If the sum is not zero, the formula is not correct.
Final answer
The correct aluminum sulfide formula is: \[ \mathrm{Al_2S_3} \]