The atomic number of A is 13 and the atomic number of B is 17. The chemical…

An ionic compound forms only when atoms combine in the ratio that makes the total positive charge exactly cancel the total negative charge. Each element’s valency — the charge on the ion it forms — fixes that ratio, and the valency itself follows from the atom’s electron configuration, which is set by its atomic number.

1. Atomic number 13 gives the electron configuration 2, 8, 3. The 3 electrons in the outermost shell are lost, so element A is Aluminium (Al), forming the ion Al3+ with valency +3.

2. Atomic number 17 gives the electron configuration 2, 8, 7. One electron is gained to complete the octet, so element B is Chlorine (Cl), forming the ion Cl− with valency −1.

3. To keep the compound neutral, the magnitude of each ion’s charge becomes the other ion’s subscript: A takes the subscript 1 (from B’s charge magnitude 1) and B takes the subscript 3 (from A’s charge magnitude 3).

4. This gives the ratio A : B = 1 : 3, so the formula is AB3.

Cross-check by adding the charges: 1 × (+3) + 3 × (−1) = +3 − 3 = 0. The positive and negative charges cancel exactly, confirming AB3 as the neutral formula.