The formula of the compound formed between a trivalent metal, M and a divalent non-metal, Y is

Analyze the question: A trivalent metal M has a valency of 3 (it forms M³⁺). A divalent non-metal Y has a valency of 2 (it forms Y²⁻). We need the formula of the compound they form.

Concept: the criss-cross method For an ionic compound to be neutral, the total positive charge must equal the total negative charge. The quick way is to swap the valencies and use them as subscripts.

Solution:

1. Write the ions: M³⁺ and Y²⁻
2. Criss-cross the numbers: the 3 from M becomes the subscript of Y, and the 2 from Y becomes the subscript of M.
3. This gives M₂Y₃

Check the charges:

• 2 atoms of M: 2 × (+3) = +6
• 3 atoms of Y: 3 × (−2) = −6
• Total = 0, so the compound is neutral ✔

Check the other options:

• B. M₃Y₂: (+9) + (−4) = +5, not neutral
• C. MY: (+3) + (−2) = +1, not neutral
• D. M₃Y: (+9) + (−2) = +7, not neutral

M₂Y₃**.

Exam tip: Remember "swap and drop". Swap the valencies and drop them as subscripts. Compare with Al₂O₃ (aluminium is 3+, oxygen is 2−), which has the same pattern as M₂Y₃. If the question asks about a known compound like this, you can use it as a quick reference.