If the formula of a compound is M2O3 and oxygen has valency 2, then valency of M is

1. Use criss-cross: subscripts come from swapped valencies. 2. Oxygen 2 became the subscript on M, so M's valency = subscript on O = 3. 3. So valency of M equals 3. 4. Hence option A is correct. _Source: Selina Concise Chemistry Class 9, Ch 4 The Language of Chemistry (Mistral OCR'd PDF), section on chemical formula_