A metal M forms as oxide having the formula `M_2O_3`. It belongs to third period. Write the atomic number and valency of the metal.
Period no. = no. of shells, so n = 3
From the formula `M_2O_3` its valency is 3.
Since it is a metal, its valence shell has 3 electrons.
So its electronic configuration is 2, 8, 3
Atomic number =13
Hence the metal is Aluminum with valency 3.