Diatomic molecules are those molecules which have two atoms in them.
O2 (oxygen), Hydrogen H2, Chlorine Cl2 exist as diatomic gases. These molecules contain atoms of the same element. These are homonuclear diatomic molecules.
CO, NO etc. also contain two atoms, but of different elements. So they are called heteronuclear diatomic molecules.
Valency is the oxidation state of an atom of molecule (of an element or a compound). It is the number of positive or negative charge it carries on it.
Monovalent compound is that which has lost an electron or has gained one electron. Or, has an oxidation state of 1. Or, that which can donate one electron (for sharing or permanently) or can receive one electron only.