How are "valence" and "valency" defined?

Mar 16, 2017

And thus carbon has a valency of $4$ in methane, nitrogen has a valence of $3$ in ammonia..........
Now valency is a fluid concept, and it is usually better to use oxidation states and number, which is another concept you have to absorb. See here for a start. With metal ions, i.e. ${M}^{n +}$, when we speak of the univalent, $n = 1$, bivalent, $n = 2$, tervalent, $n = 3$, ions, we refer to the ${M}^{+}$, ${M}^{2 +}$, and ${M}^{3 +}$ ions. And, here, clearly, valence reflects oxidation state.