Question

How are `"valence"` and `"valency"` defined?

Answer 1

Traditionally, valence was the number of hydrogens (hydrides or protons) that combined with the element. It is a bit of an old-fashioned term.

Explanation:

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.

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