How are #"valence"# and #"valency"# defined?
Traditionally, valence was the number of hydrogens (hydrides or protons) that combined with the element. It is a bit of an old-fashioned term.
And thus carbon has a valency of
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.
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