The term "valence" or "valency" is used in chemistry to describe the potential an element or molecule has to bond. Similar to oxidation number and formal charge of an ion, the valency of an atom or molecule can be described as how many hydrogen atoms it can bond with. Radicals are similar to polyatomic ions, only without a formal charge. They are groups of atoms that can readily react with other elements and compounds.
Use the Octet Rule
Determine the electrons in the outer shells of the elements in the radical. This can be determined by counting how many columns on the periodic table the element is from a noble gas. For example, the cyanide radical (CN) has four outer electrons for carbon and five outer electrons for nitrogen.
Combine the atoms with covalent bonds, so they share as many electrons as possible without exceeding eight electrons. For cyanide, both carbon and nitrogen can share three electrons each. When nitrogen adds these three electrons to its existing five, it has eight electrons, known as an octet. Carbon ends up with seven electrons.
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Determine how many electrons would need to be added to the molecule to make an octet for all the elements. This number is the radical's valence. In the example, one electron would be needed to give carbon an octet. Therefore, the cyanide radical has a valence of one.
Use Existing Chemical Formula
Find a known hydrogen-containing formula with the radical in it. For example, to determine the valency of the sulfate radical, consider hydrogen sulfate: H2SO4.
Count how many hydrogen atoms are in the formula. This is the valency of the radical. For example, H2SO4 has two hydrogen atoms, so the valency of sulfate is two. Because sulfate can bond with two positive hydrogen atom, its valency is the opposite charge and often expressed as 2-.
If no hydrogen-containing compound is available, use a compound with a known valence. For example, aluminum sulfate has the formula Al2(SO4)3. Aluminum has a valence of 3+. Because there are two aluminum atoms in the formula, the total valence is 6+. Because there are three sulfate ions in the formula, 6 divided by 3 yields a valence number of 2 for sulfate. Aluminum makes ions with a positive charge, which is why the sulfate ion has a negative charge, and that makes the sulfate radical have a 2- valency.
TL;DR (Too Long; Didn't Read)
In general, the valency of a radical is the same as the charge on the polyatomic ion of the same formula.