How to Calculate Valency of Radicals

How to Calculate Valency of Radicals
••• DragonImages/iStock/GettyImages

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.

    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.


    • In general, the valency of a radical is the same as the charge on the polyatomic ion of the same formula.

Related Articles

How to Use the Octet Rule
How to Write a Chemical Compound Formula
How to Calculate the Unsaturation Number
How to Calculate the Charge of an Ion
Examples of Elements Without a Stable Electron Configuration
How to Calculate a Fraction Covalent
How to Remember the Charges of Polyatomic Ions
How to Balance Redox Equations
How to Memorize Ionic Compounds
How to Figure Out the Chemical Symbol for Ions
How to Determine How Many Hybrid Orbitals
Easy Way to Learn Chemistry Formulas
How to Calculate How Many Rings in an Atom
How to Determine the Charge of Transition Metal Ions
What Is the Valence of Hydrogen?
How to Calculate a Steric Number
How to Calculate Valence
How to Calculate the Formal Charge of CoCl2
Difference Between Sulfide and Sulfite
Non Neutral Atoms Examples