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Tips for Assigning Oxidation Numbers - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

So, when you're doing redox reactions, before you even get started, you're going to have to assign oxidation numbers to these compounds and atoms. What does it even mean? What are oxidation numbers?

Well, oxidation numbers are actually the relative charges that each of the atoms have in a chemical compound. So the more electronic atom are most likely going to have a negative charge. Well not most likely, definitely going to have the negative charge because electronegativity means those electrons are going to be around that particular atom more so, than the other atom in the compound.

So the other atoms can be positive. So these are relative numbers to decide where electrons are and where they're spending their time, basically. They're very similar to charges when you're looking for actual charges on atoms and they're various too. So these are the rules we're going to follow, to make it easier for ourselves, to really understand what charge an atom is in a compound.

If an atom is by itself, looking at the first one, it has an awful time of spending, there's only one place electrons can spend and there's equal number of protons and electrons. So it's an overall charge of 0, including the diatomics. So if you see aluminum by itself, it has an oxidation number of 0. Cl2, even though it has a 2 there, 2 has an oxidation number of 0, equal number of protons and electrons around that, in that particular atom.

Fluorine is extremely electronegative. it is always going to be a -1 charge. Oxygen is the second most electronegative atom in the periodic table. It is going to be a -2 charge, unless it's bonded to Fluorine, then it's going to be whatever it needs to be for Fluorine to be -1.

Number 4, anything alkaline metals, anything in group 1, is going to be a +1 charge. Alkaline earth metals, group 2, are +2 charge, aluminum is always a 3. Hydrogen is almost always +1 unless it's bonded with a metal halide and we'll talk about that in a second.