Remembering electronegativity values by heart is difficult and drawing an electron dot diagram does take time.Because of this, chemists have devised a shortcut for assigning oxidation numbers.For instance, what is the oxidation number of chromium (Cr) in the following chemical formula (dichromate ion): Solution Among the rules outlined above, we have no specific rule for Cr.
This shortcut does not apply to all compounds, but we can use it to assign oxidation numbers to many compounds we encounter in chemistry.
To use this shortcut, we must consider the following rules: Rule 1. As you can see, elements including iodine, bromine, oxygen, chlorine, hydrogen are all diatomic.
If the oxidation state (number) of the atom increases, that atom is oxidized (loses electrons).
If the oxidation state decreases from left to right, that atom is reduced.
And since we have 7 atoms of O in the formula, we must multiply 7 by -2. Also, since the overall charge of the formula is 2-, then it follows from rule 6 that the oxidation state of the chemical formula is -2.
Now, from rule 7 again, we must ensure the sum of the oxidation numbers of atoms in the formula add up to the oxidation state of the formula.
The chart below should help you to visualize the possible oxidation numbers that can occur for the first 39 atoms.
then compare the oxidation number of any given atom on both the left and right sides of the reaction.
But since we don’t know the oxidation number of Cr, we will use the letter x to represent this number.
But since we have two atoms of Cr in the formula, it follows that we must multiply x by 2. Now, let’s translate all that into an algebraic expression.