GoalKnow the concept of oxidation state and find the oxidation states of elements in compounds.
New words
oxidation state · oksidlanish darajasiconditional charge · shartli zaryadpositive and negative oxidation state · musbat va manfiy oksidlanish darajasisum of charges · zaryadlar yig‘indisi
Explanation
The oxidation state is the conditional charge of an atom in a compound, calculated as if all bonds in the compound were ionic. Rules: in simple substances (H₂, O₂, Fe, S₈) it is zero; hydrogen is usually +1 (only –1 in metal hydrides such as NaH); oxygen is usually –2 (–1 in peroxides, e.g. H₂O₂); alkali metals are +1, alkaline-earth metals +2, aluminium +3, and fluorine is always –1. In a molecule the oxidation states of all atoms add up to zero, in a complex ion to the charge of the ion. The sign is written before the number: +6, –2 (an ion charge is written with the number first, like Ca²⁺). The oxidation state differs from valency: it can be negative, zero, or even fractional.
Worked examples
S in H₂SO₄: 2·(+1) + x + 4·(–2) = 0, x = +6. S in Na₂SO₃: 2·(+1) + x + 3·(–2) = 0, x = +4.
N in NH₄⁺: x + 4·(+1) = +1, x = –3. N in HNO₃: (+1) + x + 3·(–2) = 0, x = +5. S is –2 in H₂S, 0 in S₈, +4 in SO₂: the oxidation states of sulfur range from –2 to +6.
Class activity
“Oxidation-state bingo”: write 10 formulas (H₂O, CO₂, NH₃, HCl, SO₃, KMnO₄, H₂O₂, NaH, Cl₂, HNO₃) on squared paper. Write the oxidation state next to each and let a friend check. Only pencil and paper.
Practice
1
What is the oxidation state and how does it differ from valency?
The conditional charge of an atom in a compound; it has a sign and can be zero.
2
Oxidation state of carbon in CO₂, CH₄ and CO?
+4, –4, +2
3
Find the oxidation state of chlorine in Cl₂O₇.
2x + 7·(–2) = 0, x = +7.
4
Why is the oxidation state of oxygen –1, not –2, in H₂O₂?
The two H give +2, the molecule is neutral, so the two O share –2, i.e. –1 each (there is an O–O bond).