Question

6. What is the oxidation number for the atom indicated in the following compounds.

Answer 1

Answer:

a. +6;

b. +5;

c. +3.

Explanation:

Start with elements with well-known oxidation states.

The oxidation state on oxygen O in compounds is mostly -2. Common exceptions include:

• -1 in peroxides and
• positive when oxygen bonds to fluorine.

The oxidation state on group 1 metals (Li, Na, K, etc.) in compounds is mostly +1.

The oxidation state on group 2 metals (Be, Mg, Ca, etc.) in compounds is mostly +2.

Barium Ba is a group 2 metal. The oxidation state on Ba in the compound BaSO₄ is expected to be +2.

The oxidation state on hydrogen H in compounds is mostly +1. The oxidation state on H might be negative when it is bonded to metals.  

The oxidation state on halogens (F, Cl, Br, etc.) is mostly -1. The oxidation state may vary when the halogen is bonded to oxygen or another halogen element.

Compounds are neutral. The oxidation state on all atoms in a compound shall add up to 0. Both BaSO₄ and HClO₂ are neutral.

BaSO₄

Oxidation states:

• Ba: +2;
• The oxidation state on sulfur S is to be determined;
• O: -2.

Let the oxidation state on S be x.

2 + x + 4 × (-2) = 0;

x = 6.

Hence, the oxidation state on S in BaSO₄ is +6.

HClO₂

Oxidation states:

• H: +1;
• Cl here is bonded to oxygen. The oxidation state on chlorine Cl is to be determined;
• O: -2.

Let the oxidation state on Cl be x.

Refer to the equation in BaSO₄ as an example. Try setting up the equation on your own.

x = 3.

Hence, the oxidation state on Cl is +3.

PO₄³⁻

Ions carry charge. Oxidation states on atoms in an ion shall add up to the charge of the ion. The superscript of an ion shows its charge. The superscript 3- in the phosphate ion shows that the ion carries a charge of -3.

Oxidation states:

• The oxidation state on P is to be found;
• O: -2.

Let the oxidation state on P be x.

x + 4 × (-2) = -3;

x = 5.

Hence, the oxidation state on P is +5.