Difference Between Valency and Oxidation State

Main Difference – Valency vs Oxidation State

All compounds are made of atoms of different elements. These atoms are combined in different ratios according to the number of available free orbitals they have or the number of electrons that a particular atom can release. Valency and oxidation state are two terms used to explain the reasons for these different combinations of atoms. The main difference between valency and oxidation state is that valency is the number of electrons present in the outermost shell of an atom of a particular element whereas oxidation state is the number of electrons that an element in a particular compound has lost or gained.

Key Areas Covered

1. What is Valency
      – Definition, Indication, Examples
2. What is Oxidation State
      – Definition, Indication, Rules to Determine the Oxidation State, Examples
3. What is the Difference Between Valency and Oxidation State
      – Comparison of Key Differences

Key Terms: Atom, Compound, Element, Shell, Oxidation State, Valency

What is Valency

Valency can be defined as the number of electrons present in the outermost shell of a particular element. Valency determines the maximum number of bonds that a particular atom can have. The elements in the periodic table are grouped according to the number of valence electrons (electrons in the outermost orbitals) an atom can have when it is in the neutral and gaseous state. The following table will explain this.

Elements of the same group in the periodic table have the same valency since elements are grouped according to the atomic number and the number of electrons present in the outermost shell are the same when considering their electron configurations.

Figure 1: Valency of Potassium is 1

What is Oxidation State

The oxidation state can be defined as the number of electrons a particular atom can lose, gain or share with another atom. The loss or gain of electrons causes the charge of an atom to be changed since electrons are negatively charged, and each negative charge is neutralized by the positive charge of protons in the nucleus. When an atom is losing or gaining electrons, there would be an imbalance of electrical charges. Therefore, the oxidation state of that atom is the charge of that atom. The oxidation state can be used to describe the charge of an atom that is in a compound.

The oxidation state is given using positive (+) or negative (-) symbol along with a number. The symbol describes the charge of that atom and the number describes the number of electrons that have been exchanged.

Figure 2: 0 and +2 are the only oxidation states of Zinc

How to Determine the Oxidation State

The oxidation state of an atom is calculated under seven rules. Those rules and some examples for oxidation states are given below.

Rule 1

The oxidation state of an element is always zero. Elements in their elemental form are also in zero oxidation state.

Ex: Oxidation State of Sodium (Na)  is 0 and that of Hydrogen (H₂)  is 0

Rule 2

The total charge of the molecule or ion should be the sum of the charges of each atom in the compound.

Ex:    KCl total charge is 0

          Charge (oxidation state) of K  is +1

          Charge (oxidation state) of Cl is -1

Rule 3

The oxidation state of group 1 elements is always +1.

Rule 4

The oxidation state of group 2 elements is always +2.

Rule 5

The negative charge is given to the most electronegative atom compared to the other atoms bonded to it.

Ex: Fluorine is the most electronegative element. Therefore, whenever it is in a bond with another element, Fluorine is given -1 oxidation state.

Rule 6

The oxidation state of hydrogen (H) is +1.

But when it is with group 1 metals, the oxidation state of hydrogen is -1.

Rule 7

The oxidation state of oxygen (O) is -2.

But in peroxides, the oxidation state of O is -1.

Difference Between Valency and Oxidation State

Definition

Valency: Valency can be defined as the number of electrons present in the outermost shell of a particular element.

Oxidation: Oxidation state can be defined as the number of electrons that a particular atom can lose, gain or share with another atom.

Electrical Charge 

Valency: Valency does not indicate the electrical charge of an atom in a compound.

Oxidation: Oxidation state indicates the electrical charge of an atom in a compound.

Bonding

Valency: Valency determines the maximum number of bonds that an atom can have.

Oxidation: Oxidation state does not indicate the number of bonds that a particular atom can have.

Pure Elements

Valency: Valency of a pure element depends on the number of electrons it has in its outermost shell of an atom (an atom in gaseous phase).

Oxidation: Oxidation state of a pure element is always zero.

Conclusion

The valency and oxidation state may have the same values or different values. Valency is the number of electrons present in the outermost shell of a particular element whereas oxidation state is the number of electrons that an element in a particular compound has lost or gained. This is the main difference between valency and oxidation state.

References:

1.“Oxidation states (oxidation numbers).” Oxidation states (oxidation numbers) N.p., n.d. Web. Available here. 06 July 2017.
2.”Rules for Assigning Oxidation Numbers to Elements.” Dummies. N.p., n.d. Web. Available here. 06 July 2017. 

Image Courtesy:

1. “Electron shell 019 Potassium” By Pumbaa (original work by Greg Robson) – File:Electron shell 019 potassium.png (CC BY-SA 2.0 uk) via Commons Wikimedia
2. “Electron shell 030 zinc” (CC BY-SA 2.0 uk) via Commons Wikimedia