The key difference between valency and oxidation number is that valency is the maximum number of electrons an atom can lose, gain or share to become stable, whereas oxidation number is the number of electrons an atom can lose or gain to form a bond with another atom.
The terms oxidation number and valency are related to the valence electrons of an atom. Valence electrons are the electrons that occupy the outermost orbitals of an atom. These electrons have a weak attraction towards the atomic nucleus; thus, atoms can easily remove or share these electrons with other atoms. This loss, gain or sharing of electrons cause a particular atom to have an oxidation number and valency, and eventually it forms a chemical bond between the two atoms.
CONTENTS
1. Overview and Key Difference
2. What is Valency
3. What is Oxidation Number
4. Side by Side Comparison – Valency vs Oxidation Number in Tabular Form
5. Summary
What is Valency?
Valency is the maximum number of electrons an atom loses, gains or shares to become stable. For metals and nonmetals, the octet rule describes the most stable form of an atom. Here, if the number of the outermost shell of an atom is filled completely (need eight electrons for this completion), that electron configuration is stable. In other words, if the s and p sub-orbitals are completely filled having ns2np6 configuration, the atom is stable.
Naturally, noble gas atoms have this electron configuration. Therefore, other elements need to either lose, gain or share electrons to obey the octet rule. The maximum number of electrons an atom needs to lose or gain or share in this stabilization is the valency of that atom.
For instance, let us consider Silicon. The electron configuration of silicon is 1s22s22p63s23p2. The outermost shell is n=3, and it has 4 electrons. Therefore, it should obtain four more electrons to complete the octet. Generally, Silicon can share 4 electrons with other elements to complete the octet. Thus, the valency of silicon is 4.
For different chemical elements, the valency differs. It is because electrons are filled to the orbitals according to the energy levels of those orbitals. However, most transition metals have the same valency; often it is 2. But, some elements can have difference valencies because the atom can get stabilized in different electron configurations by removing electrons.
For example, in Iron (Fe), the electron configuration is [Ar]3d64s2. Therefore, the valency of iron is 2 (2 electrons in 4s2). But sometimes, the valency of iron becomes 3. It is because the 3d5 electron configuration is more stable than 3d6. Thus, removing one more electron along with 4s electrons will stabilize Iron more.
What is Oxidation Number?
The oxidation number is the number of electrons an atom can lose or gain to form a bond with another atom. Sometimes, we use the terms oxidation state and oxidation number interchangeably, but they have slight differences.
Figure 01: Some Chemical Elements Can Show Different Oxidation Numbers
Most of the times, the term oxidation number is applied for coordination complexes. In coordination complexes, the oxidation number is the charge of the central atom of a coordination compound if all bonds around that atom were ionic bonds. Coordination complexes are almost always composed of transition metal atoms in the centre of the complex. This metal atom has chemical groups in its surrounding, which we name as ligands. These ligands have lone electron pairs that can be shared with metal atoms to form coordination bonds.
After the formation of the coordination bond, it is similar to a covalent bond. It is because the two atoms in the coordination bonds share a pair of electrons, just like a covalent bond. However, we have to calculate the oxidation number of the central metal atom considering the coordination bonds as ionic bonds.
What is the Difference Between Valency and Oxidation Number?
The terms oxidation number and valency are related to the valence electrons of an atom. The key difference between valency and oxidation number is that valency is the maximum number of electrons an atom can lose, gain or share to become stable whereas oxidation number is the number of electrons an atom can lose or gain to form a bond with another atom. Moreover, the term valency is applied for any chemical element, but the term oxidation number is applied mainly regarding coordination complexes.
Below infographic summarizes the difference between valency and oxidation number.
Summary – Valency vs Oxidation Number
Both oxidation number and valency are terms related to the valence electrons of an atom. The key difference between valency and oxidation number is that valency is the maximum number of electrons an atom can lose, gain or share to become stable whereas oxidation number is the number of electrons an atom can lose or gain to form a bond with another atom.
Reference:
1. “Oxidation Number.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.
Image Courtesy:
1. “Period 2 oxidation numbers” By Albris – Own work (CC BY-SA 4.0) via Commons Wikimedia