Question

Why is it that the metals are reducing agents and non-metals are oxidising agents?

Answer 1

1. Oxidising agent

Definition:

An oxidising agent is an agent which causes the oxidation of others and themselves get reduced by gaining electrons or by giving oxygen.

The one who gets reduced acts as an oxidising agent.

Example:

$2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Hydrogen Oxygen Water

• Here oxidation state of $\mathrm{H}$ in ${\mathrm{H}}_2$ is $0$and that of $\mathrm{O}$ in ${\mathrm{O}}_2$ is$0$ at the reactant side but the oxidation state of $\mathrm{H}$ in ${\mathrm{H}}_2\mathrm{O}$ is $+1$ and that of $\mathrm{O}$ in ${\mathrm{H}}_2\mathrm{O}$ is $-2$ at the product side.
• So, we can say that ${\mathrm{H}}_2$ is getting oxidised since its oxidation number is increasing from $0$ to $+1$ and ${\mathrm{O}}_2$ is getting reduced since its oxidation number is decreasing from $0$ to $-2$.
• The one who gets oxidised acts as a reducing agent and the one who gets reduced acts as an oxidising agent.
• So here ${\mathrm{H}}_2$ is a reducing agent and ${\mathrm{O}}_2$ is an oxidising agent.

Metals are good reducing agents:

• From above example it is clear that reducing agent need to donate electrons.
• We know that metals have $1,2\mathrm{or}3$ electrons in its outermost shell.
• To attain noble gas configuration that is $8$ electrons in its outermost shell they need to donate electrons.
• Example: Sodium ($\mathrm{Na}$) it has $11$electrons.
• Electronic configuration: $\mathrm{K}\mathrm{L}\mathrm{M}\mathrm{N}$

$281$

• Since sodium has $1$ electron in its outermost shell it will donate this $1$ electron and attain electronic configuration like neon($\mathrm{Ne}$) and become stable.
• Same goes with $\mathrm{Mg}$ ($2,8,2$)and $\mathrm{Al}$ ($2,8,3$) they will donate $2\mathrm{and}3$ electrons respectively and attain electronic configuration like neon($\mathrm{Ne}$) and become stable.

So we can say that to attain stability metals donate electrons and therefore they act as good reducing agents.

2. Reducing agent

Definition:

Reducing agent is an agent which causes the reduction of others and themselves get oxidised by donating electrons or by gaining oxygen.

The ones that get oxidised act as a reducing agent.

Example:

${\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{F}}_2\left(\mathrm{g}\right)\to 2\mathrm{HF}\left(\mathrm{g}\right)$

Hydrogen Fluorine Hydrogen fluoride

• Here oxidation state of $\mathrm{H}$ in ${\mathrm{H}}_2$ is $0$ and that of $\mathrm{F}$ in ${\mathrm{F}}_2$ is $0$ at the reactant side but the oxidation state of $\mathrm{H}$ in $\mathrm{HF}$ is $+1$ and that of $\mathrm{F}$ in $\mathrm{HF}$ is $-1$ at the product side.
• So, we can say that ${\mathrm{H}}_2$ is getting oxidised since its oxidation number is increasing from $0$ to $+1$ and ${\mathrm{F}}_2$ is getting reduced since its oxidation number is decreasing from $0$ to $-1$.
• The one who gets oxidised acts as a reducing agent and the one who gets reduced acts as an oxidising agent.
• So here ${\mathrm{H}}_2$ is a reducing agent and ${\mathrm{F}}_2$is an oxidising agent.

Non-metals are good oxidising agents:

• From above example it is clear that reducing agent need to gain electrons.
• We know that non-metals have $5,6\mathrm{or}7$ electrons in its outermost shell.
• To attain noble gas configuration that is $8$ electrons in its outermost shell they need to gain electrons.
• Example: Chlorine ($\mathrm{Cl}$) it has $17$electrons.
• Electronic configuration: $\mathrm{K}\mathrm{L}\mathrm{M}\mathrm{N}$

$287$

• Since chlorine has $7$ electron in its outermost shell it will gain $1$ electron and attain electronic configuration like argon($\mathrm{Ar}$) and become stable.
• Same goes with $\mathrm{P}$ ($2,8,5$)and $\mathrm{S}$ ($2,8,6$) they will gain $3\mathrm{and}2$ electrons respectively and attain electronic configuration like argon($\mathrm{Ar}$) and become stable.

So we can say that to attain stability non-metals gain electrons and therefore they act as good oxidising agents.