Redox: Oxidation and Reduction I don't get the difference between the gain or loss of electrons in a reaction and changes in oxidation state. Based on my notes and some research:


Gain or loss of electrons


A substance is oxidised if it loses electrons in a reaction.
A substance is reduced if it gains electrons in a reaction.

Changes in oxidation state


A substance is oxidised if the oxidation state of the element increases.
A substance is reduced if the oxidation state of the element decreases.



Whenever I see a chemical equation, I am confused as I do not know if it is a gain or loss of electrons or changes in oxidation state. I have an assignment to do which involves redox reaction, but it is the holidays and I can't contact my teacher. Could someone explain so that I could understand? For example,

$$\ce{2 Ca (s) + O2 (g) → 2CaO (s)}$$

I thought there is a change in oxidation state as on the reactants side, the oxidation state of both $\ce{Ca}$ and $\ce{O2}$ are $0$. As for the product, the oxidation state for $\ce{Ca}$ is $+2$ and $\ce{O}$ is $-2$. Therefore, $\ce{Ca}$ is oxidised and $\ce{O2}$ is reduced.

But one of the questions on my worksheet asked about electron transfer for this chemical equation, and I do not understand why it is a electron transfer.
Look this as:

$$\ce{2Ca + O2 -&gt; 2CaO}\qquad\text{(complete reaction)}$$ 

then we can this split into two sub-reactions:

$$\ce{Ca - 2 e- -&gt; \overset{+2}{Ca}}\tag{1}$$

Here "$+2$" means that $\ce{Ca}$ lost $2$ electrons (or two negative units) and therefore it was oxidized. Since it lost something negative it has a higher number of protons (positive particles) remained. In other words the atoms is now positively charged. The initial state was $0$ (the same numbers of protons and electrons, or positive and negative particles/units) and it was increased by loosing two electrons (the number increased).

$$\ce{O + 2 e- -&gt; \overset{-2}{O}}\tag{2}$$

The oxygen got two electrons or two negative particles. Now oxygen has two negative particles more than positive particles (protons) and therefore it is $-2$. In other words, the number (oxydation state) decreased ($0 \to -2$).