When a substance is oxidized, it

1. Analyze the Question

The question asks for the fundamental definition of oxidation in chemical reactions. In chemistry, oxidation is one half of a redox (reduction-oxidation) reaction, and it is defined by the movement of electrons.

2. Explain the Concept

To understand oxidation, we use the popular mnemonic OIL RIG:

• Oxidation Is Loss (of electrons)
• Reduction Is Gain (of electrons)

While oxidation can also involve the gain of oxygen or the loss of hydrogen, the most comprehensive and modern definition used in chemistry (especially for JAMB/WASSCE) is based on electron transfer. When an atom, ion, or molecule loses electrons, its oxidation state increases, which is the hallmark of oxidation.

3. Show the Solution

• Option A (Loses electrons): This is the definition of oxidation. For example, when Sodium ($Na$) becomes a Sodium ion ($Na^+$), it loses one electron: $Na \rightarrow Na^+ + e^-$. This is an oxidation process.
• Option B (Gains oxygen atoms): While this is an older definition of oxidation, it is not the most fundamental. Not all oxidation reactions involve oxygen (e.g., the reaction between Sodium and Chlorine).
• Option C (Gains electrons): This is the definition of reduction, not oxidation.
• Option D (Loses protons): Losing protons ($H^+$) is often associated with oxidation in organic chemistry (dehydrogenation), but it is not the universal definition for all chemical substances.

Comparing these, Option A is the most accurate and universal definition of oxidation used in chemistry examinations.

4. Identify the Answer

The correct option is A.

5. Exam Tip

Always remember OIL RIG. If you are ever confused during the exam, look at the charge of the element. If the charge becomes more positive (e.g., from $0$ to $+2$), the substance has lost electrons and has been oxidized. If the charge becomes more negative (e.g., from $0$ to $-1$), it has gained electrons and has been reduced.