Key ideas: chemical reactions take place in only three different ways:

The idea

At a fundamental level, chemical changes are categorised by how subatomic particles interact. The first mechanism is proton transfer, occurring in acid-base neutralisation where an acid donates a hydrogen ion (H⁺) to a base. The second is electron transfer, characteristic of redox reactions and ionic bonding. For example, when sodium reacts with chlorine, sodium loses an electron to form a positive ion while chlorine gains it to form a negative ion. The third mechanism is electron sharing, which happens when non-metal atoms bond covalently, such as two hydrogen atoms sharing a pair of electrons to form H₂. Understanding these particle interactions helps explain macroscopic changes.

What earns the marks

• Identify proton transfer as the mechanism in acid-base reactions, where H⁺ ions are donated and accepted.
• Describe electron transfer in redox reactions or ionic bonding, where one species loses electrons and another gains them.
• Explain electron sharing as the mechanism for covalent bonding between non-metal atoms to achieve full outer shells.
• Apply these concepts to specific examples, such as identifying electron transfer in the reaction between a metal and a non-metal.
• Recognise that a hydrogen ion (H⁺) is equivalent to a proton in the context of acid-base chemistry.

Common mistakes and how to put them right

• Confusing electron transfer with electron sharing.

  Put it right by remembering that transfer creates oppositely charged ions (ionic bonding), whereas sharing keeps atoms joined in molecules (covalent bonding).

• Thinking proton transfer involves the nucleus breaking apart.

  Put it right by understanding that in chemistry, a 'proton' refers to a hydrogen ion (H⁺) that has lost its single electron, not a proton escaping from a heavier nucleus.

• Assuming oxidation and reduction can happen independently.

  Put it right by noting that electron transfer must involve both simultaneously; if one reactant loses electrons, another must gain them.

Exam tips

• When asked to explain a reaction in terms of electrons, explicitly state which atom loses electrons and which gains them, or state that they are shared.
• Look for the terms 'acid' or 'alkali' in the question prompt as a strong hint that the reaction involves proton transfer.