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Radioactive Decay & Half-Life - explained simply! (IB Physics / IBDP)

Why do unstable nuclei decay? Two forces compete inside the nucleus: the strong nuclear force pulling protons and neutrons together, and the electrostatic repulsion pushing the positive protons apart. When repulsion wins, the nucleus breaks down, emitting alpha, beta and gamma radiation, until it becomes a smaller, more stable nucleus.

What is a half-life? It is the time taken for half the radioactive nuclei in a sample to decay: 100 to 50 to 25 to 12.5... and it never quite reaches zero. Every isotope has its own half-life, from minutes to thousands of years.

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🔬 Chlorine in Redox | OCR A-Level Chemistry

Why can chlorine take on so many different oxidation states in redox reactions? The answer lies in one key idea: expanded octet.

Unlike elements in Period 2 (like fluorine), chlorine is in Period 3 — meaning it has access to empty 3d orbitals. This allows chlorine to expand beyond 8 electrons in its outer shell, giving it the ability to form multiple oxidation states: -1, 0, +1, +3, +5, and +7.

This is why chlorine is such a versatile element in redox chemistry — it can act as both an oxidising agent and a reducing agent depending on the reaction conditions.

📌 Key Concept: Expanded octet = many oxidation states

✅ What you need to know for OCR A-Level:
• Chlorine's common oxidation states in redox: -1, 0, +1, +3, +5, +7
• Cl₂ undergoes disproportionation in water and alkali (simultaneously oxidised and reduced)
• In reactions with NaOH (cold, dilute): Cl₂ → NaCl + NaOCl (oxidation states: 0 → -1 and 0 → +1)
• Chlorine as an oxidising agent: gains electrons, oxidation state decreases (e.g. 0 → -1)
• Chlorine as a reducing agent: loses electrons, oxidation state increases (e.g. -1 → 0)

💡 Exam Tip: In OCR A-Level redox questions, always assign oxidation states step by step. If the same element appears as both the oxidised and reduced species, it's disproportionation — a favourite exam topic!

📚 Part of the Mega Lecture "Exam Concept in 50 Seconds" series — designed to help you master high-yield A-Level topics fast.

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