Lesson Notes By Weeks and Term v4 - SHS 3
MATTER AND ITS PROPERTIES
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MATTER AND ITS PROPERTIES
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Subject: Chemistry
Class: SHS 3
Term: 1st Term
Week: 9
Grade code: 1.1.1.LI.9
Strand code: 1
Sub-strand code: 1
Content standard code: 1.1.1.CS.1
Indicator code: 1.1.1.LI.9
Theme: PHYSICAL CHEMISTRY
Subtheme: MATTER AND ITS PROPERTIES
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This lesson introduces the fascinating world of radioactivity, a natural phenomenon where the very core of an atom, the nucleus, undergoes change. While the word "radiation" might sound scary, we will learn that it has incredible applications that save lives, help us grow more food, and understand the history of our world. We will explore why some atoms are stable while others are not, the different types of radiation they emit, and how we can use this knowledge responsibly in Ghana and beyond. We will also learn the mathematics behind radioactive decay, specifically the concept of half-life.
A. Introduction to Radioactivity Radioactivity (or radioactive decay) is the spontaneous and random process by which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. Unstable Nucleus: The nucleus of an atom contains protons and neutrons. Protons are positively charged and repel each other. The strong nuclear force is an attractive force that holds the nucleus together, acting over very short distances. In some atoms, the balance between the repulsive electrostatic force of the protons and the attractive strong nuclear force is not right, making the nucleus unstable. Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons. For example, Carbon-12 (6 protons, 6 neutrons) is stable, but Carbon-14 (6 protons, 8 neutrons) is unstable and therefore radioactive. We call Carbon-14 a radioisotope or radionuclide. B. Nuclear Reactions vs. Chemical Reactions It is crucial to understand that radioactivity involves nuclear reactions, which are very different from the chemical reactions we have studied so far.
| Feature | Chemical Reaction | Nuclear Reaction | | :--- | :--- | :--- | | Particles Involved | Valence (outer shell) electrons. | Protons, neutrons, and other particles within the nucleus. | | Identity of Element | The element does not change. Atoms are rearranged. | The element often changes (transmutation). New isotopes are formed. | | Energy Change| Relatively small amounts of energy are absorbed or released. | Enormous amounts of energy are released (millions of times greater). | | Effect of Conditions | Rate is affected by temperature, pressure, concentration, and catalysts. | Rate is unaffected by temperature, pressure, or catalysts. It is a spontaneous process. | | Example| `2H₂(g) + O₂(g) → 2H₂O(l)` | `${^{238}_{92}}U → {^{234}_{90}}Th + {^{4}_{2}}He` | C. Types and Properties of Radiation When a nucleus decays, it emits radiation. The three main types are Alpha, Beta, and Gamma.
| Property | Alpha (α) Radiation | Beta (β) Radiation | Gamma (γ) Radiation | | :--- | :--- | :--- | :--- | | Nature | Particle (Helium nucleus) | Particle (High-speed electron) | Electromagnetic wave (Photon) | | Symbol | `${^{4}_{2}}He` or `${^{4}_{2}}α` | `${^{0}_{-1}}e` or `${^{0}_{-1}}β` | `${^{0}_{0}}γ` | | Charge| +2 | -1 | 0 (Neutral) | | Relative Mass | 4 | ~1/1840 (negligible) | 0 | | Penetrating Power| Low. Stopped by a sheet of paper or a few cm of air. | Medium. Stopped by a thin sheet of aluminium (~5 mm). | High. Stopped by several cm of lead or thick concrete. | | Ionising Power| High. Its large mass and charge easily knock electrons off other atoms. | Medium. Less effective at ionising than alpha particles. | Low. Least effective at ionising atoms. | | Deflection in Fields| Deflected towards the negative plate in an electric field. | Deflected strongly towards the positive plate. | Not deflected. |
*(Teacher can draw this on the board, showing a lead block with a radioactive source, and paths of α, β, and γ rays between two charged plates).* D. Nuclear Stability and the Neutron-to-Proton (n/p) Ratio The stability of a nucleus depends heavily on its neutron-to-proton (n/p) ratio. Light Elements (Z 20): As the number of protons increases, the electrostatic repulsion becomes much stronger. More neutrons are needed to provide the extra "glue" (strong nuclear force) to hold the nucleus together. The stable n/p ratio gradually increases, reaching about 1.5:1 for the heaviest stable elements like Lead-208 (82p, 126n; n/p ≈ 1.54). Unstable Nuclei: Nuclei with too many neutrons (high n/p ratio): These tend to decay by beta emission. A neutron is converted into a proton and an electron. The electron is ejected (beta particle), increasing the proton number and decreasing the neutron number, thus lowering the n/p ratio. Nuclei with too many protons (low n/p ratio): These tend to decay by alpha emission (common for Z > 83) or positron emission. Alpha decay removes 2 protons and 2 neutrons, which increases the n/p ratio. E. Balancing Nuclear Equations Two simple rules must be followed: Conservation of Mass Number (A): The sum of the mass numbers on the reactant side must equal the sum of the mass numbers on the product side. Conservation of Atomic Number (Z): The sum of the atomic numbers on the reactant side must equal the sum of the atomic numbers on the product side.