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: 7
Grade code: 1.1.1.LI.7
Strand code: 1
Sub-strand code: 1
Content standard code: 1.1.1.CS.1
Indicator code: 1.1.1.LI.7
Theme: PHYSICAL CHEMISTRY
Subtheme: MATTER AND ITS PROPERTIES
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This lesson transitions from earlier, simpler models of the atom (like Rutherford's) to the more sophisticated models proposed by Niels Bohr and later developed into the quantum mechanical model. Understanding this evolution is crucial because it explains *why* elements behave the way they do—why sodium is a reactive metal and neon is an inert gas, why fireworks explode in brilliant colours, and how technologies like solar panels work. In Ghana, as we embrace more technology in energy and health, understanding the behaviour of electrons at this fundamental level becomes increasingly important.
Part A: The Problem with Rutherford's Model and Bohr's Solution
Before Niels Bohr, the accepted model was Rutherford's nuclear atom: a dense positive nucleus with electrons orbiting it like planets around the sun. However, classical physics predicted a major problem: a moving charged particle (the electron) should continuously lose energy as radiation, causing it to spiral into the nucleus and the atom to collapse. This, of course, does not happen.
In 1913, Niels Bohr proposed a revolutionary model for the hydrogen atom that incorporated the new idea of "quanta" (discrete packets of energy) introduced by Max Planck.
Bohr's Planetary Theory: Main Postulates Fixed Energy Levels (Orbits): Electrons revolve around the nucleus in fixed, circular paths called orbits or energy shells. Each orbit has a definite, fixed amount of energy. An electron in a particular orbit does not radiate energy and therefore does not spiral into the nucleus. These orbits are labelled n=1, 2, 3, ... or K, L, M, ... starting from the one closest to the nucleus. Quantization of Energy: An electron can only gain or lose energy in discrete amounts (quanta) by "jumping" from one allowed orbit to another. To move to a higher energy level (further from the nucleus), an electron must absorb a specific amount of energy. To move to a lower energy level (closer to the nucleus), an electron must emit a specific amount of energy, usually as a photon of light. Quantization of Angular Momentum: The angular momentum of an electron in an orbit is quantized, meaning it can only have certain discrete values. This is the mathematical reason why only certain orbits are allowed. (For SHS level, the key takeaway is simply that not just any orbit is possible). Part B: Evidence for Bohr's Model - The Hydrogen Spectrum