Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Explain the Aufbau principle, Pauli's exclusion principle, and Hund's rule.
• Write electron configurations for the first thirty elements.
• Explain the significance of quantum numbers.
• Describe the shapes of s and p orbitals.

Lesson summary

This lesson delves into the modern understanding of the atom, moving beyond the simple Bohr model to the more accurate quantum mechanical model. We will explore the "rules" that govern how electrons arrange themselves within an atom. Understanding this arrangement, known as electron configuration, is the key to explaining almost everything in chemistry: why certain elements react violently while others are stable, why metals are conductive, why compounds have specific shapes, and how the periodic table is structured.

Lesson notes

Part 1: From Orbits to Orbitals - A New Model of the Atom

In SHS1 and SHS2, we often talked about the Bohr model, where electrons move in fixed, circular paths around the nucleus, much like planets around the sun. These paths were called orbits. Orbit: A well-defined, two-dimensional circular path around the nucleus where an electron is expected to be found. This model is simple but not entirely accurate.

The modern Quantum Mechanical Model tells us that we cannot know the exact path of an electron. Instead, we can only talk about the probability of finding an electron in a certain region of space. This three-dimensional region of high probability is called an orbital. Orbital: A three-dimensional region of space around the nucleus where there is a high probability (usually 90-95%) of finding an electron.

Analogy: Think of an orbit as a race track. You know the exact path the car will follow. An orbital is more like the area around a beehive. You don't know the exact path of any single bee, but you know the region where you are most likely to find bees. Part 2: Quantum Numbers - An Electron's Address

Evaluation guide

• Apply Aufbaus principle, Pauli's exc lusion principle and Hund's rule of maximum
• multiplicity to write electron configuration of the fi rst thirty elements of the periodic
• table.
• Group Learning: Research f rom the In ternet, library, books and other sources about Aufbaus