MYP Chemistry Electronic Configuration and Valency Notes

Electrons Occupy Energy Levels in Atoms

1. Electrons in atoms do not move randomly anywhere around the nucleus.
2. They occupy specific energy levels, also called shells or principal energy levels, labelled with numbers: $$n=1,2,3,4, \ldots$$
3. These shells are arranged around the nucleus of an atom.
   1. Electrons in shells closer to the nucleus (smaller nn) have lower energy.
   2. Electrons in shells further from the nucleus (larger nn) have higher energy.

Maximum Number of Electrons in a Shell

Each shell can hold a maximum number of electrons given (in theory) by: $$ \text { Maximum electrons in shell } n=2 n^2$$

Orbitals Are Regions That Describe Electron Positions

Each energy level (shell) is divided into subshells (s, p, d, f), and each subshell contains one or more orbitals.

1. We often show orbitals in diagrams, but they are not fixed paths like planets around the Sun.
2. They are probability clouds showing where an electron is likely to be found.

Electrons and Spin

1. Each orbital can hold a maximum of two electrons.
2. When two electrons share an orbital, they must have opposite spins.
3. This is a consequence of the Pauli exclusion principle: no two electrons in an atom can have exactly the same set of quantum numbers.

Types of Orbitals

Orbitals come in different shapes, linked to different subshells:

1. s orbitals
   1. Shape: spherical
   2. Present in every energy level (1s, 2s, 3s, …)
2. p orbitals
   1. Shape: dumbbell-shaped
   2. First appear from the second energy level (2p, 3p, …)
   3. There are three p orbitals in each p subshell, oriented along the x, y, and z axes.

Subshells, Orbitals and Electron Capacity

1. Subshells are labelled s, p, d and f.