Chapter 4 – Forces

Explaining chemistry

The periodic table helps explain how elements behave. Chemical reactivity is based on arrangement of electrons around an atom

• 1^st shell only allows 2 electrons
• 2^nd shell allows 8
• 3^rd shell allows 18, etc.

This structure leads to placement of atoms on the periodic table. But why is there this structure?

In 1925, Wolfgang Pauli described the Pauli Exclusion Principle – two identical matter particles cannot share the same quantum state. The exclusion principle comes from the mathematics where the wave function cancels out if two particles share identical state.

This limits each electron orbit around the nucleus to two electrons: with spin +½ and -½

• The 1st shell has one orbit allowing 2 electrons.
• The 2nd has four distinct orbits for 8 electrons, etc.