# Summary of Atomic Models – HSC Physics

This topic is part of the HSC Physics course under the section Structure of The Atom

### HSC Physics Syllabus

• investigate, assess and model the experimental evidence supporting the nuclear model of the atom, including:

– the Geiger-Marsden experiment
– Rutherford’s atomic model
– Chadwick’s discovery of the neutron (ACSPH026)
• assess the limitations of the Rutherford and Bohr atomic models

• analyse the contribution of Schrödinger to the current model of the atom

### Why are there so many models of the atom?

Numerous atomic models represent our evolving understanding of the atom over time. This understanding was built on different experiments that aimed to investigate the atomic structure.

### Evolution of Atomic Models: From Thomson to Schrödinger

1. Thomson's "Plum Pudding" Model (1897):

• Description: J.J. Thomson proposed an atomic model that resembled a "plum pudding." He envisioned the atom as a positively charged sphere with negatively charged electrons (the 'plums') scattered throughout.
• Key Discovery: Cathode rays were found to consist of negatively charged particles (electrons).

2. Rutherford's Nuclear Model (1911):

• Description: Ernest Rutherford's gold foil experiment led him to propose an atomic model with a tiny, dense, positively charged nucleus at the centre. Electrons orbited this nucleus, much like planets orbit the sun.
• Key Discovery: Most of the atom's mass and positive charge are concentrated in the nucleus, while most of its volume is empty space where electrons move.

3. Bohr's Quantum Model (1913):

• Description: Niels Bohr introduced quantum theory into atomic structure. In his model, electrons orbit the nucleus in specific, quantised energy levels or shells. Electrons could "jump" between these levels by absorbing or emitting energy in discrete amounts (quanta).
• Key Aspect: Introduced the idea of quantised energy levels, addressing issues like why atoms only emit specific frequencies of light.

4. Schrödinger's Quantum Mechanical Model (1926):

• Description: Erwin Schrödinger advanced the atomic model by incorporating wave mechanics. Rather than precise orbits, electrons exist in "clouds" or probability regions called orbitals.
• Key Aspect: Introduced the concept of the electron as both a particle and a wave, described by a wavefunction. The exact position of an electron is not deterministic but is given by a probability distribution.