Atomic theory - modern models
Rutherford atomic Model The Bohr atomic model The modern quantum atomic theory

Rutherford atomic model
In the early part of the 20th Century physicist Ernest Rutherford, and his team, carried out a series of experiments exploring the atom by shooting high speed positively charged alpha particles at a thin piece of gold foil. Alpha particles were known from earlier work that had been done in radioactivity, but scientists were still not sure if they were bits of atoms. Alpha particles were later discovered to be the nuclei of helium atoms.

What Rutherford discovered was that most of the alpha particle "bullets" passed straight through the gold foil, as though it were empty space. The occasional particle was diverted from its course by something very small and dense in the gold atoms.

The Rutherford atomic model was also known as the "Rutherford nuclear atom" and the "Rutherford Planetary Model". In 1911, Rutherford described the atom as having a tiny, dense, and positively charged core called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus. The light, negatively charged, electrons circulated around this nucleus, much like planets revolving around the Sun.

Rutherford's model varied from Dalton's and Thompson's models in that he considered that most of the atom was empty space, whereas in the earlier models the atom was considered solid.

Rutherford's atom

The Bohr atomic model
After working with both Thomson and Rutherford, Niels Bohr, in 1913, realised that Rutherford's model was unstable. According to classical theory, electrons moving on an orbit should emit electromagnetic radiation and the electrons would therefore lose energy and spiral into the nucleus. Bohr realised this was not the case because atoms did not continually glow or spontaneously disappear.

Rutherford's model demise

Instead, Bohr suggested that an atom's electrons move in orbits of fixed size and energy and that the energy of an electron depends on the size of the orbit. Radiation could only be emitted from an atom when an electron dropped from a higher energy orbit to another lower orbit. Stable atoms have their electrons in the lowest possible energy orbits.

This was the first "quantum atomic theory", allowing only certain energy levels to exist within the atom. Today's modern quantum theories are very close to Bohr's ideas.

Bohr's Atom

The modern quantum atomic theory
Today it is generally agreed that the atom consists mostly of empty space except for its nucleus which is tiny, dense and positively charged. The nucleus is surrounded by a cloud of fast moving, negatively charged, electrons that are bound to it by electric forces.

The change from Bohr's model is how electrons occupy space. Scientists now believe that electrons move in regions rather than in orbits and use mathematics and quantum numbers to describe this.

Bohr's model is quite adequate for describing the chemical properties of an atom. However, the modern quantum theory allows a deeper understanding of the strange subatomic world of wave/particles and electromagnetic energy.

Quantum atom

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