**Chemistry is the study of matter and the changes that it undergoes. Since all matter consists of atoms, it is important that you understand the atom and its components especially the electron.**

Although models are not perfect there are five models of the atom that lead to the understanding of matter. The more you understand these models the better you will be at chemistry. The five models are:

The Solid Sphere model

The Plum Pudding Model

The Nuclear Model

The Bohr Model

The Wave Mechanical Model

**Solid Sphere Model**

The solid sphere model was proposed in 1803 by John Dalton. John Dalton was an English chemistry and math tutor who took the ideas of his day and put them into a model of how matter was arranged. He stated that matter was made of tiny indivisible particles called atoms and atoms of the same element were all alike. Also, he included the

**law**of multiple proportions and the

**law**of definite composition in his model. Although his model was not perfect it was the main main model for over 90 years.

**Plum Pudding**

In 1897 English physicist, J. J. Thomson, suggested that cathode rays consisted of negatively charged particles even smaller than atoms. As a matter of fact he suggested these particles were subatomic parts of the atom. Thus, Dalton’s theory of an indivisible particle was no longer workable. A new theory that allowed for subatomic particles was needed.

So, Thomson proposed that the atom was a

**mass**of positive charge with negative electrons placed into it like raisins in a pudding, and dubbed it the Plum Pudding model. Thomson’s model, although attractive, nevertheless had many shortcomings.

**The Nuclear Model**

In 1906 Ernest Rutherford and his assistants, performed the famous Gold Foil experiment that led to the discoveries of the atomic nucleus and that the atom is mostly space.

The Gold Foil experiment demonstrated that the

**mass**of the atom was the same as predicted by Thompson’s model, but the volume of the

**mass**was much smaller and seemed to be located in the center of the atom. Thomson’s Plum Pudding model was no longer workable. So, in 1911, Rutherford proposed the Nuclear model of the atom.

**The Bohr Model**

In 1913 Danish physicist, Neils Bohr, suggested applying the quantum theory to the Rutherford model. Bohr’s new model proposed that electrons are in fixed energy levels he called orbits. The energy of these orbits is quantized and electrons must absorb or release energy (photons) at certain wavelengths to move between energy levels.

**The Wave Mechanical Model**

In 1921 Louis de Broglie stated, without any empirical evidence, that if waves have matter properties then matter should have wave properties. Later, Bell Laboratories would prove that De Broglie’s hypothesis was indeed correct. De Broglie’s hypothesis was a revelation that even impressed Einstein and changed the view of the atom forever.

Combining Louis de Broglie’s hypothesis with Bohr’s model, Erwin Schrodinger proposed the electron was a 3-D waveform circling the nucleus in a whole number of wavelengths allowing the waveform to repeat itself as a stable standing wave representing the energy levels of the Bohr model.

In support of his hypothesis, SchrÃ¶dinger developed a mathematical equation to describe the wave-like behavior of the electron. The SchrÃ¶dinger wave equation not only gave the correct energy levels for the hydrogen atom, but also was somewhat useful in atoms with more than one electron. This mathematical description of the details of atomic behavior became known as the “Wave Mechanical Model”.

Chemistry – The Atomic Models and Chemistry

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