THE HISTORY OF THE ATOMIC THEORY

THE FOUR MAIN MODELS

• Plum pudding model
• Planetary model
• Bohr model
• Electron cloud model

THE PLUM PUDDING MODEL

The plum pudding model was a model of the atom that incorporated the recently discovered electron, and was proposed by J. J. Thomson in 1904.

In this model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "raisins" surrounded by positively charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons.

THE PLANETARY MODEL

This theory was proposed by the Nobel Prize winning chemist Ernest Rutherford in 1911 and is sometimes called the Rutherford model. Based on experiments that showed the atom appeared to contain a small core of positive charge, Rutherford postulated that the atom consisted of a small, dense and positively charged nucleus, around which electrons orbited in circular rings. This model was one of the first to propose the odd idea that atoms are mostly made up of empty space through which the electrons move.

BOHR MODEL

The Bohr model was devised by Neils Bohr, a physicist from Denmark who received the Nobel prize for his work on the atom. In some ways it is a more sophisticated enhancement of the Rutherford model. Bohr proposed, as did Rutherford, that the atom had a small, positive nucleus where most of its mass resided. He stated that the electrons orbited around this nucleus like planets around the sun. The main improvement of Bohr's model was that the electrons were confined to set orbits around the nucleus, each having a specific energy level, which explained experimental observations such as electromagnetic radiation

THE ELECTRON CLOUD MODEL

The electron cloud model is currently the most sophisticated and widely accepted model of the atom. It retains the concept of the nucleus from Bohr and Rutherford's models, but introduces a different definition of the motion of electrons around the nucleus. The movement of electrons around the nucleus in this model is defined by regions where there is a greater probability of finding the electron at any given moment. These regions of probability around the nucleus are associated with specific energy levels and take on a variety of odd shapes as the energy of the electrons increase.

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