What is the Plum Pudding Model?
The plum pudding model (also known as Thomson’s plum pudding model) is a historical scientific models of the atom. The plum pudding model is defined by electrons surrounded by a volume of positive charge, like negatively-charged “plums” embedded in a positively-charged “pudding” (hence the name).
The plum pudding model was first proposed by the English Physicist Sir Joseph John J.J. Thomson soon after the discovery of the electron, but before the discovery of the atomic nucleus. The model tried to explain two properties of atoms then known: that electrons are negatively-charged particles and that atoms have no net electric charge.
At the time of discovery, J.J. Thomson called this negatively charged particle a corpuscles.
Thomson’s Plum Pudding Model is the first model to represent the atomic structure of matter.
According to Thomson’s Plum Pudding Model, a substance is consists of small spheres which are having the radius of about 10-10 m in diameter. The positive charge is spread uniformly throughout the volume of sphere called pudding. The negatively charged particles Electrons called Plums are distributed as point charges in shells as shown in figure below-
The positively charged sphere exerts the force on negatively charged electrons. The direction of the net force on negatively charged electrons due to positively charged sphere is towards the center of the sphere. These negatively charged electrons repel each other and form the shells.
Thomson’s Plum Pudding Model hold sway for few years until the Ernest Rutherford announced the nuclear model of the atom in the year of 1911. After Ernest Rutherford’s nuclear model of the atom in 1911, the interest in Thomson’s Plum Pudding Model fell off rapidly. But the Thomson’s Plum Pudding Model assumed its place in history as the first modern attempt to construct a theory of atomic structure.
The Thomson’s Plum Pudding Model was given up as it failed to explain the existence of some observed phenomena. This model failed to explain the emission of electron spectrum consisting of different frequencies from Thomson’s atom when it is subjected to external frequencies emitted from other substances.
It also failed to explain the existence of light spectrum. For example, lets consider the emission of light from hydrogen atom having a single electron. According to Thomson’s Plum Pudding Model, it can emit the light at single frequency whereas, practically it emits the light spectrum which consists of different frequencies. Due to the failure of Thomson’s Plum Pudding Model for explaining the above phenomenon, this model got rejected.
Even though the Thomson’s Plum Pudding Model represents a considerable progress towards the truth of the matter but it failed to explain these facts.
Moreover, it fails to provide the satisfactory mechanism to explain the deflection of α – particle.
In the year of 1911, British Physicist Ernest Rutherford gave an atomic model which is capable of explaining the above phenomenon such as hydrogen spectrum consisting of different frequencies, light spectrum consisting of different frequencies, deflation of α – particles in the external field. Hence the Ernest Rutherford’s nuclear model of the atom in 1911 replaced the Thomson’s Plum Pudding Model.