Describe the Rutherford Model of atom?
Answers
Rutherford observed that:
Most of the α-particles passed through the gold foil undeflected.
Some of the α-particles were deflected by small angles.
A few particles were either deflected by very large angles or were actually reflected back along their path.
On the basis of experiment Rutherford put forward nuclear model of atom. Main points of this model are:
There is positively charged centre in an atom called nucleus.
The electrons revolve around the nucleus in well-defined path.
The size of the nucleus is very small as compared to size of the atom.
Rutherford proposed that an atom is composed of empty space mostly with electrons orbiting in a set, predictable paths around fixed, positively charged nucleus.
Rutherford's Model of an Atom
Rutherford’s Atomic Model (Source Credit: Britannica)
History
The concept of atom dates back to 400 BCE when Greek philosopher Democritus first conceived the idea. However, it was not until 1803 John Dalton proposed again the idea of the atom. But at that point of time, atoms were considered indivisible. This idea of an atom as indivisible particles continued until the year 1897 when British Physicist J.J. Thomson discovered negatively charged particles which were later named electrons.
He proposed a model on the basis of that where he explained electrons were embedded uniformly in a positively charged matrix. The model was named plum pudding model. However, J.J. Thomson’s plum pudding model had some limitations. It failed to explain certain experimental results related to the atomic structure of elements.
A British Physicist “Ernest Rutherford” proposed a model of the atomic structure known as Rutherford’s Model of Atoms. He conducted an experiment where he bombarded α-particles in a thin sheet of gold. In this experiment, he studied the trajectory of the α-particles after interaction with the thin sheet of gold.
Rutherford Atomic Model Experiment
In Rutherford’s experiment, he bombarded high energy streams of α-particles on a thin gold foil of 100 nm thickness. The streams of α-particles were directed from a radioactive source. He conducted the experiment to study the deflection produced in the trajectory of α-particles after interaction with the thin sheet of gold. To study the deflection, he placed a screen made up of zinc sulfide around the gold foil. The observations made by Rutherford contradicted the plum pudding model given by J.J. Thomson.
rutherford atomic model, Rutherford Gold foil experiment
Rutherford’s Gold Foil Experiment
Observations of Rutherford Model Experiment
On the basis of the observations made during the experiment, Rutherford concluded that
Major space in an atom is empty – A large fraction of α-particles passed through the gold sheet without getting deflected. Therefore, the major part of an atom must be empty.
The positive charge in an atom is not distributed uniformly and it is concentrated in a very small volume – Few α-particles when bombarded were deflected by the gold sheet. They were deflected minutely and at very small angles. Therefore he made the above conclusion.
Very few α-particles had deflected at large angles or deflected back. Moreover, very few particles had deflected at 180o. Therefore, he concluded that the positively charged particles covered a small volume of an atom in comparison to the total volume of an atom.
Postulates of Rutherford atomic model based on observations and conclusions
An atom is composed of positively charged particles. Majority of the mass of an atom was concentrated in a very small region. This region of the atom was called as the nucleus of an atom. It was found out later that the very small and dense nucleus of an atom is composed of neutrons and protons.
Atoms nucleus is surrounded by negatively charged particles called electrons. The electrons revolve around the nucleus in a fixed circular path at very high speed. These fixed circular paths were termed as “orbits.”
An atom has no net charge or they are electrically neutral because electrons are negatively charged and the densely concentrated nucleus is positively charged. A strong electrostatic force of attractions holds together the nucleus and electrons.
The size of the nucleus of an atom is very small in comparison to the total size of an atom.
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