atomic-models

Atomic Models of John Dalton, Thomson’s Plum pudding, Rutherford & Bohr

Atomic models

There has been a wide range of atomic models throughout the history of atomic physics, that refers primarily to a period from the beginning of the 19th century to the very first half of the 20th century. Here are some standard atomic models.

atomic-models

John Dalton’s atomic model

John Dalton was an English scientist, who came up with an idea that all matter is made up of really small things. It was the very first total effort to explain all matter in regard to particles. He called these particles atoms and formed an atomic theory. In this theory, he declares that:

  1. All matter is made from atoms. Atoms are indivisible and unbreakable.
  2. All atoms of a given element are equal in mass and properties.
  3. Substances are formed by a mix of 2 or more different sorts of atoms.
  4. A chemical reaction is a rearrangement of atoms
Plum pudding model

After the discovery of an electron in 1897, people understood that atoms are made up of even smaller particles. Quickly after in 1904 J. J. Thomson proposed his famous “plum pudding model”. In this model, atoms were understood to include negatively charged electrons, nevertheless, the atomic nucleus had not been discovered yet.

Thomson understood that the atom had an overall neutral charge. He thought that there need to be something to counterbalance the negative charge of an electron. He came up with an idea that negative particles are floating within a soup of scattered positive charge. His model is typically called the plum pudding model, because of his similarity to a popular English dessert.

Rutherford’s Atomic Model

Rutherford carried out the ‘Gold Foil’ experiment to comprehend how negative and positive charges could exist side-by-side in an atom. He bombarded alpha particles on a 0.00004 cm thick gold foil. Alpha particles are given off by radioactive elements like radium and polonium.

These are in fact helium nuclei (He2+). They can penetrate through matter to some extent. He observed the impacts of- particles on a photographic plate or a screen coated with zinc sulphide. He proved that the ‘plum-pudding’ model of the atom was not accurate.

Observations made by Rutherford were as follows:

  1. Almost all the particles passed through the foil un-deflected.
  2. Out of 20000 particles, just a couple of were deflected at fairly large angles and very few recovered on striking the gold foil.

atomic-model-Rutherford

Outcomes of the experiment

Keeping in view the experiment, Rutherford proposed a planetary model for an atom and concluded the following outcomes:

  • Because most of the particles passed through the foil undeflected, for that reason most of the volume occupied by an atom is empty.
  • The deflection of a couple of particles showed that there is a ‘center of positive charges’ in an atom, which is called ‘nuclei’ of an atom.
  • The total rebounce of a few particles shows that the nucleus is very dense and hard.
  • Considering that a couple of particles were deflected, it shows that the size of the nucleus is really small as compared to the total volume of an atom.
  • The electrons move around the nucleus.
  • An atom as a whole is neutral, for that reason the number of electrons in an atom is equal to the number of protons.

Other than electrons, all other fundamental particles that lie within the nucleus, are referred to as nucleons.

Defects in Rutherford’s Model

Although Rutherford’s experiment showed that the ‘plum-pudding’ model of an atom was not correct, yet it had the following problems:

  1. According to the classical theory of radiation, electrons being the charged particles need to release or emit energy constantly and they should ultimately fall under the nucleus.
  2. If the electrons give off energy constantly, they need to form a constant spectrum but in fact, the line spectrum was observed.
Bohr’s Atomic Theory

Keeping in view the defects in Rutherford’s Atomic Model, Neil Bohr provided another model of the atom in 1913.

The Quantum Theory of Max Planck was utilized as the foundation for this model According to Bohr’s model, revolving electron in an atom does not soak up or release energy continuously. The energy of a revolving electron is ‘quantized’ as it revolves just in orbits of fixed energy, called ‘energy levels’ by him.

atomic-model-Bohr

Postulates of Bohr’s atomic model

The Bohr’s atomic model was based upon the following postulates:

i. The hydrogen atom includes a small nucleus and electrons are revolving in one of the circular orbits of radius ‘r’ around the nucleus.

ii. Each orbit has set energy that is quantized.

iii. As long as electron stays in a particular orbit, it does not emit or absorb energy. The energy is released or taken in just when electron leaps from one orbit to another.

iv. When an electron jumps from lower orbit to higher orbit, it absorbs energy and when it jumps from a higher orbit to lower orbit it radiates energy. This change in energy, E is given by following Planck’s formula

atomic-formula-Bohr

Where, h is Planck’s consistent equal to 6.63 10-34 Js, and v is the frequency of light.

 

v. Electron can revolve just in orbits of a fixed angular moment mvr, given as:

formula-Electron

Where ‘n’ is the quantum number or orbit number having values 1,2,3 and so on.

Distinction between Rutherford and Bohr’s atomic models
Rutherford’s Model    Bohr’s Model
It was based upon classical theory.

 

It was based upon quantum theory.

 

Electrons revolve around the nucleus.

 

Electrons revolve around the nucleus in orbits of fixed energy.

 

No concept about orbits was introduced.

 

Orbits had angular momentum.

 

Atoms ought to produce constant spectrum.

 

Atoms should produce a line spectrum.

 

Atoms should collapse.

 

Atoms need to exist.