A Short Presentation on

ATOMIC STRUCTURE

OUTLINE

Thomson’s Atom Model

Rutherford Atom Model

Bohr's Atom Model

Thomson’s Atom Model (1897)

Atoms are positive charged spheres where electrons are embedded. This model was proposed just after his discovery of the electron, for which he was awarded the Nobel Prize in 1906. However, this model was found to be incorrect by Rutherford’s experiment.

J. J. Thomson (1856-1940): A British physicist who was awarded the Nobel Prize in 1906 for his discovery of the electron.

Rutherford Atom Model (1906): Rutherford’s students Geiger & Marsden conducted the classic

gold foil alpha particle scattering experiment which showed large deflections for a very small fraction of incident particles.

This led Rutherford to propose that atom is composed of very high dense positively charged nucleus and very light electrons.

Rutherford was awarded the Nobel Prize in 1908 in chemistry

for this discovery. He was very sad that the prize was in chemistry rather than in physics, and in his acceptance speech he remarked that he had seen many transformations in his studies, but never one more rapid than his own from physicist to chemist.

Rutherford’s Experiment (1906):

Experimental observation: Most particles flew right through the foil as if there were nothing there: The

foil was mostly empty space.

A small number of particles were bounced back to their source: There must be a small, dense nucleus with a positive charge.

Interpretation:

The atom is composed of very high dense positively charged nucleus (of size 10-15 to 10-14 meter) and very light electrons.

The nucleus contains the whole positive charge and almost the whole atom's mass.

The electrons have to orbit around the nucleus, not to fell down on the nucleus. The area of the orbits (order of 10-10 meter), depends on electron energy.

The optical spectrum of atoms of the same element is the same.

How does Rutherford’s Atom look-like?

Rutherford (1871-1937) New Zealander-English physicist who was born in Nelson, New Zealand, attended school in Nelson and Marlborough, and finished his tertiary education in Canterbury, New Zealand before traveling to England.

Limitations

1st limitation: If the electrons orbit the nucleus, according to classical physics they should emit electromagnetic waves and loose energy. If this happens, electrons will spiral into the nucleus, and no stable atom should exist! But atoms are stable.

2nd limitation: According to this model the radiation should be continuous. The spectral lines shouldn't occur. This is because: in accordance with the electron energy loss, the time of electron's cycle should change continuously. But atomic spectral lines are experimentally observed:

Bohr's Atom Model (1913): Bohr’s atom model retains two essential features of

Rutherford’s atom model: i) atom has a very small, massive, positively charged nucleus at its center, and ii) negatively charged electrons circulate around the nucleus. But there are some completely new ideas:

There are some orbits called stationary orbits, where moving electrons don't emit energy and where orbital angular momentum is an integral multiple of ħ.

Each emission or absorption of radiation energy represents the electron transition from one stationery orbit to another. The frequency of emitted photon is given by the formula hv = E1-E2, whereas the frequency of the absorbed photon is hv = E2-E1,

h is Planck’s constant, and E1 (E2) is the energy of the first (second) stationary state of the system.

Limitations of Bohr Atomic Model Theory: .

It violates the Heisenberg Uncertainty Principle. The Bohr atomic model theory considers electrons to have both a known radius and orbit i.e. known position and momentum at the same time, which is impossible according to Heisenberg.

The Bohr atomic model theory made correct predictions for smaller sized atoms like hydrogen, but poor spectral predictions are obtained when larger atoms are considered.

Heisenberg Uncertainty Principle

The Heisenberg uncertainty principle states that it is impossible to know simultaneously the exact position and momentum of a particle. That is, the more exactly the position is determined, the less known the momentum, and vice versa.

Momentum: the quantity of motion of a moving body, measured as a product of its mass and velocity.

Thank you all