Structure of Atom | Part- 2

DALTON’S ATOMIC THEORY

Dalton proposed the atomic theory on the basis of the law of conservation of mass and law of definite proportions. He also proposed the law of multiple proportions as a logical consequence of this theory. The salient features of this theory are

(a) Each element is composed of extremely small particles called atoms.
(b) Atoms of a particular element are all alike but differ with the atoms of other elements.
(c) Atom of each element is an ultimate particle, and has a characteristic mass but is structureless.
(d) Atom is indestructible i.e. it can neither be destroyed nor created by simple chemical reactions.
(e) Atoms of an element take part in chemical reactions to form molecules.
(f) In a given compound, the relative number and kind of atom are the same.
(g) Atoms of different elements combine in a fixed ratio of small whole numbers to form compound atoms (now called molecules).

Why Dalton’s Atomic Theory Failed ?
According to John Dalton, matter is made up of the smallest particle that is known as atom that can neither be created nor destroyed and are indivisible but the discovery of two fundamental particles (electrons and protons) inside the atom lead to the failure of this aspect of Dalton’s Theory.

Thomson’s Model Of An Atom :
This model was proposed by “Joseph James Thomson” in 1897.
This model proposed that an atom is a uniform sphere of positive charges (due to presence of protons) as well as the negative charges (due to the presence of  the electrons).
We can relate this model to that of a watermelon, the positive charge in the atom is spread all over like the red edible part of watermelon while the electrons are studded in the positively charged sphere, like the seeds in the watermelon. 
The atom as a whole is electrically neutral as the negative and positive charges are of equal magnitude.
This model is further known as the watermelon model, plum pudding model and apple pie model.

Rutherford’s Model Of An Atom :
This model was proposed by “Ernest Rutherford” in 1909.He was also known as the father of nuclear physics.
This model is based on theory, where fast moving alpha-particles were made to fall on a thin gold foil.
Add image of Rutherford alpha particles scattering experiment.

OBSERVATIONS FOUND :
Most of the fast moving alpha-particles passed straight through the gold foil.
Some of the particles get deflected by small angles.
It was also found that one out of every 12000 particles appeared to rebound.

CONCLUSIONS MADE :
Atoms contain a lot of empty space due to which most of the alpha-particles passed without getting deflected through the gold foil.
The positive charge of the atom occupies very little space due to which very few particles were deflected from their path.
The positive charges are concentrated in a very small volume within the atom due to which a very small fraction of alpha-particles were deflected by 180°.
He also concluded that the radius of the nucleus is about 105 times less than the radius of the atom.

The nuclear model of an atom on the basis of Rutherford’s experiment :       
The nucleus of an atom is located at the center.
All the mass of an atom resides in the nucleus.
The electrons revolve around the nucleus in a circular path, which is known as orbit.
If we compare the size of the nucleus and the atom. 
We will find, nucleus is very small in comparison to that of the atom.

Drawbacks of Rutherford’s model of the atom :
The nuclear model of an atom suggested by Rutherford failed as it couldn’t justify the statement that how an atom remains stable after having positive and negative charge.
As per Maxwell’s theory, if any charged particle would move in a circular orbit it will radiate energy. 
So, if electrons start moving in a circular motion around the nucleus they would also radiate (lose) some energy and as a result they would fall into the nucleus due to presence of high positive charge.
This theory proves that if this were so, the atom should be highly unstable and hence, the matter would not exist in the form that we know, as we know matter is quite unstable.

What do you mean by nucleons?
The Protons and the Neutrons are together known as nucleons.

Bohr’s Model Of The Atom :
This model was proposed by Niels Bohr in 1913.

This model of an atom proposed that :
The electron spins around the nucleus of the atom in a certain special orbit known as discrete orbits of electrons.
The electrons do not emit any energy while revolving in these discrete orbits.
These shells of orbits are known as energy levels which are represented by the letters K,L,M,N,… 
and the numbers, n = 1,2,3,4,…