Ancient Indian and Greek philosophershave always wondered about the unknown and unseen form of matter. An Indian philosopherMaharishi Kanad, postulated that if we go ondividing matter (padarth), we shall get smallerand smaller particles. He named these particles as

paramaanu.

Glimpses about An Atom

Around the same era, ancient Greek philosophers :

Democritus and Leucippus suggested that if we go

on dividing matter, a stage will come when particles

obtained cannot be divided further. Democrituscalled these indivisible particles atoms (meaningindivisible).

A step forward:By the end of the eighteenth century,

scientistsrecognized the difference between

elements andcompounds and naturally became

interested infinding out how and why elements

combine andwhat happens when they combine.

Dalton’s viewIn 1808, John Dalton presented his

atomic theorywhich was a turning point in the study of

matter. According to Dalton’s atomic theory, all

matter, whether an element, a compound or

a mixture is composed of small particlescalled atoms.

The postulates of this theory may be stated as follows:

(i) All matter is made of very tiny particlescalled Atoms.(ii) Atoms are indivisible particles, which

cannot be createdor destroyed in a chemical reaction.(iii) Atoms of a given element are identicalin mass and chemical properties.(iv) Atoms of different elements havedifferent masses and chemicalproperties.

(v) Atoms combine in the ratio of small whole numbers to form compounds.

(vi) The relative number and kinds of atoms are constant in a given compound.

Discovery of electron & Plum pudding model

J J Thomson in 1897 discovered the sub atomic particle “electron”. Thomson discovered this through his explorations on the properties of cathode rays.

In this model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively-charged "plums" surrounded by positively-charged "pudding".

The electrons were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons .Instead of a soup, the atom was also sometimes said to have had a "cloud" of positive charge.

Thomson’s Plum pudding model

Rutherford model By 1911 the components of the atom had

been discovered. The atom consisted of subatomic particles called protons and electrons. However, it was not clear how these protons and electrons were arranged within the atom. Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom.

Then, if he shot high velocity alpha particles (helium nuclei) at an atom then there would be very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. As expected, most alpha particles went right through the gold foil but to his amazement a few alpha particles rebounded almost directly backwards.

These deflections were not consistent with Thomson's model. Rutherford was forced to discard the Plum Pudding model and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus. He thus developed the planetary model of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun.

Gold foil experiment

Visualiztion of Rutherford’s model

Bohr’s Theory According to the Rutherford's model of atom

the electrons places outside the nucleus must circulate around it on some orbits. Otherwise they would fall on it. If there is some energy transferred to the atom than its electrons would circulate in a farther distance from the nucleus (their potential and kinetic energy increases). They would circulate on the more external orbits. According to the classic laws of the electrodynamics a circulating electron should cause some electromagnetic radiation.

The radiation transfers out some of the energy of electron. So the electron loosing its energy should move on the smaller and the smaller orbits and finally fall on the nucleus. But no such phenomena occurs. If it had been so than the electron would has been placed on any orbit which would has been permanently changed by the electron as the energy would has been emitted. According to that all the atoms should be on different energy states and emit the radiation of all waves lengths.

The spectrum should be continuous one not the line one. That divergences between theory and practice led to the one of the most famous physicists of the beginning of the 20th century - Niels Bohr to a new theory describing the laws governing the atom

Postulates of Bohr’s theory Electrons orbit the nucleus. They are

held in orbit by an electrostatic force. Electrons can only be in certain,

permitted orbits and an electron does not emit radiation when it is in one of these orbits.

An electron only emits radiation when it "falls" from a higher energy state to a lower state.

The radii of the allowed orbits are also quantized - each energy state has a specific radius proportional to h/2π.

Angular momentum of electron will be mvr = nh/2π, n is

principle quantum number.

Limitations of Bohr’s model• electron couldn’t circle around

nucleus like a planet! Because they would lose energy (by emitting electromagnetic radiation) & spiral into nucleus. 

• Bohr was not able to explain electron orbits of large atom with many electrons.

Questions How did the discovery of sub atomic

particle affect or leads to failure of Dalton’s postulates?

How according to different scientist’s electrons and protons are arranged in an atom?

How can an atom be represented symbolically?

How are electrons distributed in different orbits?