21.THE ATOMIC STRUCTURE- History,Discovery of Electron.

Today we start a long discourse on a very fundamental topic which is at the core of not only Chemistry but to the understanding of the whole universe  – ATOMIC STRUCTURE. We know , that all the matter in this universe is made up of matter.So what is this matter made up of ? Let us now begin our journey into the sub atomic realm. Let us begin learning about  the wonderful discoveries, made by various scientists , that helped us learn the secrets of this universe.Let us raise the curtain and study various phenomenon ,which were earlier enshrouded in mystery.

It all started in 5th century  BC ,when a Greek philosopher named Leucippus ,stated that everything is composed of imperishable,indivisible elements called ‘atoms’.(In Greek, ‘atomos’ = indivisible/uncuttable).So, the word ‘ATOM’ has its origins in Greek language).His pupil, Democritus later adopted Leucippus’s theory .

democritus by IonP2012

According to Democritus –
♦ The physical world consists of VOID(vacuum) and BEING(infinity of atoms).
♦ All matter consists of atoms which are indivisible and indestructible.


As already mentioned earlier (in Post 2 – Ancient Indian Chemistry), the atomic theory was also proposed in India by Acharya Kannada (originally known by the name Kashyap). He was regarded  ‘The father of atomic theory’.He formulated the theory of very small ,invisible particles – ‘Anu’(comparable to atoms),which is an  indivisible entity and cannot be sensed through any human organ.

Let us now take a leap from the ancient civilisation to the modern world and begin to take a look at the discoveries made in the 18th century.

In 1803, another atomic theory  was proposed by John Dalton .

John Dalton.

The Dalton’s theory states –

♦ All matter is composed of atoms, which are the smallest indivisible part of matter.
♦ Atoms cannot be created or destroyed.
♦ Atoms of different elements have different chemical properties and weights.
♦ Chemical reactions occur when atoms are rearranged,combined or separated.
♦ Atoms of different elements combine in simple whole number ratios to form compounds.

Dalton rearranged the elements known at that time according to ascending order of chemical mass and coined symbols for them .He was the first one to get a sense of stoichiometry in chemical reactions and he was the one who established a correlation between the difference in atomic structure and chemical behaviour of elements!  He also published a table of relative atomic weights.He suggested that atoms of elements have characteristic mass.Thus, the concept of forming a table for elements began, which much later lead to formation of a periodic table! John Dalton was a polymath who worked on gas laws, reflection  and refraction of light, human vision, etc.He  suffered from red-green color blindness and thus , he also did work on human eye and color blindness.

Dalton’s table of elements.
Sir J.J.Thompson.

In 1897, Sir J.J. Thompson was studying cathode rays.He showed that cathode rays are negatively charged particles .He applied high voltage to two electrodes fitted in a glass tube with vacuum inside.He observed  rays of negatively charged particles traveling from cathode to anode.As these rays were coming out of the cathode, he termed them as ‘Cathode Rays‘.He later applied another electric field in the path of these cathode rays and found out that these rays were deflected to the positive electric plate.Thus, he concluded that these rays consisted of negatively charged particles.He thus discovered the first subatomic particle – THE ELECTRON !(He called them negatively charged corpuscles).

He used to say a toast at dinner time,-

“To the electron! May it never be of any use to anybody !”

He just didn’t discover the electron but also characterized it- he found out the charge to mass ratio(e/m) for an electron. He won the NOBLE PRIZE for this discovery in 1906.Thompson never coined the term electron, he called the negatively charged particles ‘corpuscles’. It was another British elctrochemist , J. Stoney , who coined the word ‘ ‘Electron’.This term comes from the Greek word ‘electra’ which means amber – if you rub amber you build static chargeHe also found out that the electron was about 2000 times lighter than the hydrogen atom.Though he could get a sense of electron’s mass , he could not exactly determine the charge on the electron.

(https://youtu.be/4QAzu6fe8rE –  Animation Video of Cathode Ray Experiment).

Cathode ray tube.
Effect of electric field on Cathode rays.

One can measure the charge , mass and spin of an electron.One can study the effect of electrons on other particles.However, THE ELECTRON ITSELF HAS NEVER ACTUALLY BEEN OBSERVED!! THE ELECTRON IS SMALLER THAN ANYTHING EVER MEASURED !

After the discovery of electron, in 1904, J.J. Thompson proposed another atomic model – THE PLUM PUDDING MODEL.

Plum pudding is a common English dessert served especially during Christmas time.

Plum Pudding.

Sir J.J.Thompson proposed that the structure of the atom is like a plum pudding – the negative corpuscles(electrons) were like plums in a postively charged pudding.The electrons were spread throughout the atom.According to this theory, the electrons were distributed in spherical cloud of positive charge as follows –


However,this model could not explain the spectral lines that were observed for some elements.As the proposed model did not corroborate spectroscopic data, this theory couldnot be considered foolproof.Although, the theory didnot correctly explain the structure of an atom, it paved the way for  one of J.J.Thompson’s student’s research in the Cavendish Lab at the University of Cambridge.This student of J.J.Thompson did some superlative work in the field of nuclear physics and came to be known as ‘The Father of Nuclear Physics’ ! Who was this great scientist? What were his acheievements? What progress did the 19th century witness with respect to the atomic structure studies? We shall study all of this is my coming posts.Till then ,

Be a perpetual student of life and keep learning ..

Good Day!

References and Further Reading –

  1. https://en.wikipedia.org/wiki/Atomic_theory
  2. https://en.wikipedia.org/wiki/Democritus
  3. https://the-history-of-the-atom.wikispaces.com/Democritus
  4.  MIT 3.091SC Introduction to Solid State Chemistry, Fall 2010 Lecture by Professor Donald Sadoway.
  5. http://padakshep.org/otp/subjects/chemistry/physical-chemistry/discovery-of-electrons-protons-and-neutrons/
  6. The Inexplicable Universe documentary.

Image sources –


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