Discovery and History of an Electron in an Atom

           Discovery and History of an Electron in an Atom

Sub-atomic particles:

Small particles are present inside an atom. These small particles are called Sub-atomic particles or Fundamental particles. These small particles are smaller than the atom.

                                     Discovery of Electron (e):

Joseph John Thomson: (1856-1940) :

A British physicist discovered electrons in 1897 -the first subatomic particle. He named the particle first ‘corpuscles’ which were later known as the electron. He was awarded the Noble Prize in Physics for his discovery in1906.

From his experiments he drew revolutionary conclusions:

• Cathode ray particles are smaller than atoms.
• Cathode ray particles are negatively charged.
• Cathode ray consists of negatively charged particles.
• Cathode ray particles are at least a thousand times lighter than a hydrogen atom.

Whatever source was used to generate cathode ray, all cathode ray particles are of identical mass and identical charge.

He noticed an atom can be divided.

An electron appears to be a truly fundamental particle that cannot be divided further.

In 1907, Thomson established using a variety of methods that every atom of hydrogen has only one electron

In 1912, Thomson found the first evidence that stable elements can exist as isotopes. In other words, the same element could exist with different atomic masses.

Thomson invented one of the most powerful tools in analytical chemistry – the mass spectrometer.

Charge of electron (e) = 1.60X10^-19C.

Mass of electron = 9.1X10^-28 gram.

e/m for electron =1.76X10⁸ coulomb per gram

The construction of working of the experiment is as follows:

Cathode ray tube experiment.

In 1897, Thomson carried out a famous experiment. He constructed a glass tube, the air in the hollow glass tube is pumped out to create a vacuum. At the ends of the discharge tube, two electrodes were placed, connected to a battery for high voltage supply. The electrode connected to the negative end was known as the cathode and that to the positive as anode. He then observed a stream of particles (rays) coming out from the negatively charged electrode (cathode) to a positively charged electrode (anode) through the vacuum, creating a green glow. These rays are called cathode rays. Cathode ray particles were smaller than atoms. The whole construction is called a cathode ray tube.

To discover the charge of a cathode ray particle is positive or negative:

Applying the external electric field in the path of cathode rays, the cathode rays get deflected towards the positive electrode, but in the absence of an electric field, they travel in a straight line. Hence cathode-ray consists of negatively charged particles.

The cathode rays are made up of particles of a certain mass and some energy:

When rotor blades are placed in the path of the cathode rays, they seem to rotate. This proves that the cathode rays are made up of particles of a certain mass so that they have some energy.

Thomson found the charge to mass ratio (e/m) of electro:

On applying the electric and magnetic field upon the cathode ray particles (electrons), these particles are deflected in both fields. Thomson found the charge to mass ratio (e/m) of the electron. (e/m) for electron 1.76X10⁸ coulomb per gram.

In 1909, American physicist R. Millikan measured the charge of an electron using negatively charged oil droplets.

Charge of electron (e) is 1.60X10^-19C

From e/m ratio, we can calculate the mass of an electron.

Mass of electron(m) is 9.1X10^-28 gram.

                        J. J. Thomson in his laboratory operating a cathode ray tube