Discovery of Electron, Proton & Neutron

Lets have discussion about the discovery of fundamental particles of atom i.e electron, proton and neutron.

Discovery of Electron

1.  Cathode Rays:

During the latter half of the nineteenth century, it was found that while normally dry gases do not conduct an electric current, they do so under very low pressure and then patches of light are seen. The passage of electricity through gases as studied by a number of physicists, particularly by Faraday, Davy, Crookes and J.J. Thomson.

When a current of high voltage (10,000 volts) is passed through a gas of air kept at a very low pressure (0.01 – 0.03 mm) blue rays are seen emerging from the case. These rays are called “Cathode Rays”.

Some of the important properties of the cathode rays studied by Sir J.J. Thomson and others are given below:

Cathode rays come out at right angles to the surface of the cathode and move in straight lines.

Their path is independent on the position of the anode.

They produce phosphorescence on certain salts like ZnS and fluorescence on glass.

They blacken photographic plates.

The rays pass through thin sheet of metals. If the metal sheet is too thick to be penetrated the rays cast a shadow.

They produce X-ray when they strike a metal.

The rays ionize a gas through which they pass.

They heat a substance on which they fall.

They rotate a light wheel placed in their paths. This shows that cathode rays contain material particles having both mass and velocity.

The mass of a particle present in cathode rays is found to be 1/1837 of H-atom. This shows that the particle is of sub-atomic nature.

Cathode rays are deflected by a magnetic or an electric field showing the particle to be electrically charged, the direction of deflection  shows that they are negatively charged.

Cathode rays contain the smallest unit of negative charge.

No cathode ray was produced when the tube was completely evacuated.

Different gases produce same cathode rays as they have the same e/m (charge/mass) ratio. This indicates that the particles present in cathode rays are fundamental constituent of all matter.

Sir J.J. Thomson named these negatively charged sub-atomic particles as electron. “A sub-atomic particle which is a fundamental constituent of all matter having a mass 1/1837th of a H-atom and which carries the smallest unit of negative charge is called an electron”.

2.  Determination of Velocity and Charge/mass (e/m) ratio of Electrons:

Sir J.J. Thomson (1897) who discovered electron) extended the cathode ray experiment for the determination of velocity of electrons and their charge/mass ratio by applying electrical and magnetic field perpendicular to each other as well as to the path of electrons.

According to Thomson the amount of deviation of the particles from their path in the presence of electrical or magnetic field depends upon:

the magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater is the interaction with the electric or magnetic field and thus greater is the deflection.

the mass of the particle : Lighter the particle, greater the deflection.

the strength of the electrical or magnetic field : The deflection of electrons from  its original path increases with the increase in the voltage across the electrodes, or the strength of the magnetic field.

The value of e/m for an electron = 1.76 x 108 C/g. For the H+ ion (proton),
e/m = 96500/1.008 C/g.

Millikan’s Oil Drop Method: Determination of Charge on an Electron

In 1909, Millikan measured the charge on an electron by his oil drop method. In this method a spray of oil droplets is produced by an atomizer, some of which pass through an opening into a viewing chamber, where we can observe them with a microscope. Often these droplets have an electric charge, which is picked up from the friction forming the oil droplets. A droplet may have one or more additional electrons in it, giving it a negative charge.

As the droplet falls to the bottom of the chamber, it passes between two electrically charged plates. The droplet can be suspended between them; we adjust the voltage in the plates so that the electrical attraction upward just balances the force of gravity downward. We then use the voltage needed to establish this balance to calculate the mass - to charge ratio for the droplet. Because we already know the mass of the droplet we can find the charge on it.

Millikan's found that the charge on all droplets could be expressed as whole number multiples of e, where the value of e is 1.602 x10-19 C. By combining e/m. ratio and 'e' we calculate mass of the electron

Me  = e / (e/m) = 1.6022 x 10-19 / 1.76 x 108

= 9.104´10-31 kg

This very small value shows that the electron is a subatomic particle. Thus charge on an electron = 1.602 x 10–19C.

Discovery of Proton:

Positive rays or Canal rays

Atoms are electrically neutral. Hence after the discovery of the negatively charged constituent (electron) of an atom, attempts were made to discover the positively charged counterpart of electrons. By using a discharge tube containing a perforated cathode. Goldstein (1886) found that some rays passed through these holes in a direction opposite to that of the cathode rays.These are also known as anode rays  because they seems to be coming from a perforated anode used in a discharge tube experiment. These are  made up of positively charged  particles known as protons.

The charge to the mass ratio (e/m ratio ) is not constant for these rays, and has been found different for different gases used in the discharge tube.

These are called the positive rays or canal rays. J.J. Thomson (1910) measured their charge by mass ratio from which he was able to deduce that these contain positive ions. Their properties are:

They are positively charged.

The positive charge is either equal to or whole number multiple of the charge on an electron.

When hydrogen gas was filled in the discharge tube the positive charge on the positive rays was equal to the negative charge on an  electron, and the mass was less than the hydrogen atom.

Unlike cathode rays the properties of positive rays are characteristics of the gas in the tube.

The deflection of positive rays under the influence of an electric or magnetic field is smaller than that of the cathode rays for the same  strength of field. This shows that the positive rays have a greater mass than that of electrons.

The mass of the positive rays depends on the atomic weights or molecular weights of the gases in the discharge tube. The charge/mass  ratio also varies because the change in positive charge on the rays. It may be either equal to or integral multiple of the charge on an  electron.

The lightest of all particles identified in positive rays from different elements was one with a mass very slightly less than that of hydrogen atom (or nearly equal to H-atom). The lightest positively charged particle is called a proton (P or P+). Positive rays are atomic or molecular resides from which some electrons have been removed. The removed electrons constitute the cathode rays and the positive residues form the positive or canal rays.

Positive Rays

Cathode Rays

H

H+

e–

O →

O+

e–

O2 →

O2+

e–

O2 →

O22+

2e–

The mass of a proton is very slightly less than that of a H-atom. This shows that protons are sub-atomic particles. Protons are fundamental constituent of matter because positive rays are produced by all substances.

“A sub-atomic particle, which is a fundamental constituent of all matter having a mass slightly less than that of H-atom and which carries a positive charge equal in magnitude to the charge on an electron, is called a proton”. A proton is denoted by p or p+ of +1p.

Comparison of Positive (Canal) Rays and Cathode Rays:

Properties

Cathode Rays

Canal Rays

Sign of Charge

Negative

Positive

Mag. of Charge

Always –1

Mostly +1, but also +2, +3…

Mass

Definite value

Variable, depends on ions

e/m

Definite value

Variable, depends on ions

Discovery of Neutron

After the discovery of electron and proton Rutherford (1920) had predicted the existence of a neutral fundamental particle. In 1932, Chadwick bombarded the element Beryllium with a-particles and noticed the emission of a radiation having the following characteristics.

The radiation was unaffected by magnetic and electric fields which show that it is electrically neutral.

It was found to have approximately the same mass as the protons.

The name ‘neutron’ was given to this sub-atomic particle. It is denoted by n or 1on. Bombardment of beryllium by α-particles results in the formation of carbon and neutrons are emitted.



At present there are a number of evidences which confirm that like electron, proton and neutron is also a fundamental constituent of atoms (a single exception is 11H atom which does not contain any neutron)

Mass of a neutron is 1.008930 amu (1.6753 x 10–24g or 1.6753 x 10–27 kg)

Neutron “A sub-atomic particle, which is a fundamental constituent of matter having mass approximately equal to the hydrogen atom and which is electrically neutral, is called a neutron”.

You can also refer to

IIT JEE Chemistry Syllabus

Reference books of Physical Chemistry


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