Dalton's atomic theory, proposed in the year 1803, considered the atom to be the smallest indivisible constituent of all matter. The Dalton's theory could explain the law of conservation of mass, law of constant composition and law of multiple proportions known at that time.
However, towards the end of nineteenth century, certain experiments showed that an atom is neither the smallest nor indivisible particle of matter as stated by Dalton. It was shown to be made up of even smaller particles. These particles were called electrons, protons and neutrons. The electrons are negatively charged whereas the protons are positively charged. The neutrons are uncharged in nature.
In 1885, Sir William Crookes carried out a series of experiments to study the behaviour of metals heated in a vacuum using cathode ray tubes. A cathode ray tube consists of two metal electrodes in a partially evacuated glass tube. An evacuated tube is the one from which most of the air has been removed. The negatively charged electrode is called cathode whereas the positively charged electrode is called anode. These electrodes are connected to a high voltage source.
It was observed that when very high voltage was passed across the electrodes in evacuated tube, the cathode produced a stream of particles. These particles were shown to travel from cathode to anode and were called cathode rays. In the absence of external magnetic or electric field these rays travel in straight line.
In 1897, an English physicist Sir J.J. Thomson showed that the rays were made up of a stream of negatively charged particles. This conclusion was drawn from the experimental observations when the experiment was done in the presence of an external electric field.
Following are the important properties of cathode rays:
Cathode rays travel in straight line
The particles constituting cathode rays carry mass and possess kinetic energy
The particles constituting cathode rays have negligible mass but travel very fast
Cathode ray particles carry negative charge and are attracted towards positively charged plate when an external electric field is applied
The nature of cathode rays generated was independent of the nature of the gas filled in the cathode ray tube as well as the nature of metal used for making cathode and anode. In all the cases the charge to mass ratio (e/m) was found to be the same
These particles constituting the cathode rays were later called electrons. Since it was observed that the nature of cathode rays was the same irrespective of the metal used for the cathode or the gas filled in the cathode ray tube. This led Thomson to conclude that all atoms must contain electrons. This meant that the atom is not indivisible as was believed by Dalton and others. In other words, Dalton’s theory of atomic structure failed partially.
This conclusion raised a question, "If the atom was divisible, then what were its constituents?". Today a number of smaller particles are found to constitute atoms. These particles constituting the atom are called subatomic particles.