How was protons discovered??
In most popular texts on particle physics you see a long laundry list of particles and their discoverers :
1897 Thomson discovers the electron
1911 Rutherford discovers the nucleus
1932 Chadwick discovers the neutron
and so on and so on. Somewhere between Thomson and Chadwick, physicists realized that there are positively charged constituents of the nucleus, which we call 'protons'. The way this happened was a gradual process, and that is why it is hard to say exactly who discovered the proton, although if you had put a name against it, it would be Ruthford, sort of.
After the discovery of the electron, it was realized that there must be positive charge centers within the atom to balance the negative electrons and create electrically neutral atoms. Rutherford's discovery of the nucleus demonstrated that these positive charges were concentrated in a very small fraction of the atoms' volume. In 1919 Rutherford discovered that he could change one element into another by striking it with energetic alpha particles (which we now know are just helium nuclei). In the early 1920's Rutherford and other physicists made a number experiments, transmuting one atom into another. In every case, hydrogen nuclei were emitted in the process. It was apparent that the hydrogen nucleus played a fundamental role in atomic structure, and by comparing nuclear masses to charges, it was realized that the positive charge of any nucleus could be accounted for by an integer number of hydrogen nuclei. By the late 1920's physicists were regularly referring to hydrogen nuclei as 'protons'. The term proton itself seems to have been coined by Rutherford, and first appears in print in 1920. - 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
1897 Thomson discovers the electron
1911 Rutherford discovers the nucleus
1932 Chadwick discovers the neutron
and so on and so on. Somewhere between Thomson and Chadwick, physicists realized that there are positively charged constituents of the nucleus, which we call 'protons'. The way this happened was a gradual process, and that is why it is hard to say exactly who discovered the proton, although if you had put a name against it, it would be Ruthford, sort of.
After the discovery of the electron, it was realized that there must be positive charge centers within the atom to balance the negative electrons and create electrically neutral atoms. Rutherford's discovery of the nucleus demonstrated that these positive charges were concentrated in a very small fraction of the atoms' volume. In 1919 Rutherford discovered that he could change one element into another by striking it with energetic alpha particles (which we now know are just helium nuclei). In the early 1920's Rutherford and other physicists made a number experiments, transmuting one atom into another. In every case, hydrogen nuclei were emitted in the process. It was apparent that the hydrogen nucleus played a fundamental role in atomic structure, and by comparing nuclear masses to charges, it was realized that the positive charge of any nucleus could be accounted for by an integer number of hydrogen nuclei. By the late 1920's physicists were regularly referring to hydrogen nuclei as 'protons'. The term proton itself seems to have been coined by Rutherford, and first appears in print in 1920.
- Discovery of Proton
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 |
