Discovery of Proton – Who Discovered It

In this chapter, we will explore the discovery of proton. As we know, atoms are the fundamental building blocks of matter, composed of three main subatomic particles: protons, neutrons, and electrons.

Among these, protons play a critical role in defining the identity of an atom in the periodic table. The number of protons in the nucleus of an atom determines the atomic number of an element. For example, an atom with one proton is hydrogen, while an atom with six protons is carbon.

Two key figures, Eugen Goldstein and Ernest Rutherford, played a crucial role in the discovery of proton.

In 1886, German physicist Eugen Goldstein first observed the presence of positively charged particles in an atom. His predicted was based on the concept that atoms are electrically neutral, which means that an atom contains the same number of positive and negative charges to balance each other.

Eugen Goldstein performed a series of experiments and observed that when high voltage electricity passed through a perforated cathode in the discharge containing hydrogen gas at low pressure, a new type of rays was produced from the positive electrode (anode) which moves towards the cathode. These new rays he named as canal rays, anode rays or positive rays. 

This discovery of Goldstein was an early indication of the existence of positive charges within atoms. However, he did not identify the proton itself.

Who Discovered Protons?

The discovery of the proton is credited to Ernest Rutherford, who first identified the proton in 1917 during his experiment conducted at the University of Manchester. He proved that the nucleus of a hydrogen atom, which is just a proton, is present in the nuclei of all atoms. This means that every atom contains protons in its nucleus.

Sir E. Rutherford bombarded nitrogen atoms with alpha particles and observed the release of hydrogen nuclei. He concluded that these hydrogen nuclei were fundamental components of all atomic nuclei and named them protons.

Rutherford’s Proton Discovery Experiment

Let’s break down the experiment of Sir Ernest Rutherford that led to the discovery of the proton into simple steps:

Step 1: Preparing the Experiment

• Objective: Sir E. Rutherford wanted to explore what happens when he bombards nitrogen atoms with alpha particles.
• Alpha particles are small, positively charged particles made up of 2 protons and 2 neutrons bound together. They are also known as alpha rays or alpha radiation. These particles are emitted spontaneously from the nuclei of certain radioactive elements, such as uranium or radium, during radioactive decay.
• Setup: Rutherford used a special detector called a scintillation detector to observe the particles that were released when alpha particles hit a target. This detector allowed him to see and identify the tiny flashes of light (scintillations) produced by the particles during the experiment.

Step 2: Shooting Alpha Particles into the Air

• Rutherford shot a beam of alpha particles into the air and used scintillation detectors to observe what particles were produced.
• He noticed that hydrogen nuclei had produced from the nitrogen atoms present in the atmosphere.

Step 3: Discovering the Source

• Rutherford realized that these hydrogen nuclei must be coming from nitrogen atoms in the air, not from hydrogen itself.
• This was surprising for him because hydrogen nuclei were being released even though he was working with nitrogen gas.

Step 4: Testing Pure Nitrogen Gas

• To confirm his findings, Rutherford proceeded to fire beams of alpha particles into pure nitrogen gas.
• He observed that even more hydrogen nuclei (protons) were produced in this experiment.

Step 5: Conclusion

• Rutherford concluded that the hydrogen nuclei (which were later called protons) came from the nitrogen atoms.
• This meant that the proton is a fundamental part of all atoms, not just hydrogen. The nitrogen atoms were breaking apart, releasing protons when hit by beams of alpha particles.

Step 6: Understanding the Reaction

• Rutherford observed the first nuclear reaction during his experiment. The equation for the reaction can be written as:
  • ¹⁴N + α → ¹⁷O + p
• In this equation:
  • ¹⁴N represents a nitrogen atom.
  • α is an alpha particle (which contains 2 protons and 2 neutrons).
  • ¹⁷O is the oxygen atom produced.
  • p is the proton (hydrogen nucleus) released.
• This was the first time a nuclear reaction was observed, showing that protons are a part of atomic nuclei.

Step 7: The Discovery of Proton

• Rutherford’s experiment showed that the hydrogen nucleus which was later named protons, are the fundamental constituents of the nucleus of all atoms. His findings helped confirm that the proton is one of the building blocks of matter and is present in the nucleus of every atom.

Properties of Protons

There are the following characteristic properties of protons that you should remember. They are as follows:

(1) Positive Charge:

• Protons carry a positive electric charge of +1 unit (i.e. +1.602 * 10^-19 coulombs). This charge is equal in magnitude but opposite to the charge of electron, which is a negative sign.

(2) Mass:

• A proton has a mass of approximately 1.6726 × 10^^-27 kilograms (1.0072 amu). Its mass is about 1,836 times heavier than an electron, but nearly the same as the mass of neutron.

(3) Location in the Atom:

• Protons are found in the nucleus at the center of every atom, along with neutrons. Together, they make up almost the entire mass of the atom.

(4) stability:

• Protons are highly stable particles. They do not decay under normal conditions and have an incredibly long lifespan.

(4) Atomic Number:

• The number of protons in the nucleus of an atom determines the atomic number of the element. For example, hydrogen has 1 proton, and oxygen has 8 protons.

(5) Quark Composition:

• A proton is made up of three smaller particles called quarks—two up quarks and one down quark held together by the strong nuclear force.

(6) Proton-Proton Repulsion:

• Protons are all positively charged and repel each other because of this like charge. However, they are held together in the nucleus by the strong nuclear force.

(7) Interchangeability:

• A proton can sometimes transform into a neutron through a process called beta decay, but this typically occurs in certain radioactive conditions.

Charge to Mass Ratio of Protons

The charge-to-mass ratio (e/m) of the positively charged particles present in the anode or canal rays varies depending on the type of gas used in the discharge tube. It was observed that the e/m ratio is maximum when hydrogen gas is used in the discharge tube.

This is because the mass of the positive particles in the canal rays depends on the atomic or molecular weight of the gas present in the tube. Hydrogen, being the lightest element, has the smallest atomic weight, which results in a higher e/m ratio compared to other gases.

• H —-> H⁺ + e^–
• O —-> O⁺ + e^–
• N —-> N⁺ + e^–

Here, the charge (e) of the electrons in the cathode rays is always the same. Therefore, the e/m ratio remains the same for cathode rays, regardless of the gas used in the discharge tube. The mass of cation (positive particles) in canal rays varies depending on the type of gas used. Consequently, the e/m ratio differs for different gases.

The e/m ratio is the highest for H2 gas because the mass of hydrogen is lowest and charge is the same. Thus, the smallest and lightest positive ions were obtained from the hydrogen. These lightest positively charged particles were named as protons.

Determination of the Charge on a Proton

The charge-to-mass (e/m) ratio for the cathode rays obtained from hydrogen gas was found to be the highest, with a value of:

e/m = 9.58 × 10⁴ Cg⁻¹ = 9.58 × 10⁷ C/Kg

Since the charge of a proton is equal in magnitude but opposite in sign to the charge of an electron, the mass of the positive particle (proton) from hydrogen gas can be calculated using the formula:

Mass of proton=e / (e/m) = e / me​

Substituting the known values:

Mass of proton = 1.602×10⁻¹⁹ C / 9.58×10⁷ C/Kg = 1.67×10⁻²⁷ Kg

This value is practically the same as the mass of a hydrogen atom, which is about 1837 times the mass of an electron. Therefore, the proton is recognized as the second fundamental subatomic particle of an atom, carrying one unit of positive charge and having a mass nearly equal to the mass of a hydrogen atom, not the electron.

The discovery of proton by Ernest Rutherford in 1919 was a landmark moment in the history of science. It provided a deeper understanding of atomic structure and clarified the nature of chemical elements.

Frequently Asked Questions on Protons