Structure of Atom


An atom’s nucleus comprises two major particles. An atom’s nuclei, these two types of stable particles are present and they are known as protons and neutrons. These subatomic particles are denoted by ‘p’ for proton and ‘n’ for neutron. The mass of the proton is slightly less than the mass of the neutron.

The total mass of protons and neutrons makes the mass of an atom. Protons and neutrons are collectively known as nucleons. The number of protons and neutrons in an atom can be one or more. The atomic number is the number of protons in an atom’s nucleus that defines a property of an element.

Since each element has a unique atomic number, therefore, each element has a different number of protons in its atom’s nuclei. A proton carries a positive electric charge which is equivalent to an electron’s negative charge. The word “Proton” is a Greek word and its meaning is “first” and this name was given by New Zealand physicist Ernest Rutherford in 1920.



Discovery of Proton

The discovery of proton was initiated when Ernest Rutherford started the experiments to discover hydrogen-like atoms in other atoms. He noticed that when alpha particles are shot in the air the scintillation detectors showed the typical hydrogen nuclei as a product. He found that when alpha particles bombed on nitrogen gas the alpha particles were captured and absorbed by nitrogen gas and the nitrogen nucleus converted to a heavy oxygen atom by releasing hydrogen nucleus as a result. In this way, a hydrogen-like nucleus with a positive charge was produced and he termed this particle as “Proton” first time in 1920.

This experiment supported Prout’s hypothesis that hydrogen is the building block of all elements and it is the simplest and lightest element. He named this fundamental building block of the nucleus as “Proton” which is a Greek word and has meaning as “first”.  Each element’s atom has the same number of protons in its nucleus as it has the atomic number. This number defines the element’s position in the periodic table. As the protons are positively charged sub-atomic particles therefore in a neutral atom, the number of protons is equal to the number of electrons which are negatively charged sub-atomic particles.

The ionised hydrogen which is simply proton has high velocity and this property of proton is used to study the nuclear reactions. Protons are the constituent of cosmic rays. These rays are created artificially in the nuclear reactions and therefore protons produced are utilised as projectiles for other reactions or field.

In the late 20th century, the structural understanding of protons and neutrons were refined and discovered that these particles are further made up of small particles termed baryons and these baryons are composed of other smaller particles known as quarks.

Particle Analysis of Proton

After the discovery of an atom’s nuclei and its constituent particles, protons and neutrons, it was assumed that the protons are single particles and cannot be further broken into other sub-particles. They were considered elementary particles. In the modern standards of high-energy particle physics, protons are composite particles and are composed of elementary particles, quarks.

A proton has two up quarks and one down quark. The electric charge on up quark is equal to \(\frac{2}{3}e\), which is a positive fractional charge. On the other hand, the electric charge on down quark is equal to \(-\frac{1}{3}e\) which is a negative fractional charge. Where ‘e’ the value of charge on a single electron without the sign. As protons are not fundamental particles they have a measurable size. Its size is \(0.84 \times 10^{-15}\) to \(0.87 \times 10^{-15} m\), which is the root mean square charge radius.

The mass of the proton consists of the mass of quark particles and the gluon fields which bind the quarks together. Though the masses of quarks contribute nearly 1% of the total mass of the proton, the rest mass consists of gluons. The nuclei of an atom consist of protons and neutrons, collectively known as nucleons, bind together with nuclear forces within the nuclei. The total mass of protons and neutrons make the mass of an atom and it is represented as the mass number of atom.

Constituent Particles of Proton

Protons are made from other fundamental and elementary particles referred to as Quarks. The instability of these subatomic particles, quarks, makes them pair with each other and therefore, they are never found in isolation. They combine to form stable hadrons. In the protons and neutrons, elementary particles like baryons and mesons are found within the quark-gluon plasma.

Quarks are always paired with their antiparticles. These antiparticles are antiquarks. They combine to form hadrons which in turn, combine to form other subatomic particles like mesons and baryons. To date, quarks are supposed to be the elementary particles that are not broken down to other smaller particles as they have no structure. In particle physics, only quarks are the basic elementary particles that exist in nature.

In Gell-Mann’s model, there are six types of quarks and are found in three pairs. The type of quarks is additionally termed as “flavour” in order that there are six flavours of quarks. These six flavours are up and down, top and bottom and charm and strange. Out of those flavours, up and down quarks are the lightest particles and 4 other heavier particles which tend to convert rapidly to up and down quarks.

It means up and down quarks are commonest within the universe as they’re stable. Whereas other elementary particles charm, strange, top and bottom quarks can only produce when high energy collisions occur like cosmic rays and particle accelerators like cyclotrons.

The electric charge on up, charm and top quarks are equal to \(\frac{2}{3}e\), which is a positive fractional charge. On the other hand, the electric charge on down, strange and bottom quarks is equal to \(-\frac{1}{3}e\) which is a negative fractional charge. All flavours of quarks have antiparticles (or antiquarks) to each and the electrical charges on these antiparticles are opposite to their corresponding quarks.

FAQs on Protons

Q.1:  Are protons elementary particles?

Answer: No, protons are not elementary particles as they can be broken down into other small particles. Protons further consist of small particles, baryons and these baryons consist of other smaller particles, quarks. These quarks occur in pairs and there are three pairs, “flavours”. This way, there are sis flavours, namely, up and down, top and bottom and charm and strange.

Q.2: How do the atomic number and atomic mass correlate with protons?

Answer: Atomic number of an element is equal to the number of protons it bears inside the nucleus. And on the other hand, the atomic mass of an element is equal to the total mass of protons and neutrons inside the nucleus. Thus both atomic number and atomic mass depend on the number of protons.

Q.3: How protons are useful in the medical field?

Answer: Protons are useful in the treatment of diseases and they are part of proton therapy or proton radiotherapy. In this, a beam of protons is used to irradiate the diseased tissue. A disease that is treated with this therapy is cancer. The main advantage of proton therapy is that protons penetrate to smaller depth and do not produce scattered radiation which helps to save nearby healthy tissues.

Share with friends

Customize your course in 30 seconds

Which class are you in?
Get ready for all-new Live Classes!
Now learn Live with India's best teachers. Join courses with the best schedule and enjoy fun and interactive classes.
Ashhar Firdausi
IIT Roorkee
Dr. Nazma Shaik
Gaurav Tiwari
Get Started

Leave a Reply

Notify of

Customize your course in 30 seconds

Which class are you in?
No thanks.