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Atomic Nucleus – Definition, Structure, Discovery, Characteristics

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  • Last Updated : 20 Oct, 2021
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In physics, the atomic nucleus is the central component of an atom. It is much smaller than an atom and contains the majority of the atom’s mass. The atomic nucleus also contains all of the positive electric charge (in protons), while the electron cloud contains all of the negative charges.

The nucleus of an atom is the central region of an atom that contains the majority of its mass. Rutherford’s scattering of alpha particles experiment revealed that the nucleus of an atom contains the majority of the atom’s mass. The nucleus of an atom occupies nearly 10-14 times the volume of the atom but contains 99.99% of the atomic mass.

Structure of Atom

An atom is a complex arrangement of negatively charged electrons organised in defined shells around a positively charged nucleus.

This nucleus, which is made up of protons and neutrons, contains the majority of the atom’s mass (except for common hydrogen which has only one proton). Every atom is roughly the same size. The Angstrom (Å), defined as 1 x 10-10 m, is a useful unit of length for measuring atomic sizes. An atom has a diameter of about 2-3 Å.

Discovery of Nucleus

Thomson started experimenting with cathode ray tubes. Cathode ray tubes are vacuum-sealed glass tubes with the majority of the air removed. At one end of the tube, a high voltage is applied across two electrodes, causing a beam of particles to flow from the cathode (negatively charged) to the anode (positively charged). Because the particle beam, or ‘cathode ray,’ originates at the cathode, the tubes are known as cathode ray tubes. 

Thomson used two oppositely charged electric plates to surround the cathode ray. The cathode ray was redirected from the negatively charged to the positively charged electric plate. This meant that the cathode ray was made up of negatively charged particles. Cathode rays are particles that are negatively charged. Scientists gradually accepted Thomson’s discoveries. Cathode ray particles became well-known and were given the name electrons.

Rutherford’s gold foil experiment demonstrated that the atom is mostly empty space with a tiny, densely packed, positively charged nucleus. Rutherford proposed the nuclear model of the atom based on these findings. Rutherford’s gold foil experiment involved firing an alpha beam at a thin sheet of gold foil. The majority of the particles passed through the gold foil without being deflected, but a small number were slightly deflected and an even smaller fraction was deflected more than 90 degrees from their path. 

This prompted Rutherford to propose the nuclear model, which states that an atom is made up of a very small, positively charged nucleus surrounded by negatively charged electrons. Based on the number of alpha particles deflected in his experiment, Rutherford calculated that the nucleus took up a tiny fraction of the volume of the atom.

Composition of a Nucleus

An atom’s nucleus is made up of a densely packed arrangement of protons and neutrons. Because these are the two heavy particles in an atom, 99.9% of the mass is concentrated in the nucleus. Because protons have a net positive charge, the nucleus of an atom is positively charged overall, while negatively charged electrons revolve around the central nucleus. Because the mass concentration at an atom’s nucleus is enormous, the nuclear forces that hold protons and neutrons together are also enormous. 

Since, the protons are so close to each other inside the tiny nucleus, electrostatic forces of repulsion also act inside the nucleus. Because the total number of protons in a nucleus equals the total number of electrons revolving around the nucleus, the atom as a whole is electrically neutral.

Characteristics of the Nucleus

  1. The nucleus is located in the centre of the atom and contains protons and neutrons.
  2. Because of the presence of positively charged protons, the nucleus determines the overall charge on the atom.
  3. It is also in charge of the chemical properties of the element, such as the valency of the atom, the element’s reactivity to other elements, and the atomic number and mass number of the atom.
  4. While the neutron is electrically neutral, it contributes to the atom’s mass.
  5. The numeric sum of protons and neutrons is the atomic mass. The mass of an atom determines physical properties such as melting point, boiling point, density, and so on.
  6. The nucleus remains stationary at the centre of the atom, while the electron revolves around it due to the attraction between the positively charged nucleus and the negatively charged electron.
  7. The electron spins on its axis as it moves around the nucleus. An electron’s mass is considered negligible in comparison to the nucleus, but it is responsible for the valency of the atom and determines the atom’s ability to participate in any chemical reaction.

Mass of a Nucleus

As previously stated, the nucleus contains almost all of an atom’s mass, with only a minor contribution from the electron cloud. The mass of an atom is related with its atomic mass number, which is the total number of protons and neutrons in its nucleus. A chemical element’s isotopes each have a distinct mass number. The mass number is written after the element name or as a superscript to the left of an element’s symbol. Carbon-12, or 12C, is the most common carbon isotope.

Atoms are so small in size and mass that using standard measuring units, while possible, is frequently inconvenient. On the atomic scale, units of mass and energy have been defined to make measurements easier to express. The atomic mass unit is the unit of mass measurement (amu). 1.66 x 10-24 grams is one atomic mass unit.

Sample Questions

Question 1: What are subatomic particles?


Subatomic particles are the particles that make up an atom. This term generally refers to protons, electrons, and neutrons.

Question 2: What are the shortcomings of Bohr’s atomic model?


According to this atomic model, the structure of the atom offers poor spectrum predictions for bigger atoms. It also neglected to take the Zeeman effect into account. It could only successfully describe the hydrogen spectrum.

Question 3: How can the total number of neutrons in the nucleus of a given isotope be determined?


The total number of protons and neutrons in an isotope is used to calculate its mass number. The total number of protons in the nucleus is described by the atomic number. As a result, subtracting the atomic number from the mass number yields the number of neutrons.

Question 4: What are quarks?


Individual protons and neutrons contain internal structure known as quarks. There are six different kinds of quarks. These subatomic particles cannot be liberated and studied separately. Current research into the structure of the atom is ongoing.

Question 5: What are the reasons for the difference between mass number and isotopic mass?


The difference between mass number and isotopic mass, known as the mass defect, is caused by two factors:

  1. The neutron weighs slightly more than the proton. In comparison to the atomic mass unit scale with equal numbers of protons and neutrons based on 12C, this increases the mass of nuclei with more neutrons than protons.
  2. The nuclear binding energy varies depending on the nucleus. According to Einstein’s mass-energy equivalence relation E = mc2, a nucleus with a higher binding energy has a lower total energy and thus a lower mass. As the atomic mass of 63Cu is less than 63, this must be the determining factor.
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