Liquid - Drop Model

Liquid Drop Model in Nuclear Physics

The aim is to explain nuclei's masses and binding energies. The Liquid Drop Model is named after the fact that nuclei are thought to behave similarly to liquids (at least to first order). A liquid's molecules are kept together by the Van der Waals force, which exists only between close neighbours.

 

Liquid Drop Model of the Nucleus

The liquid drop model of the nucleus explains forces in atomic nuclei as if they were created by a tiny liquid drop in nuclear physics. On a nuclear scale, however, the fluid is made up of nucleons (protons and neutrons). The liquid drop model accounts for the fact that the forces acting on nucleons on the surface vary from those acting on nucleons on the inside, where they are absolutely surrounded by attracting nucleons. This is analogous to considering surface tension as a factor in calculating the energy of a tiny liquid drop.

 

Key Facts of Nuclear Liquid Drop Model

  1. Nuclei seem to have a constant density, according to scattering studies.

  2. Nuclei have their own volume and surface, where various forces work.

  3. The nucleus is spherical in its ground state.

  4. This spherical nucleus can be twisted into a dumbbell shape and then broken into two fragments if enough kinetic or binding energy is applied.

  5. For the binding energy of nuclei, the Weizsaecker formula is an empirically refined variant of the liquid drop model. The terms are as follows:

    1. Volume term

    2. Surface term

    3. Asymmetry term

    4. Pairing term

    5.  Coulomb term.

  6. Binding energies and masses of atomic nuclei can be calculated using the Weizsaecker formula. As a result, we can calculate the energy released per fission.


Semi-Empirical Formula

The von Weizsäcker mass formula (also known as the semi-empirical mass formula – SEMF) was published in 1935 by German physicist Carl Friedrich von Weizsäcker and was one of the first models that could accurately explain the action of nuclear binding energies and thus nuclear masses. The liquid drop model proposed by George Gamow is the basis for this theory.

The atomic nucleus, according to this model, behaves like the molecules in a drop of liquid. The fluid, on the other hand, is made up of nucleons (protons and neutrons) held together by a strong nuclear force. The nucleus' liquid drop model takes into account the fact that nuclear forces on nucleons on the surface vary from those on nucleons in the nucleus' interior. Other attracting nucleons completely surround the interior nucleons. The forces that shape a drop of liquid can be compared to this.

The nucleus is spherical in its ground state. This spherical nucleus can be distorted into a dumbbell shape and then broken into two fragments if enough kinetic or binding energy is applied. The splitting of such heavy nuclei must be followed by energy release since these fragments have a more stable configuration. This model does not account for all of the atomic nucleus’s properties, but it does account for the expected nuclear binding energies.

 

The Liquid Drop Model

George Gamow suggested the liquid drop model, which was further developed by Niels Bohr and John Archibald Wheeler. It treats the nucleus as an incompressible fluid drop with a very high density that is kept together by the nuclear force (a residual effect of the strong force), with a structure that resembles that of a spherical liquid drop. The liquid drop model, though crude, accounts for most nuclei's spherical shape and allows a rough prediction of binding energy.

The mass formula is solely determined in terms of the number of protons and neutrons it includes. Five terms are described in the original Weizsäcker formula:

  • Interior nucleon has a certain number of other nucleons in contact with it when an arrangement of nucleons of the same size is packed together into the smallest volume. As a result, the amount of nuclear energy emitted is proportional to the volume.

  • The presumption that each nucleon interacts with the same number of other nucleons is corrected by surface energy. Since this definition is negative and proportional to the surface area, it is approximately equal to liquid surface tension.

  • The potential energy of each pair of protons is known as Coulomb energy. The binding energy is diminished since this is a repulsive force.

  • The Pauli exclusion theory is explained by asymmetry energy (also known as Pauli Energy). Uneven numbers of neutrons and protons mean that one form of the particle will fill higher energy levels while the other will leave lower energy levels empty.

  • The propensity for proton and neutron pairs to form is explained by pairing energy. Due to spin coupling, an even number of particles is more stable than an odd number.


What are the major achievements of the liquid-drop model?

The liquid-drop model was an efficient model that helped scientists study atomic nuclei. The major achievements of the liquid-drop model are:

  • It helps in predicting the atomic mass of the nuclei accurately.

  • It helps in predicting the binding energies of various nuclei accurately.

  • It helps scientists in predicting the emission of alpha particles and beta particles in radioactivity.

  • It also efficiently explains the basic features of the process of fission.


What properties can be studied using a liquid-drop model?

The liquid-drop model can be efficiently used to study various properties of nuclear physics. This model considers the nucleus of an atom as a liquid. Nuclear properties, like binding energy, are studied through a liquid-drop model and are expressed in parameters that are associated with a liquid such as a volume energy, compressibility, and surface energy. The model was also used to explain how a nucleus performs when it undergoes fission.


What are the major advantages of the liquid-drop model?

The liquid-drop model has various advantages in the field of nuclear physics. The major advantages of the liquid-drop model are:

  •  The liquid-drop model explains the spherical shape and stability of nuclei very well.

  • The model helps in predicting the binding energy of the nucleus and also helps us to know how much energy is available for consumption purposes.

  • The model helps in explaining the radioactive phenomenon of artificial radioactivity and nuclear fission.


Some terminologies related to nuclear physics

  • Nucleon: it refers to a proton or a neutron in the nucleus of an atom.

  • Atomic number: it refers to the number of protons present in a nucleus and is denoted as Z.

  • Atomic mass number: it refers to the number of nucleons present in a nucleus and is denoted as A =N+Z.

  • Nuclide: it is the term given to a nucleus that contains a specific value of A and Z. the values of A and Z are written next to the chemical symbol in the form of subscript and superscript. The atomic mass number is written in the superscript and the atomic number is written in the subscript.

  • Isotope: it refers to a nucleus that has the same atomic number but a different atomic mass number. The isotopes generally have similar chemical and atomic behavior but may possess different nuclear properties.

  • Isotone: it refers to a nucleus that contains a different number of protons but the same number of neutrons.

  • Isobar: these are just the opposite of isotopes and refer to a nucleus that has the same atomic mass number but a different atomic number.

  • Mirror nuclei: these refer to a pair of unique nuclei wherein the number of protons in one nucleus, say A is equal to the number of neutrons in the other nuclei, say B, and the number of neutrons in A is equal to the number of protons in B.

Learn more about the liquid drop model of the atomic nucleus by Vedantu from the top subject matter experts and build a strong foundation of nuclear physics. Find out deeper insights into this topic and proceed with better concepts to study advanced subjects.

FAQs on Liquid - Drop Model

1. Explain the Liquid Drop Model of the Nucleus?

The liquid drop model of the nucleus explains forces in atomic nuclei as if they were created by a tiny liquid drop in nuclear physics. On a nuclear scale, however, the fluid is made up of nucleons (protons and neutrons). The liquid drop model accounts for the fact that the forces acting on nucleons on the surface vary from those acting on nucleons on the inside, where they are absolutely surrounded by attracting nucleons. This is analogous to considering surface tension as a factor in calculating the energy of a tiny liquid drop.

2. How Does the Liquid Drop Model Account For Nuclear Fission?

The nucleus is thought to be like a drop of liquid, with some analogies, according to the liquid drop model. When a thermal neutron collides with the nucleus, a compound nucleus with a certain amount of exciting energy is produced as a result of the extra neutron. This energy induces rapid oscillations in the compound liquid drop nucleus. As a consequence, the nucleus assumes the shape of a dumbbell and breaks into two fragments due to positive charge repulsion on the two ends, forming two daughter nuclei. This is the phenomenon of nuclear fission, which is well explained by the liquid drop model.

3. What are the assumptions made in the liquid drop model?

The liquid-drop model is a model describing the atomic nuclei in nuclear physics. The molecules in a drop of liquid behave like nucleons, that is, neutrons and protons. The model is designed on the basis of certain assumptions. There are four major assumptions made while designing the liquid-drop model. The assumptions made were that the nucleus consists of incompressible matter, for every nucleon the nuclear force is identical, the nuclear force saturates and the nuclei of the atom remain spherically symmetrical in a state of equilibrium under the action of strong attractive nuclear forces.

4. Why is the liquid-drop model compared to the atomic nucleus?

The nucleons in the atomic nucleus and the molecules in a drop of liquid resemble each other and the idea was accepted due to the following similarities between them.

  • The surface tension force of a liquid is similar to the nuclear force.

  • The molecules in a liquid and nucleons behave in a similar manner.

  • The density of nuclear matter is independent of A just like a drop of liquid.

  • The latent heat of vaporization of the liquid and the constant binding energy per nucleon are analogous to each other.

  • The phenomenon of evaporation of molecules from the surface of the liquid and the disintegration of nuclei due to the emission of particles are similar to each other.

  • The internal thermal vibrations of drop molecules correspond to the energy of nuclei.

  • The phenomenon of condensation of drops corresponds to the absorption of bombarding particles by a nucleus.

5. Does the liquid-drop model have any failures?

Yes, there are certain failures associated with the liquid-drop model. These failures are:

  • The liquid-drop model could not explain the extra stability of certain nuclei specifically in the nucleus where the number of protons or neutrons were magic numbers, that is, 2, 8, 20, 28, 50, 82, or 126.

  • The model also could not explain the measured magnetic moments of many nuclei.

  • The model also could not explain the spin of nuclei.

  • The model was also not successful in giving a proper explanation for the excited states in most of the nuclei.

  • The model was also not successful in explaining the low lying excited states of the nucleus.

6. Explain the Liquid Drop Model of the Nucleus?

The liquid drop model of the nucleus explains forces in atomic nuclei as if they were created by a tiny liquid drop in nuclear physics. On a nuclear scale, however, the fluid is made up of nucleons (protons and neutrons). The liquid drop model accounts for the fact that the forces acting on nucleons on the surface vary from those acting on nucleons on the inside, where they are absolutely surrounded by attracting nucleons. This is analogous to considering surface tension as a factor in calculating the energy of a tiny liquid drop.

7. How Does the Liquid Drop Model Account For Nuclear Fission?

The nucleus is thought to be like a drop of liquid, with some analogies, according to the liquid drop model. When a thermal neutron collides with the nucleus, a compound nucleus with a certain amount of exciting energy is produced as a result of the extra neutron. This energy induces rapid oscillations in the compound liquid drop nucleus. As a consequence, the nucleus assumes the shape of a dumbbell and breaks into two fragments due to positive charge repulsion on the two ends, forming two daughter nuclei. This is the phenomenon of nuclear fission, which is well explained by the liquid drop model.

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