The plum pudding model is one of several historical

scientific models Scientific modelling is a scientific activity, the aim of which is to make a particular part or feature of the world easier to understand, define, quantify, visualize, or simulate by referencing it to existing and usually commonly accepted ...

of the

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

. First proposed by

J. J. Thomson Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered. In 1897, Thomson showed that ...

in 1904 soon after the discovery of the

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

, but before the discovery of the

atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron ...

, the model tried to explain two properties of atoms then known: that electrons are negatively charged particles and that atoms have no net electric charge. The plum pudding model has electrons surrounded by a volume of positive charge, like negatively charged "plums" embedded in a positively charged " pudding".

Overview

It had been known for many years that atoms contain negatively charged subatomic particles. Thomson called them "corpuscles" (

particle In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from ...

s), but they were more commonly called "electrons", the name G. J. Stoney had coined for the " fundamental unit quantity of electricity" in 1891. It had also been known for many years that atoms have no net electric charge. Thomson held that atoms must also contain some positive charge that cancels out the negative charge of its electrons. Thomson published his proposed model in the March 1904 edition of the ''

Philosophical Magazine The ''Philosophical Magazine'' is one of the oldest scientific journals published in English. It was established by Alexander Tilloch in 1798;John Burnett"Tilloch, Alexander (1759–1825)" Oxford Dictionary of National Biography, Oxford Univer ...

'', the leading British science journal of the day. In Thomson's view:

... the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, ...

Thomson's model was the first to assign a specific inner structure to an atom, the only downside being that it contained many assumptions and didn't have any mathematical evidence to back it up.J.J. Thomson had followed the work of William Thomson who had written a paper proposing a vortex atom in 1867. J.J. Thomson abandoned his 1890 "nebular atom" hypothesis, based on the vortex theory of the atom, in which atoms were composed of immaterial vortices and suggested there were similarities between the arrangement of vortices and periodic regularity found among the chemical elements. Being an astute and practical scientist, Thomson based his atomic model on known experimental evidence of the day, and in fact, followed Lord Kelvin's lead again as Kelvin had proposed a positive sphere atom a year earlier. Thomson's proposal, based on Kelvin's model of a positive volume charge, reflects the nature of his scientific approach to discovery which was to propose ideas to guide future experiments. The main objective of Thomson's model after its initial publication was to account for the electrically neutral and chemically varied state of the atom. Electron orbits were stable under classical mechanics. When an electron moves away from the center of the positively charged sphere it is subjected to a greater net positive inward force due to the presence of more positive charge inside its orbit (see

Gauss's law In physics and electromagnetism, Gauss's law, also known as Gauss's flux theorem, (or sometimes simply called Gauss's theorem) is a law relating the distribution of electric charge to the resulting electric field. In its integral form, it sta ...

). Electrons were free to rotate in rings that were further stabilized by interactions among the electrons, and spectroscopic measurements were meant to account for energy differences associated with different electron rings. As for the properties of matter, Thomson believed they arose from electrical effects. He further emphasized the need of a theory to help picture the physical and chemical aspects of an atom using the theory of corpuscles and positive electricity(

Positive charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

). Thomson attempted unsuccessfully to reshape his model to account for some of the major

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s experimentally known for several elements. After the scientific discovery of radioactivity, Thomson decided to address it in his model by stating: ...we must face the problem of the constitution of the atom, and see if we can imagine a model which has in it the potentiality of explaining the remarkable properties shown by radio-active substances... As early as 1897, theoretical physicist,

Joseph Larmor Sir Joseph Larmor (11 July 1857 – 19 May 1942) was an Irish and British physicist and mathematician who made breakthroughs in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter. His most influent ...

had explained the splitting of the

s in a magnetic field by the

oscillation Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

of electrons. According to a centennial celebration of the Bohr atom in Nature magazine, it was

John William Nicholson John William Nicholson, FRS (1 November 1881 – 3 October 1955) was an English mathematician and physicist. Nicholson is noted as the first to create an atomic model that quantized angular momentum as h/2π. Nicholson was also the first to crea ...

in 1912 who first discovered that electrons radiate the spectral lines as they descend towards the nucleus and his theory was both nuclear and quantum. Thomson's model changed over the course of its initial publication, finally becoming a model with much more mobility containing electrons revolving in the dense field of positive charge(Positive Electricity as called by Thomson) rather than the static "Plum-pudding" structure. Plum_Pudding

The plum pudding model usefully guided his student,

Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand physicist who came to be known as the father of nuclear physics. ''Encyclopædia Britannica'' considers him to be the greatest ...

, to devise experiments to further explore the composition of atoms. Also, Thomson's model was an improvement upon previous solar system models of

and the Saturnian ring model for atomic electrons put forward in 1904 by Nagaoka (after

James Clerk Maxwell James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematician and scientist responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism and li ...

's model of Saturn's rings) as these could not withstand classical mechanics as in the solar system models the electrons would spiral into the nucleus, so they were dropped in favor of Thompson's model. However, all previous atomic models were useful as predecessors of the more correct 1913 solar-system-like

Bohr model In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar Syst ...

of the atom which Bohr references in his paper that he borrowed substantially from the 1912 nuclear model of

whose quantum atomic model quantized angular momentum as ''h''/2. The

Bohr Model In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar Syst ...

was initially planar as the Nagaoka model, but Sommerfeld introduced elliptical orbits in the years 1914-1925 until the theory was overthrown by modern

quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistr ...

. The colloquial nickname "plum pudding" was soon attributed to Thomson's model as the distribution of electrons within its positively charged region of space reminded many scientists of

raisin A raisin is a dried grape. Raisins are produced in many regions of the world and may be eaten raw or used in cooking, baking, and brewing. In the United Kingdom, Ireland, New Zealand, and Australia, the word ''raisin'' is reserved for the ...

s, then called "plums", in the common English dessert, plum pudding. In 1909,

Hans Geiger Johannes Wilhelm "Hans" Geiger (; ; 30 September 1882 – 24 September 1945) was a German physicist. He is best known as the co-inventor of the detector component of the Geiger counter and for the Geiger–Marsden experiment which discover ...

and

Ernest Marsden Sir Ernest Marsden (19 February 1889 – 15 December 1970) was an English-New Zealand physicist. He is recognised internationally for his contributions to science while working under Ernest Rutherford, which led to the discovery of new theories ...

conducted experiments with thin sheets of gold. Their professor, Ernest Rutherford, expected to find results consistent with Thomson's atomic model. It was not until 1911 that Rutherford correctly interpreted the experiment's results which implied the presence of a very small nucleus of positive charge at the center of each gold atom. This led to the development of the

Rutherford model The Rutherford model was devised by the New Zealand-born physicist Ernest Rutherford to describe an atom. Rutherford directed the Geiger–Marsden experiment in 1909, which suggested, upon Rutherford's 1911 analysis, that J. J. Thomson's plum ...

of the atom. Immediately after Rutherford published his results, Antonius Van den Broek made the intuitive proposal that the

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...

of an atom is the total number of units of charge present in its nucleus. Henry Moseley's 1913 experiments (see

Moseley's law Moseley's law is an empirical law concerning the characteristic x-rays emitted by atoms. The law had been discovered and published by the English physicist Henry Moseley in 1913-1914. Until Moseley's work, "atomic number" was merely an element's ...

) provided the necessary evidence to support Van den Broek's proposal. The effective nuclear charge was found to be consistent with the atomic number (Moseley found only one unit of charge difference). This work culminated in the solar-system-like (but quantum-limited) Bohr model of the atom in the same year, in which a nucleus containing an atomic number of positive charges is surrounded by an equal number of electrons in orbital shells. As Thomson's model guided Rutherford's experiments, Bohr's model guided Moseley's research.

Related scientific problems

As an important example of a

scientific model Scientific modelling is a scientific activity, the aim of which is to make a particular part or feature of the world easier to understand, define, quantify, visualize, or simulate by referencing it to existing and usually commonly accepted ...

, the plum pudding model has motivated and guided several related scientific problems.

Atomic size and scientific constants

The plum pudding model with a single electron was used in part by the physicist Arthur Erich Haas in 1910 to estimate the numerical value of Planck's constant and the

Bohr radius The Bohr radius (''a''0) is a physical constant, approximately equal to the most probable distance between the nucleus and the electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an ...

of hydrogen atoms. Haas' work was the first to estimate these values to within an order of magnitude and preceded the work of

Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922 ...

by three years. Of note, the Bohr model provides reasonable predictions only for atomic and

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

ic systems with just one effective electron.

Mathematical Thomson problem

A particularly useful mathematics problem related to the plum pudding model is the optimal distribution of equal point charges on a unit sphere, called the Thomson problem. The Thomson problem is a natural consequence of the plum pudding model in the absence of its uniform positive background charge.

Spherical quantum dots

The classical electrostatic treatment of electrons confined to spherical quantum dots is also similar to their treatment in the plum pudding model. In this classical problem, the quantum dot is modeled as a simple neutrally-charged

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

sphere (in place of a uniform, positively charged sphere of the plum pudding model) in which free, or excess, electrons reside. The electrostatic ''N''-electron configurations are found to be exceptionally close to solutions found in the Thomson problem with electrons residing at the same radius within the dielectric sphere. Notably, the plotted distribution of geometry-dependent energetics has been shown to bear remarkable resemblance to the distribution of anticipated electron orbitals in natural atoms as arranged on the periodic table of elements. Of great interest, solutions of the Thomson problem exhibit this corresponding energy distribution by comparing the energy of each ''N''-electron solution with the energy of its neighboring (''N''-1)-electron solution with one charge at the origin. However, when treated within a dielectric sphere model, the features of the distribution are much more pronounced with respect to electron orbital arrangements in real atoms as shown in the figure. Additional information may be accessed online as a large printable poster.

References

{{Atomic models Foundational quantum physics Atoms Electron Periodic table Obsolete theories in physics