The plum pudding model is an obsolete scientific model of the atom. It was first proposed by J. J. Thomson in 1904[1] following his discovery of the electron in 1897 and subsequently rendered obsolete by Ernest Rutherford's discovery of the atomic nucleus in 1911. The model tried to account for two properties of atoms then known: that there are electrons and that atoms have no net electric charge. Logically there had to be a commensurate quantity of positive charge to balance out the negative charge of the electrons, but having no clue as to the source of this positive charge, Thomson tentatively proposed it was everywhere in the atom, the atom being in the shape of a sphere. Following from this, Thomson imagined that the balance of electrostatic forces in the atom would distribute the electrons in a more or less even manner throughout this hypothetical sphere.[2]
Thomson was not able to develop a complete model that could predict any other known properties of the atom such as emission spectra or valencies. In modern textbooks the plum pudding model is only briefly mentioned to explain how the atomic nucleus was discovered.
Thomson's model is popularly referred to as the "plum pudding model" with the notion that the electrons are distributed with similar density as raisins in a plum pudding. Neither Thomson nor his colleagues ever used this analogy.[3] It seems to have been a conceit of popular science writers to make the model accessible to the layman. The analogy is perhaps misleading because Thomson likened the sphere to a liquid rather than a solid since he thought the electrons moved around in it.[4]
It had been known for many years that atoms contain negatively charged subatomic particles. Thomson called them "corpuscles" (particles), but they were more commonly called "electrons", the name G. J. Stoney had coined for the "fundamental unit quantity of electricity" in 1891.[5] 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 their electrons.[6][7] Thomson published his proposed model in the March 1904 edition of the Philosophical Magazine, 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, ...[8]
Thomson's model was the first to assign a specific inner structure to an atom, though his original description did not include mathematical formulas.[3][9] He had followed the work of William Thomson (later Lord Kelvin) who had written a paper proposing a vortex atom in 1867,[10] 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.[11] 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.[12][13] Thomson's proposal, based on Kelvin's model of a positive volume charge, served 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.[8] 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). 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 charge.[14] Thomson attempted unsuccessfully to reshape his model to account for some of the major spectral lines experimentally known for several elements.[15] 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 ...[16]
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 rather than a static structure. Despite this, 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 raisins, then called "plums", in the common English dessert, plum pudding.[3]
In a paper titled Cathode Rays, Thomson demonstrated that cathode rays are not light but made of negatively charged particles which he called corpuscles. He observed that cathode rays can be deflected by electric and magnetic fields, which does not happen with light rays. Thomson believed that atoms were made of these corpuscles, calling them primordial atoms. Thomson believed that the intense electric field around the cathode caused the surrounding gas molecules to split up into their component corpuscles, thereby generating cathode rays. Thomson thus showed evidence that atoms were in fact divisible, though he did not attempt to describe their structure at this point.
Thomson notes that he was not the first scientist to propose that atoms are actually divisible, making reference to William Prout who in 1815 noted that the atomic weights of various elements were multiples of hydrogen's atomic weight and hypothesized that all atoms were hydrogen atoms fused together. Prout's hypothesis was dismissed when it was found that some elements seemed to have a non-integer atomic weight, e.g. chlorine has an atomic weight of about 35.5, and such discrepancies wouldn't be explained until the discovery of isotopes and nuclear structure in the early 20th century.
In 1898, Thomson measured the positive charge on hydrogen ions to be roughly 6 × 10-10 electrostatic units (2 × 10-19 coulombs) and wrote that this was equal to an electron's negative charge.[17] In 1899, he showed that negative electricity created by ultraviolet light landing on a metal (known now as the photoelectric effect) has the same mass-to-charge ratio as cathode rays; then he applied his previous method for determining the charge on ions to the negative electric particles created by ultraviolet light.[18]: 86 By this combination he estimated that the electron's mass was 0.0014 times that of hydrogen ions (as a fraction: 1⁄714).[19]
Thomson provided his first description of the atom in his 1904 paper On the Structure of the Atom. Thomson believed that all the mass of the atom was carried by the electrons. This would mean that even a small atom would have to contain thousands of electrons, and the positive electrification the encapsulated them was without mass.[20]
In a lecture delivered to the Royal Institution of Great Britain in 1905,[21] Thomson stated that calculating the arrangements of thousands of electrons in a sphere of positive charge was too computationally difficult for him. However, he proposed a practical experiment to shed some light on their nature. It involved magnetized pins pushed into cork disks and set afloat in a basin of water. The magnetized pins were oriented such that they repelled each other. Above the center of the pool was suspended an electromagnet that attracted the pins towards the center. The equilibrium arrangement the pins took informed Thomson on what arrangements the electrons in an atom might take and he provided a brief table.
For instance, he observed that while five pins would arrange themselves in a stable pentagon around the center, six pins could not form a stable hexagon. Instead, one pin would move to the center and the other five would form a pentagon around the center pin, and this arrangement was stable. As he added more pins, they would arrange themselves in concentric rings around the center.
From this, Thomson believed the electrons arranged themselves in concentric shells, and the electrons could move about within these shells but did not move out of them unless electrons were added or subtracted from the atom.
In 1907, Thomson published The Corpuscular Theory of Matter which further developed his ideas on the atom's structure and proposed further avenues of research.
In Chapter 6, he further elaborates his experiment using magnetized pins in water, providing an expanded table. For instance, if 59 pins were placed in the pool, they would arrange themselves in concentric rings of the order 20-16-13-8-2 (from outermost to innermost).
In Chapter 7, Thomson proposes a number of methods by which the number of electrons in an atom could be estimated, such as X-ray emission spectra, cathode ray absorption, or optical properties. Experiments by other scientists in this field had shown that atoms contain far fewer electrons than Thomson previously thought. Thomson now believed the number of electrons in an atom was a small multiple of its atomic weight,[22] perhaps 1 to 3 times.[23]
But this would mean that most of the atom's mass was not carried by its electrons. An electron by his estimate has only 1⁄1700 the mass of a hydrogen atom (the correct figure is 1⁄1837). Thomson concluded that the rest of the atom's mass was carried by the positive electrification.[24] Thomson still did not know what substance constituted the positive electrification, though he noted that no scientist had yet found a positively-charged particle smaller than a hydrogen ion.[25]
Main article: Geiger-Marsden experiments |
Between 1908 and 1913, Ernest Rutherford, Hans Geiger, and Ernest Marsden collaborated on a series of experiments in which they bombarded metal foils with a beam of alpha particles so as to study how the foils scattered said beam. Gold was their preferred material because gold is very malleable and can therefore be made into an especially thin foil. The found that the gold foil could scatter alpha particles by more than 90 degrees. This should not have been possible according to the Thomson model. The scattering should have been negligible. The positive charge in the Thomson model is too diffuse to produce an electric field of sufficient strength to cause such scattering. Rutherford deduced that the positive charge of the atom, along with most of the atom's mass, was concentrated in a tiny nucleus at the center of the atom. Only such an intense concentration of charge, anchored by its high mass, could have scattered the alpha particle beam so dramatically.
Rutherford went on to show that the number of electrons in an atom is equal to the element's atomic number, and that the hydrogen atom is a singular particle which he called "the proton" and constitutes the positive charge of all atoms. It was later found that the protons carry only roughly half the mass of the atom, the rest being neutrons which have no charge.
As an important example of a scientific model, the plum pudding model has motivated and guided several related scientific problems.
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.[26][27]
The first known writer to compare Thomson's model to a plum pudding was an anonymous reporter who wrote an article for the British pharmaceutical magazine The Chemist and Druggist in August 1906.
While the negative electricity is concentrated on the extremely small corpuscle, the positive electricity is distributed throughout a considerable volume. An atom would thus consist of minute specks, the negative corpuscles, swimming about in a sphere of positive electrification, like raisins in a parsimonious plum-pudding, units of negative electricity being attracted toward the centre, while at the same time repelling each other.[28]
The analogy was never used by Thomson nor his colleagues. It seems to have been a conceit of popular science writers to make the model easier to understand for the layman.[3]