In the Plum Pudding Model, electrons are represented as negative plums embedded in a positive pudding. This name became popular and is still commonly used to refer to the model.
Knowledge can be acquired through acquaintance, such as knowing the color of a tree, or through description when direct acquaintance is not possible. Descriptions of phenomena are called models.
However, descriptive knowledge is based on other descriptions. For example, to understand how colors are created, we must first understand the structure of an atom, which itself can only be comprehended through descriptions, as it is a million times thinner than a strand of human hair. J.J. Thomson’s Plum Pudding Model was one such description of an atom, which was considered the most fundamental building block of matter at that time.
The Discovery of the Electron
When a voltage is applied between a positive and negative electrode in a vacuum chamber, the glass behind the positive electrode starts to glow. This phenomenon is called fluorescence. The rays that travel between the electrodes and create the glow are invisible. Physicists referred to these rays as cathode rays since they originated from the negative electrode.
(Photo Credit: Wikimedia Commons)
Earlier, Dalton proposed that atoms are the fundamental units of matter, indivisible and indestructible. Physicists of that time believed in the existence of ether, a material that filled the universe and served as a medium for the propagation of light. They thought that if waves need a medium to travel, sunlight must travel through the ether. According to this view, space was not empty but filled with ether, and atoms were disturbances in this medium, permanent vortices swirling endlessly.
However, J.J. Thomson was a practical man and well-versed in scientific principles. He was skeptical about the existence of ether due to the lack of evidence. Thus, Thomson decided to search for atoms himself. In his cathode ray experiment, when he introduced a magnet, he gained deeper insights into reality.
Previously, cathode rays were believed to be distinct and immaterial, like light. But when Thomson made them pass through magnets, he observed their deflection towards one side. He also observed their deflection by electric fields. Thomson concluded that the particles constituting cathode rays were charged, with a negative charge based on the deflection.
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Calculations revealed that the particle was incredibly tiny, at least 1000 times smaller than an atom. Thomson had discovered the electron. However, it was G.J. Stoney who coined the term “electron.” Thomson himself referred to this object as “the negative corpuscle.” As for the ether, it became unnecessary after James Clerk Maxwell successfully demonstrated that light is an electromagnetic wave and does not require a medium to propagate.
The Model of Plum Pudding
The discovery was significant. The electron was more fundamental than anything previously discovered. It was the first of the various tiny objects now known as sub-atomic particles to be found. However, atoms cannot simply contain a negative charge, as they are electrically neutral. Thomson quickly realized that there was a source of positive charge that balanced out the negative charges and made the atom completely neutral.
Joseph John Thomson received the Nobel Prize in 1906 for the discovery of the electron. (Photo Credit: Wikimedia Commons)
In the esteemed March edition of 1904 of the Philosophical Magazine, Thomson proposed a model of the atom in which the atom was a bounded region of positive charge occupied by negative charges. In his own words: “the atoms of the elements consist of a number of negatively charged corpuscles enclosed in a sphere of uniform positive electrification.” The word “uniform” is crucial here. The positivity of the space was equal to the net negative charge produced by the electrons, thus making the atom neutral.
The model was recognized by British physicists as resembling a plum pudding, a dessert beloved by the British. The negative plums represented the electrons embedded in a positive pudding. The name stuck, and the model is still commonly referred to as the Plum Pudding Model. However, it is also sometimes called the Watermelon Model. I’m sure the connotation is clear.
However, it is strange that the name stuck. In Thomson’s model, electrons are not stationary like plums or seeds in their pulp. He proposed that they constantly move, or more accurately, rotate very rapidly. The orbits were stable because, as the electrons moved away from the center of the sphere, they experienced an even greater positive force, as their orbit now included “more” positive charge. The orbits were further stabilized as the electrons interacted with other electrons. The pushing and pulling canceled each other out, allowing the electrons to rotate rapidly in circles. However, these circles turned out to be the model’s flaw.
The Flaw of the Plum Pudding Model
In the early 1850s, physicists discovered that elements, when given energy, such as through heat, emit a distinct pattern of colors. The colors appear discontinuous, but when viewed with a spectroscope, a device that separates colors by their wavelengths, it is observed that the colors are emitted intermittently: the lines of colors are separated by lines of complete darkness. This pattern is called an emission spectrum, and each element produces a unique pattern. In fact, because each element generates a unique spectrum, scientists use these spectra to identify known elements or discover new ones.
The spectrum of hydrogen is determined by comparing the predicted spectrum of an atomic model with the actual spectrum emitted by the element. If the model accurately replicates the spectrum of every element (or at least a significant number), it is considered correct. Thomson proposed that the electrons orbiting the center of the atom were responsible for the spectrum. However, his model failed to predict the spectrum of even hydrogen, which consists of only one electron. Despite his attempts to modify the model, Thomson was unsuccessful in achieving the expected results.
In 1911, Ernest Rutherford interpreted the famous Gold Foil Experiment conducted by Hans Geiger and Ernest Marsden in 1909. Rutherford successfully predicted the existence of a highly dense concentration of positive charge in the center of atoms, known as the nucleus. The atom was found to be mostly empty, with the nucleus being comparable to a baseball in a stadium.
Ironically, George Thomson, the son of J.J. Thomson, demonstrated that electrons behave not only as particles but also as waves. He was the first physicist to experimentally prove the wave nature of electrons. Despite being proven wrong, Thomson was proud of Rutherford’s achievements, as he was Thomson’s most esteemed student.
Both Rutherford and Thomson received Nobel Prizes for their groundbreaking contributions in uncovering the true nature of atoms. However, Rutherford’s success seemed fortunate, as Thomson had initially proposed three different hypotheses regarding the structure of atoms. Thomson ultimately made a choice, and the rest is history.