To explain the overall charge of the atom, which consisted of both positive and negative charges, Thompson proposed a model whereby the negatively charged “corpuscles” were distributed in a uniform sea of positive charge – known as the Plum Pudding Model. This effectively disproved the notion that the hydrogen atom was the smallest unit of matter, and Thompson went further to suggest that atoms were divisible. The Plum Pudding model of the atom proposed by J.J. He concluded that rather than being composed of light, they were made up of negatively charged particles that were 1ooo times smaller and 1800 times lighter than hydrogen. Through a series of experiments using cathode ray tubes (known as the Crookes’ Tube), Thomson observed that cathode rays could be deflected by electric and magnetic fields. By the end of the 19th century, this would change drastically, thanks to research conducted by scientists like Sir Joseph John Thomson. However, most scientists ventured that this unit would be the size of the smallest known atom – hydrogen. Discovery of the Electron:īy the late 19th century, scientists also began to theorize that the atom was made up of more than one fundamental unit. This theory expanded on the laws of conversation of mass and definite proportions and came down to five premises: elements, in their purest state, consist of particles called atoms atoms of a specific element are all the same, down to the very last atom atoms of different elements can be told apart by their atomic weights atoms of elements unite to form chemical compounds atoms can neither be created or destroyed in chemical reaction, only the grouping ever changes. Through a series of experiments involving gases, Dalton went on to develop what is known as Dalton’s Atomic Theory. For example, in the early 1800’s, English scientist John Dalton used the concept of the atom to explain why chemical elements reacted in certain observable and predictable ways. It was not until the 19th century that the theory of atoms became articulated as a scientific matter, with the first evidence-based experiments being conducted. Various atoms and molecules as depicted in John Dalton’s A New System of Chemical Philosophy (1808). However, this theory was more of a philosophical concept than a scientific one. The term “atom” was coined in ancient Greece and gave rise to the school of thought known as “atomism”. The earliest known examples of atomic theory come from ancient Greece and India, where philosophers such as Democritus postulated that all matter was composed of tiny, indivisible and indestructible units. Proposed by Danish physicist Niels Bohr in 1913, this model depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits (defined by their energy levels) around the center. Beginning in the 5th century BCE with Democritus‘ theory of indivisible “corpuscles” that interact with each other mechanically, then moving onto Dalton’s atomic model in the 18th century, and then maturing in the 20th century with the discovery of subatomic particles and quantum theory, the journey of discovery has been long and winding.Īrguably, one of the most important milestones along the way has been Bohr’ atomic model, which is sometimes referred to as the Rutherford-Bohr atomic model. However, the idea of quantized angular momentum has been retained.Atomic theory has come a long way over the past few thousand years. Modern atomic theory does not allow subatomic particles to be treated in the same way as large objects, and Bohr's reasoning is somewhat discredited. The wavelength of a particle is given by h/ mv, so nh/ mv = 2π r, which leads to mvr = nh/2π. nλ = 2π r, where λ is the wavelength and n a whole number. Each orbit has to have a whole number of wavelengths around it i.e. The idea of quantized values of angular momentum was later explained by the wave nature of the electron. This theory gave good results in predicting the lines observed in the spectrum of hydrogen and simple ions such as He +, Li 2+, etc. Each permitted value of n is associated with an orbit of different radius and Bohr assumed that when the atom emitted or absorbed radiation of frequency ν, the electron jumped from one orbit to another the energy emitted or absorbed by each jump is equal to hν. can only have certain values, each of which is a multiple of n. This means that the angular momentum is quantized, i.e. Bohr proposed that electrons could only occupy orbits in which this angular momentum had certain fixed values, h/2π, 2 h/2π, 3 h/2π,… nh/2π, where h is the Planck constant. The angular momentum of the electron would then be mvr. He assumed that a single electron of mass m travelled in a circular orbit of radius r, at a velocity v, around a positively charged nucleus. The theory published in 1913 by Niels Bohr to explain the line spectrum of hydrogen.
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