Geiger and Marsden concluded that the particles had encountered an electrostatic force far greater than that allowed for by Thomson’s model. Diagram detailing the “gold foil experiment” conducted by Hans Geiger and Ernest Marsden. However, they noted instead that while most shot straight through, some of them were scattered in various directions, with some going back in the direction of the source. If Thomson’s model were correct, the alpha particles would pass through the atomic structure of the foil unimpeded.

In this experiment, Geiger and Marsden measured the scattering pattern of the alpha particles with a fluorescent screen. Five years later, the model would be disproved by Hans Geiger and Ernest Marsden, who conducted a series of experiments using alpha particles and gold foil – aka.

For starters, there was the problem of demonstrating that the atom possessed a uniform positive background charge, which came to be known as the “Thomson Problem”. Subsequent experiments revealed a number of scientific problems with the Plum Pudding model. The concept was introduced to the world in the March 1904 edition of the UK’s Philosophical Magazine, to wide acclaim. And from this, the Plum Pudding Model was born, so named because it closely resembled the English desert that consists of plum cake and raisins. These corpuscles would later be named “electrons”, based on the theoretical particle predicted by Anglo-Irish physicist George Johnstone Stoney in 1874. 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. Atomic theory has come a long way over the past few thousand years.

Niels bohr atomic model