Structure of an atom Early atomic models and way to discovery of an atom
Greetings! I am your educator - Udit Gupta. In the first lesson, we start with the onset of atomic models given by various scientists and their approach to define the most basic structure of an element. This theory forms an integral part to the later lessons in this course.
Early atomic models It is an old concept that all matter is made of very small indivisible particles. After a long time, intelligent reasoning and classic experiments we have covered the journey from this idea to the pr sent day atomic models. Our discussion starts with the mention of scientist Robert Boyle who studied the expansion and compression of air. . The fact that air can be compressed or expanded, tells that air is made of tiny particles. When air is compr ssed, these particles get closer to each other, reducing the empty space. Mentioning Robert Boyle is important, because, with him atomism entered a new phase, from mere reasoning to experimental observations. The smallest unit of an element, which carries all the properties of the element is called an atom
Thomson's model of atom Experiments on discharge tube, measurement of e/m etc., established the presence of negatively charged electrons in the atoms. It started the search for the structure of the positive charge inside an atom because the matter as a whole is electrically neutra Thomson suggested that the atom is a positively charged solid sphere and electrons are embedded in it in sufficient number so as to make the atom electrically neutral It can be compared to a birthday cake in which cherries are embedded This model was quite attractive as it could explain several observations available at that time including why only negatively charged particles are emitted when a metal is heated and never the positively charged particles. It also explained the formation of ions and ionic compounds.
Lenard's Suggestion Lenard noted that cathode rays could pass through materials of small thickness with almost zero deviation. Had the atoms been solid spheres, most of the electrons in the cathode rays would hit them and be unable to go ahead in the forward direction. Lenard, therefore, suggested that the atom must have a lot of empty space in it He proposed that the atom is made of electrons and similar tiny particles carrying positive charge But then, the question was, why on heating a metal, these tiny positively charged particles were not ejected?
Rutherford's Model of the Atom . Thomson's model and Lenard's model, both had certain Thomson's model made the positive charge immovable by advantages and disadvantages. assuming it to spread over the total volume of the atom. On the other hand, electron were tiny particles and could be ejected on heating a metal. But the almost free passage of cathode rays through an atom was not consistent with Thomson's model.
Rutherford's Model of the Atom . For that, the atom should have a lot of empty space as .So, the positive charge should be in the form of tiny particles suggested by Lenard occupying a very small volume, yet these particles should not be able to come out on heating. It was Ernest Rutherford who solved the problem by doing a series of experiments from 1906 to 1911 on alpha particle scattering We shall discus more on his experiment in the nest lesn.
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