In the two centuries since Dalton developed his ideas, scientists have made significant progress in furthering our understanding of atomic theory. These concepts form the basis of chemistry. Atoms combine in whole-number ratios to form compounds.
Atoms of the same element are the same atoms of different elements are different.The concept that atoms play a fundamental role in chemistry is formalized by the modern atomic theory, first stated by John Dalton, an English scientist, in 1808. Atoms are so small that it is difficult to believe that all matter is made from atoms-but it is. The period at the end of a printed sentence has several million atoms in it. It would take about fifty million atoms in a row to make a line that is 1 cm long. The smallest piece of an element that maintains the identity of that element is called an atom. Write isotopic symbols for elements and ions.Define isotopes and give examples for several elements.Describe the three subatomic particles that compose atoms.Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford.Outline milestones in the development of modern atomic theory.(x) Cathode rays can be deflected by the electric and magnetic fields.By the end of this section, you will be able to: (ix) Cathode rays can penetrate the thin foils of metals. (viii) Cathode rays ionize the gas through which they pass. (vii) When the cathode rays strikes the heavy metal, the X rays are produced. (vi) Cathode rays affect the photographic plates. (v) When cathode rays fall on certain substances they cause fluorescence. (iv) Cathode rays carry a large amount of energy and produce heat energy when they strike an obstacle. (iii) Cathode rays exert mechanical pressure on the object they strike. (ii) Cathode rays travel in straight lines and cast sharp shadows of the obstacles placed in their path. Their direction is independent of the position of the anode. (i) The cathode rays are emitted normally from the surface of the cathode. The main properties of the cathode rays are as follows: Properties of cathode rays by J.J.Thomson cathode ray experiment This something is a stream of fast moving electrons and are called cathode rays. This shows that something is coming out from the cathode, travelling through the length of the tube and falling on the walls. This expansion of the dark space is accompanied by fluorescence of the glass walls of the discharge tube. When a high potential difference is maintained across the electrodes of the discharge tube at a pressure about 0.01 mm of mercury, the Crookes dark space fills the whole length of the tube. Anode is connected to the positive terminal and the cathode is connected to the negative terminal of the high voltage supply source. A very high potential is applied across anode and cathode by using the induction coil. A side tube P is used to pump out the enclosed gas so as to obtain the desired low pressure. It is fitted with two metal electrodes named as anode A and cathode C. It is a closed tube of length about 30 cm and diameter of about 4 cm. J.J.Thomson used a glass tube called as discharge tube. J.J.Thomson used the experimental results for defining the various properties of electrons, cathode rays. Thomson had performed discharge tube experiment in a laboratory by subjecting the electric and the magnetic fields on the cathode rays. Introduction to J.J.Thomson cathode ray experiment
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