Electrons,Protons and Neutrons
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Electrons,Protons and Neutrons

Electrons, Protons and Neutrons

Most of Dalton's atomic Theory is accepted today. One important revision is that atoms are not indivisible. Atoms can be broken down into even smaller, more fundamental particles.Dozens of these subatomic particles are unleashed when powerful atom smashers fracture atoms. In fact no single theory of atomic structure account for all of the subatomic particles: electrons, protons and neutrons.

Electrons are part of the atoms of all elements. Electrons are negatively charged particles. The English physicist Sir J.J Thomson discovered electrons in 1897. Thomson performed experiments involving the flow of electric current through gases. He sealed the gases in the glass tubes with metal disks called electrodes at each end.


 The electrodes were connected to a high voltage source of electricity. One electrode, the anode, became positively charged. The other electrode, the cathode, became negatively charged. A glowing beam formed between the electrodes. The glowing beam, which travels from the cathode to anode,is a cathode ray.

Thomson found that cathode rays were attracted to metal plates that carry a positive electrical charge. The rays repelled by plates that carry a negative electrical charge. The figure above shows a deflection of the cathode rays.In electricity, opposite charges attract and like charges repel. Therefore Thompson proposed that a cathode ray is a steam of very small negatively charged particles, all alike, moving at high speed.He called these particles electrons. Moreover, Thomson showed that cathode rays are always composed of electrons, regardless of the kind of gas in the cathode ray tube or the kind of metal in the electrodes. Thomson concluded that electrons must be a part of the atom of all elements. By 1900, Thomson and others had figured out that the electron carries roughly one unit of negative charge and that its mass is about 1/2000 the mass of a hydrogen atom.

Clever experiments enabled the American scientist Robert A. Milikan to improve earlier estimates of the charge on an electron. Because he had accurate values of both the charge and the ratio of the charge to mass of an electron, Milikan could calculate an accurate values for the mass of the electron. Milikan's values of charge and mass, reported in 1916, are very similar to those accepted today. An electron carries exactly one unit of negative charge and its mass is 1/1840 the mass of a hydrogen atom.

The most common form of the hydrogen atom is the lightest atom that exist. If an electron is only 1/1840 the mass of a common hydrogen atom, what is left over when one of these atoms loses an electron? You can think through this problem with four simple ideas about matter and electric charge. First, atoms have no electric charge; they are electrically neutral. The evidence of electrical neutrality is that you do not receive an electric shock every time you touch an object. Second, electric charges are properties of particles of matter. That is, electric charges are carried by particles of matter. Third electric charges exist in a single unit or in a multiples of single unit. There are no fractions charges. Fourth, electric charges cancel when equal numbers of negatively charged and positively charged particles combine to form an electrically neutral particle.Thus, because an electron carries one unit of negative charge, there should be a particle with one unit of positive charge left over when a common hydrogen atom loses an electron. This positively charge subatomic particles is called a proton. In 1886, E. Goldstein, using a cathode ray tube in which the cathode had holes, observed rays travelling in the opposite direction to the cathode ray. These rays, contain positively charged particles and are called canal rays.

In 1932, The English physicist Sir James Chadwick confirmed the existence of yet another subatomic particle: the neutron. Neutrons are subatomic particles with no charge, but their mass nearly equal that of the proton. Thus the fundamental building blocks of atoms are the electron,proton, and the neutron. 

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