![]() ![]() It should have been impossible for an alpha particle to bounce straight back in the plum pudding model. (Image credit: DKN0049 via Getty Images.) Instead, Rutherford, Geiger and Marsden found that the alpha particles kept getting deflected, sometimes by a small angle, but sometimes scattered by a large angle, and one in every thousand or so alpha particles bounced straight back.ĭiagram of the gold foil experiment showing alpha particles being deflected by the gold foil. If atoms were structured according to the plum pudding model, where it just contained a mixture of positive and negative charges, the alpha particles should just fly straight through. The trio, who were at the University of Manchester, fired what at the time were called alpha particles at a sheet of gold foil. Experiments by Ernest Rutherford, along with his colleagues Hans Geiger of Geiger-counter fame and Ernest Marsden, revealed the truth about atoms. What he envisaged was called the " Plum Pudding Model" with the electrons embedded like plums in the pudding of positive charge. So Thomson, reasonably, thought that there must be something inside the atom with a positive charge that was canceling out the negative charges of the electrons. ![]() However, atoms are electrically neutral, whereas electrons are negatively charged. This discovery was of immense importance because it meant that atoms are not the most fundamental particles of all and that they can be formed of smaller particles. Thomson judged that the electrons must be coming from the atoms within the cathode-ray tube that he was experimenting with. ![]() Thomson discovered electrons, by realizing that cathode rays were streams of particles and not electromagnetic waves. Things got more interesting in 1897, when J. He considered these units to be fundamental, indivisible particles, hence his use of the Greek word "atomos". It has been known that elements are broken down into basic units of a given weight since the early 1800s, in an insight that came from the English scientist John Dalton. ![]() Sometimes these isotopes are stable, but often they are not and eventually decay. The number of neutrons in the nucleus, as you may have gathered from the above example, can however change, and we call atoms of the same element with different numbers of neutrons " isotopes". So, in the above example of lead, its atomic number is 82 and that does not change if it did, it would be a different element (thallium, atomic number 81, or bismuth, atomic number 83). The number of protons in an atom of any given element is always the same. So, for example, lead has 82 protons and between 120 and 132 neutrons in its nucleus, whereas hydrogen, the lightest element, has just one proton and occasionally one or two neutrons. The more protons and neutrons contained in an atomic nucleus, the heavier that element is. ![]()
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