Physical Science 8th grade (1).pdf

The proton and the nucleus discovered in 1911 ernest

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The proton and the nucleus discovered In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did a clever experiment to test Thomson’s model of the atom. They launched positively-charged helium ions (a charged atom is called an ion ) at extremely thin gold foil (Figure 6.3). They expected the helium ions to be deflected a small amount as they passed through the foil. However, a few bounced back in the direction they came! The unexpected result prompted Rutherford to remark “ it was as if you fired a five inch (artillery) shell at a piece of tissue paper and it came back and hit you! The nuclear model of the atom The best way to explain the pass-through result was if the gold atoms were mostly empty space, allowing most of the helium ions to go through virtually undeflected. The best way to explain the bounce- back result was if nearly all the mass of a gold atom were concentrated in a tiny, hard core at the center. Further experiments confirmed Rutherford’s ideas and we know that every atom has a tiny nucleus , which contains more than 99% of the atom’s mass. The neutron The positively charged proton was soon discovered and shown to be the particle in the nucleus. But there still was a serious problem with the atomic model. Protons could only account for about half the observed mass. This problem was solved in 1932 by James Chadwick. Chadwick’s experiments revealed another particle in the nucleus which has no electric charge and similar mass as the proton. Chadwicks neutral particle was named the neutron .
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121 6.1 F UNDAMENTAL P ARTICLES AND F ORCES C HAPTER 6: T HE A TOM Three subatomic particles make up an atom Protons, neutrons, and electrons Today we know that atoms are made of three tiny subatomic particles: protons, neutrons, and electrons. Protons have positive charge. Electrons have negative charge. Neutrons add mass but have zero charge. The charge on a proton (+e) and an electron (-e) are exactly equal and opposite. Atoms that have the same number of protons and electrons have a total charge of precisely zero. The nucleus The protons and neutrons are grouped together in the nucleus, which is at the center of the atom. The mass of the nucleus determines the mass of an atom because protons and neutrons are much larger and more massive than electrons (Figure 6.4). In fact, a proton is 1,836 times heavier than an electron. All atoms have both protons and neutrons in their nuclei except the simplest type of hydrogen, which only has one proton and no neutrons. The chart below compares electrons, protons, and neutrons in terms of charge and mass. Electrons define the volume of an atom Electrons take up the region outside the nucleus in a region called the electron cloud . The diameter of an atom is really the diameter of the electron cloud (Figure 6.5). Compared to the tiny nucleus, the electron cloud is enormous, more than 10,000 times larger than the nucleus. As a comparison, if an atom were the size of a football stadium, the nucleus would be the size of a pea, and the electrons would be
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