The Structure of the Atom Section 3.2. Introduction Atom: the smallest particle of an element that...
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Transcript of The Structure of the Atom Section 3.2. Introduction Atom: the smallest particle of an element that...
![Page 1: The Structure of the Atom Section 3.2. Introduction Atom: the smallest particle of an element that retains the chemical properties of that element Nucleus:](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f2f5503460f94c48f8a/html5/thumbnails/1.jpg)
The Structure of the Atom
Section 3.2
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Introduction• Atom: the smallest particle of an
element that retains the chemical properties of that element
• Nucleus: very small region located at the center; composed of proton(s) and usually one or more neutrons
• Proton: a positively charged particle in the nucleus
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• Neutron: a neutral particle found in the nucleus
• Electron: a negatively charged particle found in a region surrounding the nucleus
• Subatomic particles: protons, neutrons, and electrons
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Discovery of the Electron
• A glass tube was prepared with a gas in it with metal disks at each end
• The cathode ray tube was connected to a vacuum pump because the charges will only flow through gases at low pressure
• An electric current was passed through various gases
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• When the current was passed through the gas, the surface of the tube opposite the cathode (negative charge) glowed
• Also, the ray of light traveled from the cathode to the anode
• The rays were deflected away from a negatively charged object
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• In 1897, J.J. Thomson used the cathode ray tube to deduce the presence of a negatively charged particle: the electron
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Charge and Mass of the Electron
• Thomson’s experiment revealed that the electron has a very large charge-to-mass ratio
• In 1909, Robert Millikan measured the charge of the electron
• More accurate experiments were conducted later to determine the mass
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Mass of the Electron
The oil drop apparatus
Mass of the electron is 9.109 x 10-31kg
Robert Millikan
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Thomson’s Model of the Atom
• Thomson’s model is called the plum pudding model
• His model is not valid• He believed that the negative electrons
were spread evenly throughout the positive charge of the rest of the atom
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J. J. Thomson and his Model
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Ernest Rutherford’s Gold Foil Experiment
• Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil
• Go
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Rutherford’s Findings
• Most of the particles passed right through• A few particles were deflected• Some were greatly deflected
“Like howitzer shells bouncing off of tissue paper
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Conclusions of the Experiment
–The nucleus is small–The nucleus is dense–The nucleus is positively charged–The atom is mostly empty space
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Rutherford’s Model–All the positive charge, and almost all the
mass is concentrated in a small area in the center. He called this a nucleus–The nucleus is composed of protons and
neutrons (they make the nucleus!)–The electrons are distributed around the
nucleus, and occupy most of the volume–His model was called a “nuclear model”
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Other Discoveries–Eugene Goldstein in 1886 observed what
is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1836 times that of an electron)–1932 – James Chadwick confirmed the
existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton
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Subatomic ParticlesParticle Relative
ChargeRelative
massLocation Actual mass
in kg
Electron (e-)
-1 __1__1836
electron cloud
9.109 x 10-31
Neutron (n0)
0 1 nucleus 1.675 x 10-27
Proton (p+)
+1 1 nucleus 1.673 x 10-27
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Other Interesting Stuff
• Atoms are electrically neutral because they contain an equal number of protons and electrons
• The number of protons determines the atom’s identity
• There are other subatomic particles, but they have little effect on the chemical properties of matter
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Forces in the Nucleus
• Protons, with their positive charge, would be expected to repel each other in the nucleus
• When they are extremely close to each other, there is a strong attraction
• A similar attraction exists when neutrons are present
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• Nuclear forces: the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus
• Nuclei are very dense
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Sizes of Atoms
• Electrons occupy the outer region known as the electron cloud
• Radius of an atom is the distance from the center of the nucleus to the outer portion of the electron cloud
• Atomic radii are extremely small so they are usually measured in picometers
• 1 x 1012 pm = 1 m