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Transcript
Atomic Structure: Basic Concepts The Electron • Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. • In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate. • Thomson’s experiments used a vacuum tube. Atomic Structure: Basic Concepts The Electron • A vacuum tube has had all gases pumped out of it. • At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube. • These electrodes become electrically charged when they are connected to a high-voltage electrical source. Atomic Structure: Basic Concepts Cathode-Ray Tube • When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode. • Because these rays originate at the cathode, they are called cathode rays. Atomic Structure: Basic Concepts Cathode-Ray Tube • Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate. • He knew that objects with like charges repel each other, and objects with unlike charges attract each other. Atomic Structure: Basic Concepts Cathode-Ray Tube • Thomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons. • These electrons had to come from the matter (atoms) of the negative electrode. Atomic Structure: Basic Concepts Cathode-Ray Tube • From Thomson’s experiments, scientists had to conclude that atoms were not just neutral spheres, but somehow were composed of electrically charged particles. • Reason should tell you that there must be a lot more to the atom than electrons. • Matter is not negatively charged, so atoms can’t be negatively charged either. Atomic Structure: Basic Concepts Cathode-Ray Tube • If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles— probably with a much greater mass than electrons. Atomic Structure: Basic Concepts Protons • In 1886, scientists discovered that a cathoderay tube emitted rays not only from the cathode but also from the positively charged anode. • These rays travel in a direction opposite to that of cathode rays. Atomic Structure: Basic Concepts Protons • Like cathode rays, they are deflected by electrical and magnetic fields, but in directions opposite to the way cathode rays are deflected. • Thomson was able to show that these rays had a positive electrical charge. • Years later, scientists determined that the rays were composed of positively charged subatomic particles called protons. Millikan’s Oil Drop Exp. •In 1909, Robert Millikan performed a famous oil drop experiment and determined the size of the charge of an electron and also it’s mass! •What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling Atomic Structure: Basic Concepts Protons • At this point, it seemed that atoms were made up of equal numbers of electrons and protons. • However, in 1910, Thomson discovered that neon consisted of atoms of two different masses. Atomic Structure: Basic Concepts Protons • Atoms of an element that are chemically alike but differ in mass are called isotopes of the element. • Today, chemists know that neon consists of three naturally occurring isotopes. • The third was too scarce for Thomson to detect. Atomic Structure: Basic Concepts Neutrons • Because of the discovery of isotopes, scientists hypothesized that atoms contained still a third type of particle that explained these differences in mass. • Calculations showed that such a particle should have a mass equal to that of a proton but no electrical charge. • The existence of this neutral particle, called a neutron, was confirmed in the early 1930s by James Chadwick who bombarded a nucleus with alpha particles (video). Atomic Structure: Basic Concepts Rutherford’s Gold Foil Experiment • In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the cookie-dough model of the atom. Atomic Structure: Basic Concepts Rutherford’s Gold Foil Experiment • The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole. Atomic Structure: Basic Concepts Rutherford’s Gold Foil Experiment • Today, we know that the particles of the beam consisted of clusters containing two protons and two neutrons and are called alpha particles. • The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam. Atomic Structure: Basic Concepts The Gold Foil Experiment Atomic Structure: Basic Concepts The Nuclear Model of the Atom • To explain the results of the experiment, Rutherford’s team proposed a new model of the atom. • Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space. Atomic Structure: Basic Concepts The Nuclear Model of the Atom • Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a nucleus. Atomic Structure: Basic Concepts The Nuclear Model of the Atom • The new model of the atom as pictured by Rutherford’s group in 1911 is shown below.