Structure of the Atom

<body> <p align="center"><a name="top"><font size="6"><strong><b>The Atom and Electromagnetic Radiation</b></strong></font></a></p> <div align="center"><center><table border="1" width="500"> <tr> <td align="center"><a href="#fund">Fundamental Subatomic Particles</a></td> <td align="center"><a href="#electro">Electromagnetic Radiation</a></td> </tr> <tr> <td align="center" colspan="2"><a href="#light">Light and Other Forms of Electromagnetic Radiation</a></td> </tr> </table> </center></div><hr> <p align="center"><a name="fund"><b><i>Fundamental Subatomic Particles</i></b></a></p> <div align="center"><center><table border="0" cellpadding="0" cellspacing="0" width="500" bordercolor="#000000"> <tr> <td align="center" width="25%"><i><u>Particle</u></i><u> </u></td> <td align="center" width="25%"><i><u>Symbol </u></i></td> <td align="center" colspan="2" width="25%"><i><u>Charge</u></i><u> </u></td> <td align="center" width="25%"><i><u>Mass</u></i><u> </u></td> </tr> <tr> <td align="center" width="25%">electron </td> <td align="center" width="25%"><i>e</i><sup>-</sup> </td> <td align="center" colspan="2" width="25%">-1 </td> <td align="center" width="25%">0.0005486 amu </td> </tr> <tr> <td align="center" width="25%">proton </td> <td align="center" width="25%"><i>p</i><sup>+</sup> </td> <td align="center" colspan="2" width="25%">+1 </td> <td align="center" width="25%">1.007276 amu </td> </tr> <tr> <td align="center" width="25%" align="center">neutron </td> <td align="center" width="25%" align="center"><i>n</i><sup>o</sup> </td> <td align="center" colspan="2" width="25%" align="center">0 </td> <td align="center" width="25%" align="center">1.008665 amu </td> </tr> </table> </center></div><p>The number of protons, neutrons, and electrons in an atom can be determined from a set of simple rules. </p> <ul> <li>The number of protons in the nucleus of the atom is equal to the atomic number (<i>Z</i>). </li> <li>The number of electrons in a <i>neutral</i> atom is equal to the number of protons. </li> <li>The mass number of the atom (<i>M</i>) is equal to the sum of the number of protons and neutrons in the nucleus. </li> <li>The number of neutrons is equal to the difference between the mass number of the atom (<i>M</i>) and the atomic number (<i>Z</i>).</li> </ul> <p>Examples: Let's determine the number of protons, neutrons, and electrons in the following isotopes. </p> <div align="center"><center><table border="0" width="200"> <tr> <td><sup>12</sup>C</td> <td> </td> <td><sup>13</sup>C</td> <td> </td> <td><sup>14</sup>C</td> <td> </td> <td><sup>14</sup>N</td> <td> </td> </tr> </table> </center></div><p>The different isotopes of an element are identified by writing the mass number of the atom in the upper left corner of the symbol for the element. <sup>12</sup>C, <sup>13</sup>C, and <sup>14</sup>C are isotopes of carbon (<i>Z</i> = 6) and therefore contain six protons. If the atoms are neutral, they also must contain six electrons. The only difference between these isotopes is the number of neutrons in the nucleus.</p> <p align="center"><sup>12</sup>C: 6 electrons, 6 protons, and 6 neutrons</p> <p align="center"><sup>13</sup>C: 6 electrons, 6 protons, and 7 neutrons </p> <p align="center"><a name="prob1"><sup></sup></a><sup>14</sup>C: 6 electrons, 6 protons, and 8 neutrons</p> <div align="center"><center><table border="5" cellpadding="10" width="100%" bordercolor="#B87333"> <tr> <td width="100%"><em><strong>Practice Problem 1:</strong></em><p>Calculate the number of electrons in the Cl<sup>-</sup> and Fe<sup>3+</sup> ions. </p> <p><a href="http://chemed.chem.purdue.edu/~genchem/topicreview/bp/ch6/problems/ex6_3a.html"><em><strong>Click here to check your answer to Practice Problem 1</strong></em></a></p> </td> </tr> </table> </center></div><p align="center"><a href="#top"><img src="graphics/top.gif" alt="Return to Top of Page" border="0" width="226" height="36" sgi_fullpath="/disk2/chemistry/genchem/public_html/topicreview/bp/ch6/graphics/top.gif"></a></p> <hr> <p align="center"><a name="electro"><em><b>Electromagnetic Radiation</b></em></a></p> <p>Much of what is known about the structure of the electrons in an atom has been obtained by studying the interaction between matter and different forms of <b>electromagnetic radiation</b>. Electromagnetic radiation has some of the properties of both a particle and a wave. </p> <p><i>Particles</i> have a definite mass and they occupy space. <i>Waves</i> have no mass and yet they carry energy as they travel through space. In addition to their ability to carry energy, waves have four other characteristic properties: speed, frequency, wavelength, and amplitude. The <b>frequency</b> (<i>v</i>) is the number of waves (or cycles) per unit of time. The frequency of a wave is reported in units of cycles per second (s<sup>-1</sup>) or hertz (Hz). </p> <p>The idealized drawing of a wave in the figure below illustrates the definitions of amplitude and wavelength. The <b>wavelength</b> (<i>l</i>) is the smallest distance between repeating points on the wave. The <i>amplitude</i> of the wave is the distance between the highest (or lowest) point on the wave and the center of gravity of the wave. </p> <p align="center"><img src="graphics/wave.gif" alt="Diagram" width="407" height="116" sgi_fullpath="/disk2/chemistry/genchem/public_html/topicreview/bp/ch6/graphics/wave.gif"></p> <p>If we measure the frequency (<i>v</i>) of a wave in cycles per second and the wavelength (<i>l</i>) in meters, the product of these two numbers has the units of meters per second. The product of the frequency (<i>v</i>) times the wavelength (<i>l</i>) of a wave is therefore the speed (<i>s</i>) at which the wave travels through space. </p> <p align="center"><a name="prob2"><i></i></a><i>vl</i> = <i>s</i> </p> <div align="center"><center><table border="5" cellpadding="10" width="100%" bordercolor="#B87333"> <tr> <td width="100%"><em><strong>Practice Problem 2:</strong></em><p>What is the speed of a wave that has a wavelength of 1 meter and a frequency of 60 cycles per second?</p> <p><a href="http://chemed.chem.purdue.edu/~genchem/topicreview/bp/ch6/problems/ex6_0a.html"><em><strong>Click here to check your answer to Practice Problem 2</strong></em></a></p> </td> </tr> </table> </center></div><p align="center"><a name="prob3"></a></p> <div align="center"><center><table border="5" cellpadding="10" width="100%" bordercolor="#B87333"> <tr> <td width="100%"><em><strong>Practice Problem 3:</strong></em><p>Orchestras in the United States tune their instruments to an "A" that has a frequency of 440 cycles per second, or 440 Hz. If the speed of sound is 1116 feet per second, what is the wavelength of this note? </p> <p><a href="http://chemed.chem.purdue.edu/~genchem/topicreview/bp/ch6/problems/ex6_4a.html"><em><strong>Click here to check your answer to Practice Problem 3</strong></em></a></p> <p><a href="http://chemed.chem.purdue.edu/~genchem/topicreview/bp/ch6/problems/ex6_4s.html"><em><strong>Click here to see a solution to Practice Problem 3</strong></em></a></p> </td> </tr> </table> </center></div><p align="center"><a href="#top"><img src="graphics/top.gif" alt="Return to Top of Page" border="0" width="226" height="36" sgi_fullpath="/disk2/chemistry/genchem/public_html/topicreview/bp/ch6/graphics/top.gif"></a></p> <hr> <p align="center"><a name="light"><em><b>Light and Other Forms of Electromagnetic Radiation</b></em></a></p> <p>Light is a wave with both <i>electric</i> and <i>magnetic</i> components. It is therefore a form of <b>electromagnetic radiation</b>. </p> <p>Visible light contains the narrow band of frequencies and wavelengths in the portion of the electro-magnetic spectrum that our eyes can detect. It includes radiation with wavelengths between about 400 nm (violet) and 700 nm (red). Because it is a wave, light is bent when it enters a glass prism. When white light is focused on a prism, the light rays of different wavelengths are bent by differing amounts and the light is transformed into a spectrum of colors. Starting from the side of the spectrum where the light is bent by the smallest angle, the colors are red, orange, yellow, green, blue, and violet. </p> <p>As we can see from the following diagram, the energy carried by light increases as we go from red to blue across the visible spectrum. </p> <p align="center"><img src="graphics/spec.gif" alt="Diagram" width="538" height="171" sgi_fullpath="/disk2/chemistry/genchem/public_html/topicreview/bp/ch6/graphics/spec.gif"></p> <p>Because the wavelength of electromagnetic radiation can be as long as 40 m or as short as 10<sup>-5</sup> nm, the visible spectrum is only a small portion of the total range of electromagnetic radiation. </p> <p align="center"><img src="graphics/spectrum.gif" alt="Diagram" width="599" height="319" sgi_setwidth sgi_setheight sgi_fullpath="/disk2/chemistry/genchem/public_html/topicreview/bp/ch6/graphics/spectrum.gif"></p> <p>The electromagnetic spectrum includes radio and TV waves, microwaves, infrared, visible light, ultraviolet, x-rays, g-rays, and cosmic rays, as shown in the figure above. These different forms of radiation all travel at the speed of light (<i>c</i>). They differ, however, in their frequencies and wavelengths. The product of the frequency times the wavelength of electromagnetic radiation is always equal to the speed of light.</p> <p align="center"><i>vl</i> = <i>c</i></p> <p><a name="prob4"></a>As a result, electromagnetic radiation that has a long wavelength has a low frequency, and radiation with a high frequency has a short wavelength. </p> <div align="center"><center><table border="5" cellpadding="10" width="100%" bordercolor="#B87333"> <tr> <td width="100%"><em><strong>Practice Problem 4:</strong></em><p>Calculate the frequency of red light that has a wavelength of 700.0 nm if the speed of light is 2.998 x 10<sup>8</sup> m/s. </p> <p><a href="http://chemed.chem.purdue.edu/~genchem/topicreview/bp/ch6/problems/ex6_5a.html"><em><strong>Click here to check your answer to Practice Problem 4</strong></em></a></p> <p><a href="http://chemed.chem.purdue.edu/~genchem/topicreview/bp/ch6/problems/ex6_5s.html"><em><strong>Click here to see a solution to Practice Problem 4</strong></em></a></p> </td> </tr> </table> </center></div><p align="center"><a href="#top"><img src="graphics/top.gif" alt="Return to Top of Page" border="0" width="226" height="36" sgi_fullpath="/disk2/chemistry/genchem/public_html/topicreview/bp/ch6/graphics/top.gif"></a></p> <hr> <p><img src="http://chemed.chem.purdue.edu/Count.cgi?df=topicrevatom_emr.dat"></p> </body>